data link layer5-1 chapter 5: the data link layer our goals: understand principles behind data link...
TRANSCRIPT
Data Link Layer 5-1
Chapter 5 The Data Link LayerOur goals understand principles behind data link layer services error detection correction sharing a broadcast channel multiple access
link layer addressing reliable data transfer flow control done
instantiation and implementation of various link layer technologies
Data Link Layer 5-2
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-3
Link Layer IntroductionTerminology hosts and routers are
nodes communication channels
that connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
Data Link Layer 5-4
Link layer context
datagram transferred by different link protocols over different links eg Ethernet on first link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment = communication link
transportation mode = link layer protocol
travel agent = routing algorithm
Data Link Layer 5-5
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted pair)
wireless links high error ratesbull Q why both link-level and end-end reliability
Data Link Layer 5-6
Link Layer Services (more) flow control
pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but not at same time
Data Link Layer 5-7
Where is the link layer implemented in each and every host link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI card 80211 card
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-8
Adaptors Communicating
sending side encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction otherwise
Data Link Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-12
Internet checksum (review)
Sender treat segment
contents as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-
zero remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by G want remainder R
R = remainder[ ]
D2r
G
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-2
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-3
Link Layer IntroductionTerminology hosts and routers are
nodes communication channels
that connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
Data Link Layer 5-4
Link layer context
datagram transferred by different link protocols over different links eg Ethernet on first link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment = communication link
transportation mode = link layer protocol
travel agent = routing algorithm
Data Link Layer 5-5
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted pair)
wireless links high error ratesbull Q why both link-level and end-end reliability
Data Link Layer 5-6
Link Layer Services (more) flow control
pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but not at same time
Data Link Layer 5-7
Where is the link layer implemented in each and every host link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI card 80211 card
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-8
Adaptors Communicating
sending side encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction otherwise
Data Link Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-12
Internet checksum (review)
Sender treat segment
contents as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-
zero remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by G want remainder R
R = remainder[ ]
D2r
G
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-3
Link Layer IntroductionTerminology hosts and routers are
nodes communication channels
that connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
Data Link Layer 5-4
Link layer context
datagram transferred by different link protocols over different links eg Ethernet on first link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment = communication link
transportation mode = link layer protocol
travel agent = routing algorithm
Data Link Layer 5-5
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted pair)
wireless links high error ratesbull Q why both link-level and end-end reliability
Data Link Layer 5-6
Link Layer Services (more) flow control
pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but not at same time
Data Link Layer 5-7
Where is the link layer implemented in each and every host link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI card 80211 card
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-8
Adaptors Communicating
sending side encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction otherwise
Data Link Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-12
Internet checksum (review)
Sender treat segment
contents as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-
zero remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by G want remainder R
R = remainder[ ]
D2r
G
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-4
Link layer context
datagram transferred by different link protocols over different links eg Ethernet on first link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment = communication link
transportation mode = link layer protocol
travel agent = routing algorithm
Data Link Layer 5-5
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted pair)
wireless links high error ratesbull Q why both link-level and end-end reliability
Data Link Layer 5-6
Link Layer Services (more) flow control
pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but not at same time
Data Link Layer 5-7
Where is the link layer implemented in each and every host link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI card 80211 card
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-8
Adaptors Communicating
sending side encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction otherwise
Data Link Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-12
Internet checksum (review)
Sender treat segment
contents as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-
zero remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by G want remainder R
R = remainder[ ]
D2r
G
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-5
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted pair)
wireless links high error ratesbull Q why both link-level and end-end reliability
Data Link Layer 5-6
Link Layer Services (more) flow control
pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but not at same time
Data Link Layer 5-7
Where is the link layer implemented in each and every host link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI card 80211 card
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-8
Adaptors Communicating
sending side encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction otherwise
Data Link Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-12
Internet checksum (review)
Sender treat segment
contents as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-
zero remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by G want remainder R
R = remainder[ ]
D2r
G
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-6
Link Layer Services (more) flow control
pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but not at same time
Data Link Layer 5-7
Where is the link layer implemented in each and every host link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI card 80211 card
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-8
Adaptors Communicating
sending side encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction otherwise
Data Link Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-12
Internet checksum (review)
Sender treat segment
contents as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-
zero remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by G want remainder R
R = remainder[ ]
D2r
G
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-7
Where is the link layer implemented in each and every host link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI card 80211 card
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
Data Link Layer 5-8
Adaptors Communicating
sending side encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction otherwise
Data Link Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-12
Internet checksum (review)
Sender treat segment
contents as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-
zero remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by G want remainder R
R = remainder[ ]
D2r
G
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-8
Adaptors Communicating
sending side encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Data Link Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction otherwise
Data Link Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-12
Internet checksum (review)
Sender treat segment
contents as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-
zero remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by G want remainder R
R = remainder[ ]
D2r
G
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction otherwise
Data Link Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-12
Internet checksum (review)
Sender treat segment
contents as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-
zero remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by G want remainder R
R = remainder[ ]
D2r
G
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction otherwise
Data Link Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-12
Internet checksum (review)
Sender treat segment
contents as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-
zero remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by G want remainder R
R = remainder[ ]
D2r
G
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 5-12
Internet checksum (review)
Sender treat segment
contents as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-
zero remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by G want remainder R
R = remainder[ ]
D2r
G
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-12
Internet checksum (review)
Sender treat segment
contents as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Data Link Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-
zero remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by G want remainder R
R = remainder[ ]
D2r
G
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-
zero remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Data Link Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by G want remainder R
R = remainder[ ]
D2r
G
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by G want remainder R
R = remainder[ ]
D2r
G
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-15
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-16
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-17
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes interference collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-18
Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-19
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code space)
allocate piece to node for exclusive use Random Access
channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can take longer turns
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-20
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-21
Channel Partitioning MAC protocols FDMAFDMA frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency bands time
FDM cable
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Wireless Mobile Networks 6-22
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Wireless Mobile Networks 6-23
CDMA EncodeDecode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zim= dicmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zim
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zimcmm=1
M
M
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Wireless Mobile Networks 6-24
CDMA two-sender interference
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Space Division Multiple Access SDMA
Control the radiated powerenergy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-26
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-27
Slotted ALOHA
Assumptions all frames same size time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh frame transmits in next slot if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-28
Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting slots
idle slots nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-29
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run
fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-30
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization
when frame first arrives transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-1t0+1]
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-31
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0p0+1]
= p (1-p)N-1
(1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then
letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Formula
(Pure ALOHA)
(Slotted ALOHA)
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Throughput
Aloha Smax=184 Slotted Aloha Smax=368
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Aloha vs Slotted Aloha
Maximum throughput for Aloha is 184 while maximum throughput for slotted Aloha is 368 slotted Aloha doubles the throughput of pure aloha
Catch slotted Aloha needs synchronization of all users a global clock
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-35
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-36
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Non-persistent CSMA
A station senses the channel If the channel is busy it will not continue sensing the channel instead it delays a random period of time repeats the same procedure
If channel is idle it transmits If a collision occurs it will delay a random period of time starts all over again
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
1-persistent CSMA
A station (a user a transmitter) senses the channel
If the channel is busy it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle it will transmit
If a collision occurs it will delay a random period of time start it all over again
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
p-persistent CSMA
Observation if you sense idle others too collision is surely to occur
p-persist CSMA applies to slotted channels
A station senses the channel If it senses idle channel it transmits with probability p with probability q=1-p it defers to next slot repeats the same procedure
If it senses busy channel it delays a random number of slots starts all over
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Comparison
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-41
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-42
CSMACD collision detection
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-43
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize channel
high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-44
ldquoTaking Turnsrdquo MAC protocolsPolling master node ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-45
ldquoTaking Turnsrdquo MAC protocolsToken passing control token passed from one node to next sequentially
token message concerns
token overhead latency single point of failure (token)
T
data
(nothingto send)
T
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-46
Summary of MAC protocols channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring (8025)
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-47
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-48
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt
retransmission attempts to estimated current load heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-49
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-50
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-51
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches LANs VLANs
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Switched Ethernet
(a) Hub (b) Switch
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-53
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-54
Switch link-layer device smarter than hubs take active role store forward Ethernet frames examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-55
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on each incoming link but no collisions full duplex each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-56
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface to reach host time stamp)
looks like a routing table Q how are entries created maintained in switch table something like a routing protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-57
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-58
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-59
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-60
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-61
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-62
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-63
Switches vs Routers
both store-and-forward devices routers network-layer devices (examine network-layer headers)
switches are link-layer devices (examine link-layer headers)
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
application
transport
networklink
physical
networklink
physical
linkphysical
switch
datagram
application
transport
networklink
physical
frame
frame
frame
datagram
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-64
VLANs motivation
What happens if CS user moves office to
EE but wants connect to CS switch
single broadcast domain all layer-2 broadcast
traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
Engineering
ComputerEngineering
Whatrsquos wrong with this picture
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-65
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-66
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined switches plus routers
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-67
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-68
Type
2-byte Tag Protocol Identifier (value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-69
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-70
Point to Point Data Link Control one sender one receiver one link easier than broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time
ability to demultiplex upwards bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to network layer
network layer address negotiation endpoint can learnconfigure each otherrsquos network address
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-72
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-73
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option)
Control does nothing in the future possible multiple control fields
Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-74
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error detection
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-75
Byte Stuffing ldquodata transparencyrdquo requirement data field must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111101gt escape byte before each lt 01111110gt data byte
Receiver 01111101 before 01111110 discard first byte continue data reception
single 01111110 flag byte
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-77
PPP Data Control ProtocolBefore exchanging network-layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-78
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP 58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-79
Virtualization of networks
Virtualization of resources powerful abstraction in systems engineering
computing examples virtual memory virtual devices Virtual machines eg java IBM VM os from 1960rsquos70rsquos
layering of abstractions donrsquot sweat the details of the lower layer only deal with lower layers abstractly
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-80
The Internet virtualizing networks1974 multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha)
packet radio network
hellip differing in addressing conventions packet formats error recovery routing
ARPAnet satellite netA Protocol for Packet Network Intercommunication V Cerf R Kahn IEEE Transactions on Communications May 1974 pp 637-648
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-81
The Internet virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP) addressing internetwork
appears as single uniform entity despite underlying local network heterogeneity
network of networks
Gateway ldquoembed internetwork
packets in local packet format or extract themrdquo
route (at internetwork level) to next gateway
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-82
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-83
R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface 6 - A 0
in out outlabel label dest interface10 6 A 1
12 9 D 0
in out outlabel label dest interface 10 A 0
12 D 0
1
in out outlabel label dest interface 8 6 A 0
0
8 A 1
MPLS forwarding tables
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-84
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches
57 PPP58 Link virtualization MPLS
59 A day in the life of a web request
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-85
Synthesis a day in the life of a web request journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-86
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-87
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-88
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-89
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth In order to send frame to router need MAC address of router interface ARP ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-90
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server demuxed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS server
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-91
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established
64233169105
web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-
Data Link Layer 5-92
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
- Chapter 5 The Data Link Layer
- Link Layer
- Link Layer Introduction
- Link layer context
- Link Layer Services
- Link Layer Services (more)
- Where is the link layer implemented
- Adaptors Communicating
- Slide 9
- Error Detection
- Parity Checking
- Internet checksum (review)
- Checksumming Cyclic Redundancy Check
- CRC Example
- Slide 15
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Code Division Multiple Access (CDMA)
- CDMA EncodeDecode
- CDMA two-sender interference
- Space Division Multiple Access SDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 28
- Slotted Aloha efficiency
- Pure (unslotted) ALOHA
- Pure Aloha efficiency
- Formula
- Throughput
- Aloha vs Slotted Aloha
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- Non-persistent CSMA
- 1-persistent CSMA
- p-persistent CSMA
- Comparison
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 44
- Slide 45
- Summary of MAC protocols
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency
- 8023 Ethernet Standards Link amp Physical Layers
- Slide 51
- Switched Ethernet
- Hubs
- Switch
- Switch allows multiple simultaneous transmissions
- Switch Table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs Routers
- VLANs motivation
- VLANs
- Port-based VLAN
- VLANS spanning multiple switches
- Slide 68
- Slide 69
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 74
- Byte Stuffing
- Slide 76
- PPP Data Control Protocol
- Slide 78
- Virtualization of networks
- The Internet virtualizing networks
- Slide 81
- Multiprotocol label switching (MPLS)
- MPLS forwarding tables
- Slide 84
- Synthesis a day in the life of a web request
- A day in the life scenario
- A day in the lifehellip connecting to the Internet
- Slide 88
- A day in the lifehellip ARP (before DNS before HTTP)
- A day in the lifehellip using DNS
- A day in the lifehellip TCP connection carrying HTTP
- A day in the lifehellip HTTP requestreply
-