Computer Networking A Top-Down Approach 8th edition Jim Kurose Keith RossPearson 2020

Chapter 6The Link Layer and LANsA note on the use of these PowerPoint slidesWersquore making these slides freely available to all (faculty students readers) Theyrsquore in PowerPoint form so you see the animations and can add modify and delete slides (including this one) and slide content to suit your needs They obviously represent a lot of work on our part In return for use we only ask the following

sect If you use these slides (eg in a class) that you mention their source (after all wersquod like people to use our book)

sect If you post any slides on a www site that you note that they are adapted from (or perhaps identical to) our slides and note our copyright of this material

For a revision history see the slide note for this page

Thanks and enjoy JFKKWR

All material copyright 1996-2020JF Kurose and KW Ross All Rights Reserved

Link layer and LANs our goals

sectunderstand principles behind link layer servicesbull error detection correctionbull sharing a broadcast channel

multiple accessbull link layer addressingbull local area networks

Ethernet VLANssectdatacenter networks

sect instantiation implementation of various link layer technologies

Link Layer 6-2

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-3

Link layer introductionterminologysect hosts and routers nodessect communication channels that

connect adjacent nodes along communication path linksbull wired bull wireless bull LANs

sect layer-2 packet frame encapsulates datagram

mobile network

enterprisenetwork

national or global ISP

datacenter network

link layer has responsibility of transferring datagram from one node to physically adjacent node over a link

Link Layer 6-4

Link layer context

sect datagram transferred by different link protocols over different linksbull eg WiFi on first link Ethernet

on next link

sect each link protocol provides different servicesbull eg may or may not provide

reliable data transfer over link

transportation analogysect trip from Princeton to Lausannebull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne

sect tourist = datagramsect transport segment =

communication linksect transportation mode = link-layer

protocolsect travel agent = routing algorithm

Link Layer 6-5

Link layer servicessect framing link accessbull encapsulate datagram into frame adding

header trailerbull channel access if shared mediumbull ldquoMACrdquo addresses in frame headers identify

source destination (different from IP address)

sect reliable delivery between adjacent nodesbull we already know how to do thisbull seldom used on low bit-error linksbull wireless links high error ratesbull Q why both link-level and end-end

reliability

hellip

hellip

Link Layer 6-6

Link layer services (more)sect flow control bull pacing between adjacent sending and

receiving nodessect error detection bull errors caused by signal attenuation noise bull receiver detects errors signals

retransmission or drops frame sect error correction bull receiver identifies and corrects bit error(s)

without retransmissionsect half-duplex and full-duplexbull with half duplex nodes at both ends of

link can transmit but not at same time

hellip

hellip

Link Layer 6-7

Where is the link layer implemented

sect in each-and-every hostsect link layer implemented in

network interface card (NIC) or on a chipbull Ethernet WiFi card or chipbull implements link physical layer

sect attaches into hostrsquos system buses

sect combination of hardware software firmware

controller

physical

cpu memory

host bus (eg PCI)

network interface

applicationtransportnetworklink

linkphysical

Link Layer 6-8

controller

physical

memory CPU

Interfaces communicating

controller

physical

cpu memory

applicationtransportnetworklink

linkphysical

applicationtransportnetworklink

linkphysical

sending sidesect encapsulates datagram in framesect adds error checking bits reliable data

transfer flow control etc

receiving sidesect looks for errors reliable data

transfer flow control etcsect extracts datagram passes to

upper layer at receiving side

linkhlinkh datagram

datagram

datagram

Link Layer 6-9

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsecterror detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-10

Error detection

Link Layer 6-11

EDC error detection and correction bits (eg redundancy)D data protected by error checking may include header fields

Error detection not 100 reliablesect protocol may miss

some errors but rarelysect larger EDC field yields

better detection and correction

datagram

D EDC

d data bits

bit-error prone link

Drsquo EDCrsquo

allbits in Drsquo

OK

Ndetected error

otherwise

datagram

Parity checking

Link Layer 6-12

single bit paritysect detect single bit errors

0111000110101011 1

paritybit

d data bits

two-dimensional bit paritysect detect and correct single bit errors

d11d21

di1

d1j+1d2j+1

dij+1

d1jd2j

dij

di+11 di+1j+1di+1j

row parity

columnparity

0 1 1 1 0 1

1 0 1 0 1 11 1 1 1 0 0

1 0 1 0 1 0

no errors

0 1 1 1 0 1

1 0 1 0 1 11 0 1 1 0 0

1 0 1 0 1 0parityerror

parityerror

detectedand

correctablesingle-bit

error

Even parity set parity bit so there is an even number of 1rsquos

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

Internet checksum (review)

sendersect treat contents of UDP

segment (including UDP header fields and IP addresses) as sequence of 16-bit integers

sect checksum addition (onersquos complement sum) of segment content

sect checksum value put into UDP checksum field

receiversect compute checksum of received

segmentsect check if computed checksum equals

checksum field valuebull not equal - error detectedbull equal - no error detected But maybe

errors nonetheless More later hellip

Goal detect errors (ie flipped bits) in transmitted segment

Transport Layer 3-13

Cyclic Redundancy Check (CRC)sectmore powerful error-detection codingsectD data bits (given think of these as a binary number)

sectG bit pattern (generator) of r+1 bits (given)

Link Layer 6-14

goal choose r CRC bits R such that ltDRgt exactly divisible by G (mod 2) bull receiver knows G divides ltDRgt by G If non-zero remainder error detectedbull can detect all burst errors less than r+1 bitsbull widely used in practice (Ethernet 80211 WiFi)

r CRC bitsd data bits

D R

ltDRgt = D 2r XOR R

bit pattern

formula for bit pattern

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Link layer and LANs our goals

sectunderstand principles behind link layer servicesbull error detection correctionbull sharing a broadcast channel

multiple accessbull link layer addressingbull local area networks

Ethernet VLANssectdatacenter networks

sect instantiation implementation of various link layer technologies

Link Layer 6-2

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-3

Link layer introductionterminologysect hosts and routers nodessect communication channels that

connect adjacent nodes along communication path linksbull wired bull wireless bull LANs

sect layer-2 packet frame encapsulates datagram

mobile network

enterprisenetwork

national or global ISP

datacenter network

link layer has responsibility of transferring datagram from one node to physically adjacent node over a link

Link Layer 6-4

Link layer context

sect datagram transferred by different link protocols over different linksbull eg WiFi on first link Ethernet

on next link

sect each link protocol provides different servicesbull eg may or may not provide

reliable data transfer over link

transportation analogysect trip from Princeton to Lausannebull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne

sect tourist = datagramsect transport segment =

communication linksect transportation mode = link-layer

protocolsect travel agent = routing algorithm

Link Layer 6-5

Link layer servicessect framing link accessbull encapsulate datagram into frame adding

header trailerbull channel access if shared mediumbull ldquoMACrdquo addresses in frame headers identify

source destination (different from IP address)

sect reliable delivery between adjacent nodesbull we already know how to do thisbull seldom used on low bit-error linksbull wireless links high error ratesbull Q why both link-level and end-end

reliability

hellip

hellip

Link Layer 6-6

Link layer services (more)sect flow control bull pacing between adjacent sending and

receiving nodessect error detection bull errors caused by signal attenuation noise bull receiver detects errors signals

retransmission or drops frame sect error correction bull receiver identifies and corrects bit error(s)

without retransmissionsect half-duplex and full-duplexbull with half duplex nodes at both ends of

link can transmit but not at same time

hellip

hellip

Link Layer 6-7

Where is the link layer implemented

sect in each-and-every hostsect link layer implemented in

network interface card (NIC) or on a chipbull Ethernet WiFi card or chipbull implements link physical layer

sect attaches into hostrsquos system buses

sect combination of hardware software firmware

controller

physical

cpu memory

host bus (eg PCI)

network interface

applicationtransportnetworklink

linkphysical

Link Layer 6-8

controller

physical

memory CPU

Interfaces communicating

controller

physical

cpu memory

applicationtransportnetworklink

linkphysical

applicationtransportnetworklink

linkphysical

sending sidesect encapsulates datagram in framesect adds error checking bits reliable data

transfer flow control etc

receiving sidesect looks for errors reliable data

transfer flow control etcsect extracts datagram passes to

upper layer at receiving side

linkhlinkh datagram

datagram

datagram

Link Layer 6-9

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsecterror detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-10

Error detection

Link Layer 6-11

EDC error detection and correction bits (eg redundancy)D data protected by error checking may include header fields

Error detection not 100 reliablesect protocol may miss

some errors but rarelysect larger EDC field yields

better detection and correction

datagram

D EDC

d data bits

bit-error prone link

Drsquo EDCrsquo

allbits in Drsquo

OK

Ndetected error

otherwise

datagram

Parity checking

Link Layer 6-12

single bit paritysect detect single bit errors

0111000110101011 1

paritybit

d data bits

two-dimensional bit paritysect detect and correct single bit errors

d11d21

di1

d1j+1d2j+1

dij+1

d1jd2j

dij

di+11 di+1j+1di+1j

row parity

columnparity

0 1 1 1 0 1

1 0 1 0 1 11 1 1 1 0 0

1 0 1 0 1 0

no errors

0 1 1 1 0 1

1 0 1 0 1 11 0 1 1 0 0

1 0 1 0 1 0parityerror

parityerror

detectedand

correctablesingle-bit

error

Even parity set parity bit so there is an even number of 1rsquos

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

Internet checksum (review)

sendersect treat contents of UDP

segment (including UDP header fields and IP addresses) as sequence of 16-bit integers

sect checksum addition (onersquos complement sum) of segment content

sect checksum value put into UDP checksum field

receiversect compute checksum of received

segmentsect check if computed checksum equals

checksum field valuebull not equal - error detectedbull equal - no error detected But maybe

errors nonetheless More later hellip

Goal detect errors (ie flipped bits) in transmitted segment

Transport Layer 3-13

Cyclic Redundancy Check (CRC)sectmore powerful error-detection codingsectD data bits (given think of these as a binary number)

sectG bit pattern (generator) of r+1 bits (given)

Link Layer 6-14

goal choose r CRC bits R such that ltDRgt exactly divisible by G (mod 2) bull receiver knows G divides ltDRgt by G If non-zero remainder error detectedbull can detect all burst errors less than r+1 bitsbull widely used in practice (Ethernet 80211 WiFi)

r CRC bitsd data bits

D R

ltDRgt = D 2r XOR R

bit pattern

formula for bit pattern

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-3

Link layer introductionterminologysect hosts and routers nodessect communication channels that

connect adjacent nodes along communication path linksbull wired bull wireless bull LANs

sect layer-2 packet frame encapsulates datagram

mobile network

enterprisenetwork

national or global ISP

datacenter network

link layer has responsibility of transferring datagram from one node to physically adjacent node over a link

Link Layer 6-4

Link layer context

sect datagram transferred by different link protocols over different linksbull eg WiFi on first link Ethernet

on next link

sect each link protocol provides different servicesbull eg may or may not provide

reliable data transfer over link

transportation analogysect trip from Princeton to Lausannebull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne

sect tourist = datagramsect transport segment =

communication linksect transportation mode = link-layer

protocolsect travel agent = routing algorithm

Link Layer 6-5

Link layer servicessect framing link accessbull encapsulate datagram into frame adding

header trailerbull channel access if shared mediumbull ldquoMACrdquo addresses in frame headers identify

source destination (different from IP address)

sect reliable delivery between adjacent nodesbull we already know how to do thisbull seldom used on low bit-error linksbull wireless links high error ratesbull Q why both link-level and end-end

reliability

hellip

hellip

Link Layer 6-6

Link layer services (more)sect flow control bull pacing between adjacent sending and

receiving nodessect error detection bull errors caused by signal attenuation noise bull receiver detects errors signals

retransmission or drops frame sect error correction bull receiver identifies and corrects bit error(s)

without retransmissionsect half-duplex and full-duplexbull with half duplex nodes at both ends of

link can transmit but not at same time

hellip

hellip

Link Layer 6-7

Where is the link layer implemented

sect in each-and-every hostsect link layer implemented in

network interface card (NIC) or on a chipbull Ethernet WiFi card or chipbull implements link physical layer

sect attaches into hostrsquos system buses

sect combination of hardware software firmware

controller

physical

cpu memory

host bus (eg PCI)

network interface

applicationtransportnetworklink

linkphysical

Link Layer 6-8

controller

physical

memory CPU

Interfaces communicating

controller

physical

cpu memory

applicationtransportnetworklink

linkphysical

applicationtransportnetworklink

linkphysical

sending sidesect encapsulates datagram in framesect adds error checking bits reliable data

transfer flow control etc

receiving sidesect looks for errors reliable data

transfer flow control etcsect extracts datagram passes to

upper layer at receiving side

linkhlinkh datagram

datagram

datagram

Link Layer 6-9

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsecterror detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-10

Error detection

Link Layer 6-11

EDC error detection and correction bits (eg redundancy)D data protected by error checking may include header fields

Error detection not 100 reliablesect protocol may miss

some errors but rarelysect larger EDC field yields

better detection and correction

datagram

D EDC

d data bits

bit-error prone link

Drsquo EDCrsquo

allbits in Drsquo

OK

Ndetected error

otherwise

datagram

Parity checking

Link Layer 6-12

single bit paritysect detect single bit errors

0111000110101011 1

paritybit

d data bits

two-dimensional bit paritysect detect and correct single bit errors

d11d21

di1

d1j+1d2j+1

dij+1

d1jd2j

dij

di+11 di+1j+1di+1j

row parity

columnparity

0 1 1 1 0 1

1 0 1 0 1 11 1 1 1 0 0

1 0 1 0 1 0

no errors

0 1 1 1 0 1

1 0 1 0 1 11 0 1 1 0 0

1 0 1 0 1 0parityerror

parityerror

detectedand

correctablesingle-bit

error

Even parity set parity bit so there is an even number of 1rsquos

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

Internet checksum (review)

sendersect treat contents of UDP

segment (including UDP header fields and IP addresses) as sequence of 16-bit integers

sect checksum addition (onersquos complement sum) of segment content

sect checksum value put into UDP checksum field

receiversect compute checksum of received

segmentsect check if computed checksum equals

checksum field valuebull not equal - error detectedbull equal - no error detected But maybe

errors nonetheless More later hellip

Goal detect errors (ie flipped bits) in transmitted segment

Transport Layer 3-13

Cyclic Redundancy Check (CRC)sectmore powerful error-detection codingsectD data bits (given think of these as a binary number)

sectG bit pattern (generator) of r+1 bits (given)

Link Layer 6-14

goal choose r CRC bits R such that ltDRgt exactly divisible by G (mod 2) bull receiver knows G divides ltDRgt by G If non-zero remainder error detectedbull can detect all burst errors less than r+1 bitsbull widely used in practice (Ethernet 80211 WiFi)

r CRC bitsd data bits

D R

ltDRgt = D 2r XOR R

bit pattern

formula for bit pattern

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Link layer introductionterminologysect hosts and routers nodessect communication channels that

connect adjacent nodes along communication path linksbull wired bull wireless bull LANs

sect layer-2 packet frame encapsulates datagram

mobile network

enterprisenetwork

national or global ISP

datacenter network

link layer has responsibility of transferring datagram from one node to physically adjacent node over a link

Link Layer 6-4

Link layer context

sect datagram transferred by different link protocols over different linksbull eg WiFi on first link Ethernet

on next link

sect each link protocol provides different servicesbull eg may or may not provide

reliable data transfer over link

transportation analogysect trip from Princeton to Lausannebull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne

sect tourist = datagramsect transport segment =

communication linksect transportation mode = link-layer

protocolsect travel agent = routing algorithm

Link Layer 6-5

Link layer servicessect framing link accessbull encapsulate datagram into frame adding

header trailerbull channel access if shared mediumbull ldquoMACrdquo addresses in frame headers identify

source destination (different from IP address)

sect reliable delivery between adjacent nodesbull we already know how to do thisbull seldom used on low bit-error linksbull wireless links high error ratesbull Q why both link-level and end-end

reliability

hellip

hellip

Link Layer 6-6

Link layer services (more)sect flow control bull pacing between adjacent sending and

receiving nodessect error detection bull errors caused by signal attenuation noise bull receiver detects errors signals

retransmission or drops frame sect error correction bull receiver identifies and corrects bit error(s)

without retransmissionsect half-duplex and full-duplexbull with half duplex nodes at both ends of

link can transmit but not at same time

hellip

hellip

Link Layer 6-7

Where is the link layer implemented

sect in each-and-every hostsect link layer implemented in

network interface card (NIC) or on a chipbull Ethernet WiFi card or chipbull implements link physical layer

sect attaches into hostrsquos system buses

sect combination of hardware software firmware

controller

physical

cpu memory

host bus (eg PCI)

network interface

applicationtransportnetworklink

linkphysical

Link Layer 6-8

controller

physical

memory CPU

Interfaces communicating

controller

physical

cpu memory

applicationtransportnetworklink

linkphysical

applicationtransportnetworklink

linkphysical

sending sidesect encapsulates datagram in framesect adds error checking bits reliable data

transfer flow control etc

receiving sidesect looks for errors reliable data

transfer flow control etcsect extracts datagram passes to

upper layer at receiving side

linkhlinkh datagram

datagram

datagram

Link Layer 6-9

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsecterror detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-10

Error detection

Link Layer 6-11

EDC error detection and correction bits (eg redundancy)D data protected by error checking may include header fields

Error detection not 100 reliablesect protocol may miss

some errors but rarelysect larger EDC field yields

better detection and correction

datagram

D EDC

d data bits

bit-error prone link

Drsquo EDCrsquo

allbits in Drsquo

OK

Ndetected error

otherwise

datagram

Parity checking

Link Layer 6-12

single bit paritysect detect single bit errors

0111000110101011 1

paritybit

d data bits

two-dimensional bit paritysect detect and correct single bit errors

d11d21

di1

d1j+1d2j+1

dij+1

d1jd2j

dij

di+11 di+1j+1di+1j

row parity

columnparity

0 1 1 1 0 1

1 0 1 0 1 11 1 1 1 0 0

1 0 1 0 1 0

no errors

0 1 1 1 0 1

1 0 1 0 1 11 0 1 1 0 0

1 0 1 0 1 0parityerror

parityerror

detectedand

correctablesingle-bit

error

Even parity set parity bit so there is an even number of 1rsquos

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

Internet checksum (review)

sendersect treat contents of UDP

segment (including UDP header fields and IP addresses) as sequence of 16-bit integers

sect checksum addition (onersquos complement sum) of segment content

sect checksum value put into UDP checksum field

receiversect compute checksum of received

segmentsect check if computed checksum equals

checksum field valuebull not equal - error detectedbull equal - no error detected But maybe

errors nonetheless More later hellip

Goal detect errors (ie flipped bits) in transmitted segment

Transport Layer 3-13

Cyclic Redundancy Check (CRC)sectmore powerful error-detection codingsectD data bits (given think of these as a binary number)

sectG bit pattern (generator) of r+1 bits (given)

Link Layer 6-14

goal choose r CRC bits R such that ltDRgt exactly divisible by G (mod 2) bull receiver knows G divides ltDRgt by G If non-zero remainder error detectedbull can detect all burst errors less than r+1 bitsbull widely used in practice (Ethernet 80211 WiFi)

r CRC bitsd data bits

D R

ltDRgt = D 2r XOR R

bit pattern

formula for bit pattern

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Link layer context

sect datagram transferred by different link protocols over different linksbull eg WiFi on first link Ethernet

on next link

sect each link protocol provides different servicesbull eg may or may not provide

reliable data transfer over link

transportation analogysect trip from Princeton to Lausannebull limo Princeton to JFKbull plane JFK to Genevabull train Geneva to Lausanne

sect tourist = datagramsect transport segment =

communication linksect transportation mode = link-layer

protocolsect travel agent = routing algorithm

Link Layer 6-5

Link layer servicessect framing link accessbull encapsulate datagram into frame adding

header trailerbull channel access if shared mediumbull ldquoMACrdquo addresses in frame headers identify

source destination (different from IP address)

sect reliable delivery between adjacent nodesbull we already know how to do thisbull seldom used on low bit-error linksbull wireless links high error ratesbull Q why both link-level and end-end

reliability

hellip

hellip

Link Layer 6-6

Link layer services (more)sect flow control bull pacing between adjacent sending and

receiving nodessect error detection bull errors caused by signal attenuation noise bull receiver detects errors signals

retransmission or drops frame sect error correction bull receiver identifies and corrects bit error(s)

without retransmissionsect half-duplex and full-duplexbull with half duplex nodes at both ends of

link can transmit but not at same time

hellip

hellip

Link Layer 6-7

Where is the link layer implemented

sect in each-and-every hostsect link layer implemented in

network interface card (NIC) or on a chipbull Ethernet WiFi card or chipbull implements link physical layer

sect attaches into hostrsquos system buses

sect combination of hardware software firmware

controller

physical

cpu memory

host bus (eg PCI)

network interface

applicationtransportnetworklink

linkphysical

Link Layer 6-8

controller

physical

memory CPU

Interfaces communicating

controller

physical

cpu memory

applicationtransportnetworklink

linkphysical

applicationtransportnetworklink

linkphysical

sending sidesect encapsulates datagram in framesect adds error checking bits reliable data

transfer flow control etc

receiving sidesect looks for errors reliable data

transfer flow control etcsect extracts datagram passes to

upper layer at receiving side

linkhlinkh datagram

datagram

datagram

Link Layer 6-9

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsecterror detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-10

Error detection

Link Layer 6-11

EDC error detection and correction bits (eg redundancy)D data protected by error checking may include header fields

Error detection not 100 reliablesect protocol may miss

some errors but rarelysect larger EDC field yields

better detection and correction

datagram

D EDC

d data bits

bit-error prone link

Drsquo EDCrsquo

allbits in Drsquo

OK

Ndetected error

otherwise

datagram

Parity checking

Link Layer 6-12

single bit paritysect detect single bit errors

0111000110101011 1

paritybit

d data bits

two-dimensional bit paritysect detect and correct single bit errors

d11d21

di1

d1j+1d2j+1

dij+1

d1jd2j

dij

di+11 di+1j+1di+1j

row parity

columnparity

0 1 1 1 0 1

1 0 1 0 1 11 1 1 1 0 0

1 0 1 0 1 0

no errors

0 1 1 1 0 1

1 0 1 0 1 11 0 1 1 0 0

1 0 1 0 1 0parityerror

parityerror

detectedand

correctablesingle-bit

error

Even parity set parity bit so there is an even number of 1rsquos

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

Internet checksum (review)

sendersect treat contents of UDP

segment (including UDP header fields and IP addresses) as sequence of 16-bit integers

sect checksum addition (onersquos complement sum) of segment content

sect checksum value put into UDP checksum field

receiversect compute checksum of received

segmentsect check if computed checksum equals

checksum field valuebull not equal - error detectedbull equal - no error detected But maybe

errors nonetheless More later hellip

Goal detect errors (ie flipped bits) in transmitted segment

Transport Layer 3-13

Cyclic Redundancy Check (CRC)sectmore powerful error-detection codingsectD data bits (given think of these as a binary number)

sectG bit pattern (generator) of r+1 bits (given)

Link Layer 6-14

goal choose r CRC bits R such that ltDRgt exactly divisible by G (mod 2) bull receiver knows G divides ltDRgt by G If non-zero remainder error detectedbull can detect all burst errors less than r+1 bitsbull widely used in practice (Ethernet 80211 WiFi)

r CRC bitsd data bits

D R

ltDRgt = D 2r XOR R

bit pattern

formula for bit pattern

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Link layer servicessect framing link accessbull encapsulate datagram into frame adding

header trailerbull channel access if shared mediumbull ldquoMACrdquo addresses in frame headers identify

source destination (different from IP address)

sect reliable delivery between adjacent nodesbull we already know how to do thisbull seldom used on low bit-error linksbull wireless links high error ratesbull Q why both link-level and end-end

reliability

hellip

hellip

Link Layer 6-6

Link layer services (more)sect flow control bull pacing between adjacent sending and

receiving nodessect error detection bull errors caused by signal attenuation noise bull receiver detects errors signals

retransmission or drops frame sect error correction bull receiver identifies and corrects bit error(s)

without retransmissionsect half-duplex and full-duplexbull with half duplex nodes at both ends of

link can transmit but not at same time

hellip

hellip

Link Layer 6-7

Where is the link layer implemented

sect in each-and-every hostsect link layer implemented in

network interface card (NIC) or on a chipbull Ethernet WiFi card or chipbull implements link physical layer

sect attaches into hostrsquos system buses

sect combination of hardware software firmware

controller

physical

cpu memory

host bus (eg PCI)

network interface

applicationtransportnetworklink

linkphysical

Link Layer 6-8

controller

physical

memory CPU

Interfaces communicating

controller

physical

cpu memory

applicationtransportnetworklink

linkphysical

applicationtransportnetworklink

linkphysical

sending sidesect encapsulates datagram in framesect adds error checking bits reliable data

transfer flow control etc

receiving sidesect looks for errors reliable data

transfer flow control etcsect extracts datagram passes to

upper layer at receiving side

linkhlinkh datagram

datagram

datagram

Link Layer 6-9

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsecterror detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-10

Error detection

Link Layer 6-11

EDC error detection and correction bits (eg redundancy)D data protected by error checking may include header fields

Error detection not 100 reliablesect protocol may miss

some errors but rarelysect larger EDC field yields

better detection and correction

datagram

D EDC

d data bits

bit-error prone link

Drsquo EDCrsquo

allbits in Drsquo

OK

Ndetected error

otherwise

datagram

Parity checking

Link Layer 6-12

single bit paritysect detect single bit errors

0111000110101011 1

paritybit

d data bits

two-dimensional bit paritysect detect and correct single bit errors

d11d21

di1

d1j+1d2j+1

dij+1

d1jd2j

dij

di+11 di+1j+1di+1j

row parity

columnparity

0 1 1 1 0 1

1 0 1 0 1 11 1 1 1 0 0

1 0 1 0 1 0

no errors

0 1 1 1 0 1

1 0 1 0 1 11 0 1 1 0 0

1 0 1 0 1 0parityerror

parityerror

detectedand

correctablesingle-bit

error

Even parity set parity bit so there is an even number of 1rsquos

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

Internet checksum (review)

sendersect treat contents of UDP

segment (including UDP header fields and IP addresses) as sequence of 16-bit integers

sect checksum addition (onersquos complement sum) of segment content

sect checksum value put into UDP checksum field

receiversect compute checksum of received

segmentsect check if computed checksum equals

checksum field valuebull not equal - error detectedbull equal - no error detected But maybe

errors nonetheless More later hellip

Goal detect errors (ie flipped bits) in transmitted segment

Transport Layer 3-13

Cyclic Redundancy Check (CRC)sectmore powerful error-detection codingsectD data bits (given think of these as a binary number)

sectG bit pattern (generator) of r+1 bits (given)

Link Layer 6-14

goal choose r CRC bits R such that ltDRgt exactly divisible by G (mod 2) bull receiver knows G divides ltDRgt by G If non-zero remainder error detectedbull can detect all burst errors less than r+1 bitsbull widely used in practice (Ethernet 80211 WiFi)

r CRC bitsd data bits

D R

ltDRgt = D 2r XOR R

bit pattern

formula for bit pattern

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Link layer services (more)sect flow control bull pacing between adjacent sending and

receiving nodessect error detection bull errors caused by signal attenuation noise bull receiver detects errors signals

retransmission or drops frame sect error correction bull receiver identifies and corrects bit error(s)

without retransmissionsect half-duplex and full-duplexbull with half duplex nodes at both ends of

link can transmit but not at same time

hellip

hellip

Link Layer 6-7

Where is the link layer implemented

sect in each-and-every hostsect link layer implemented in

network interface card (NIC) or on a chipbull Ethernet WiFi card or chipbull implements link physical layer

sect attaches into hostrsquos system buses

sect combination of hardware software firmware

controller

physical

cpu memory

host bus (eg PCI)

network interface

applicationtransportnetworklink

linkphysical

Link Layer 6-8

controller

physical

memory CPU

Interfaces communicating

controller

physical

cpu memory

applicationtransportnetworklink

linkphysical

applicationtransportnetworklink

linkphysical

sending sidesect encapsulates datagram in framesect adds error checking bits reliable data

transfer flow control etc

receiving sidesect looks for errors reliable data

transfer flow control etcsect extracts datagram passes to

upper layer at receiving side

linkhlinkh datagram

datagram

datagram

Link Layer 6-9

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsecterror detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-10

Error detection

Link Layer 6-11

EDC error detection and correction bits (eg redundancy)D data protected by error checking may include header fields

Error detection not 100 reliablesect protocol may miss

some errors but rarelysect larger EDC field yields

better detection and correction

datagram

D EDC

d data bits

bit-error prone link

Drsquo EDCrsquo

allbits in Drsquo

OK

Ndetected error

otherwise

datagram

Parity checking

Link Layer 6-12

single bit paritysect detect single bit errors

0111000110101011 1

paritybit

d data bits

two-dimensional bit paritysect detect and correct single bit errors

d11d21

di1

d1j+1d2j+1

dij+1

d1jd2j

dij

di+11 di+1j+1di+1j

row parity

columnparity

0 1 1 1 0 1

1 0 1 0 1 11 1 1 1 0 0

1 0 1 0 1 0

no errors

0 1 1 1 0 1

1 0 1 0 1 11 0 1 1 0 0

1 0 1 0 1 0parityerror

parityerror

detectedand

correctablesingle-bit

error

Even parity set parity bit so there is an even number of 1rsquos

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

Internet checksum (review)

sendersect treat contents of UDP

segment (including UDP header fields and IP addresses) as sequence of 16-bit integers

sect checksum addition (onersquos complement sum) of segment content

sect checksum value put into UDP checksum field

receiversect compute checksum of received

segmentsect check if computed checksum equals

checksum field valuebull not equal - error detectedbull equal - no error detected But maybe

errors nonetheless More later hellip

Goal detect errors (ie flipped bits) in transmitted segment

Transport Layer 3-13

Cyclic Redundancy Check (CRC)sectmore powerful error-detection codingsectD data bits (given think of these as a binary number)

sectG bit pattern (generator) of r+1 bits (given)

Link Layer 6-14

goal choose r CRC bits R such that ltDRgt exactly divisible by G (mod 2) bull receiver knows G divides ltDRgt by G If non-zero remainder error detectedbull can detect all burst errors less than r+1 bitsbull widely used in practice (Ethernet 80211 WiFi)

r CRC bitsd data bits

D R

ltDRgt = D 2r XOR R

bit pattern

formula for bit pattern

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Where is the link layer implemented

sect in each-and-every hostsect link layer implemented in

network interface card (NIC) or on a chipbull Ethernet WiFi card or chipbull implements link physical layer

sect attaches into hostrsquos system buses

sect combination of hardware software firmware

controller

physical

cpu memory

host bus (eg PCI)

network interface

applicationtransportnetworklink

linkphysical

Link Layer 6-8

controller

physical

memory CPU

Interfaces communicating

controller

physical

cpu memory

applicationtransportnetworklink

linkphysical

applicationtransportnetworklink

linkphysical

sending sidesect encapsulates datagram in framesect adds error checking bits reliable data

transfer flow control etc

receiving sidesect looks for errors reliable data

transfer flow control etcsect extracts datagram passes to

upper layer at receiving side

linkhlinkh datagram

datagram

datagram

Link Layer 6-9

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsecterror detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-10

Error detection

Link Layer 6-11

EDC error detection and correction bits (eg redundancy)D data protected by error checking may include header fields

Error detection not 100 reliablesect protocol may miss

some errors but rarelysect larger EDC field yields

better detection and correction

datagram

D EDC

d data bits

bit-error prone link

Drsquo EDCrsquo

allbits in Drsquo

OK

Ndetected error

otherwise

datagram

Parity checking

Link Layer 6-12

single bit paritysect detect single bit errors

0111000110101011 1

paritybit

d data bits

two-dimensional bit paritysect detect and correct single bit errors

d11d21

di1

d1j+1d2j+1

dij+1

d1jd2j

dij

di+11 di+1j+1di+1j

row parity

columnparity

0 1 1 1 0 1

1 0 1 0 1 11 1 1 1 0 0

1 0 1 0 1 0

no errors

0 1 1 1 0 1

1 0 1 0 1 11 0 1 1 0 0

1 0 1 0 1 0parityerror

parityerror

detectedand

correctablesingle-bit

error

Even parity set parity bit so there is an even number of 1rsquos

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

Internet checksum (review)

sendersect treat contents of UDP

segment (including UDP header fields and IP addresses) as sequence of 16-bit integers

sect checksum addition (onersquos complement sum) of segment content

sect checksum value put into UDP checksum field

receiversect compute checksum of received

segmentsect check if computed checksum equals

checksum field valuebull not equal - error detectedbull equal - no error detected But maybe

errors nonetheless More later hellip

Goal detect errors (ie flipped bits) in transmitted segment

Transport Layer 3-13

Cyclic Redundancy Check (CRC)sectmore powerful error-detection codingsectD data bits (given think of these as a binary number)

sectG bit pattern (generator) of r+1 bits (given)

Link Layer 6-14

goal choose r CRC bits R such that ltDRgt exactly divisible by G (mod 2) bull receiver knows G divides ltDRgt by G If non-zero remainder error detectedbull can detect all burst errors less than r+1 bitsbull widely used in practice (Ethernet 80211 WiFi)

r CRC bitsd data bits

D R

ltDRgt = D 2r XOR R

bit pattern

formula for bit pattern

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

controller

physical

memory CPU

Interfaces communicating

controller

physical

cpu memory

applicationtransportnetworklink

linkphysical

applicationtransportnetworklink

linkphysical

sending sidesect encapsulates datagram in framesect adds error checking bits reliable data

transfer flow control etc

receiving sidesect looks for errors reliable data

transfer flow control etcsect extracts datagram passes to

upper layer at receiving side

linkhlinkh datagram

datagram

datagram

Link Layer 6-9

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsecterror detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-10

Error detection

Link Layer 6-11

EDC error detection and correction bits (eg redundancy)D data protected by error checking may include header fields

Error detection not 100 reliablesect protocol may miss

some errors but rarelysect larger EDC field yields

better detection and correction

datagram

D EDC

d data bits

bit-error prone link

Drsquo EDCrsquo

allbits in Drsquo

OK

Ndetected error

otherwise

datagram

Parity checking

Link Layer 6-12

single bit paritysect detect single bit errors

0111000110101011 1

paritybit

d data bits

two-dimensional bit paritysect detect and correct single bit errors

d11d21

di1

d1j+1d2j+1

dij+1

d1jd2j

dij

di+11 di+1j+1di+1j

row parity

columnparity

0 1 1 1 0 1

1 0 1 0 1 11 1 1 1 0 0

1 0 1 0 1 0

no errors

0 1 1 1 0 1

1 0 1 0 1 11 0 1 1 0 0

1 0 1 0 1 0parityerror

parityerror

detectedand

correctablesingle-bit

error

Even parity set parity bit so there is an even number of 1rsquos

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

Internet checksum (review)

sendersect treat contents of UDP

segment (including UDP header fields and IP addresses) as sequence of 16-bit integers

sect checksum addition (onersquos complement sum) of segment content

sect checksum value put into UDP checksum field

receiversect compute checksum of received

segmentsect check if computed checksum equals

checksum field valuebull not equal - error detectedbull equal - no error detected But maybe

errors nonetheless More later hellip

Goal detect errors (ie flipped bits) in transmitted segment

Transport Layer 3-13

Cyclic Redundancy Check (CRC)sectmore powerful error-detection codingsectD data bits (given think of these as a binary number)

sectG bit pattern (generator) of r+1 bits (given)

Link Layer 6-14

goal choose r CRC bits R such that ltDRgt exactly divisible by G (mod 2) bull receiver knows G divides ltDRgt by G If non-zero remainder error detectedbull can detect all burst errors less than r+1 bitsbull widely used in practice (Ethernet 80211 WiFi)

r CRC bitsd data bits

D R

ltDRgt = D 2r XOR R

bit pattern

formula for bit pattern

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsecterror detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-10

Error detection

Link Layer 6-11

EDC error detection and correction bits (eg redundancy)D data protected by error checking may include header fields

Error detection not 100 reliablesect protocol may miss

some errors but rarelysect larger EDC field yields

better detection and correction

datagram

D EDC

d data bits

bit-error prone link

Drsquo EDCrsquo

allbits in Drsquo

OK

Ndetected error

otherwise

datagram

Parity checking

Link Layer 6-12

single bit paritysect detect single bit errors

0111000110101011 1

paritybit

d data bits

two-dimensional bit paritysect detect and correct single bit errors

d11d21

di1

d1j+1d2j+1

dij+1

d1jd2j

dij

di+11 di+1j+1di+1j

row parity

columnparity

0 1 1 1 0 1

1 0 1 0 1 11 1 1 1 0 0

1 0 1 0 1 0

no errors

0 1 1 1 0 1

1 0 1 0 1 11 0 1 1 0 0

1 0 1 0 1 0parityerror

parityerror

detectedand

correctablesingle-bit

error

Even parity set parity bit so there is an even number of 1rsquos

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

Internet checksum (review)

sendersect treat contents of UDP

segment (including UDP header fields and IP addresses) as sequence of 16-bit integers

sect checksum addition (onersquos complement sum) of segment content

sect checksum value put into UDP checksum field

receiversect compute checksum of received

segmentsect check if computed checksum equals

checksum field valuebull not equal - error detectedbull equal - no error detected But maybe

errors nonetheless More later hellip

Goal detect errors (ie flipped bits) in transmitted segment

Transport Layer 3-13

Cyclic Redundancy Check (CRC)sectmore powerful error-detection codingsectD data bits (given think of these as a binary number)

sectG bit pattern (generator) of r+1 bits (given)

Link Layer 6-14

goal choose r CRC bits R such that ltDRgt exactly divisible by G (mod 2) bull receiver knows G divides ltDRgt by G If non-zero remainder error detectedbull can detect all burst errors less than r+1 bitsbull widely used in practice (Ethernet 80211 WiFi)

r CRC bitsd data bits

D R

ltDRgt = D 2r XOR R

bit pattern

formula for bit pattern

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Error detection

Link Layer 6-11

EDC error detection and correction bits (eg redundancy)D data protected by error checking may include header fields

Error detection not 100 reliablesect protocol may miss

some errors but rarelysect larger EDC field yields

better detection and correction

datagram

D EDC

d data bits

bit-error prone link

Drsquo EDCrsquo

allbits in Drsquo

OK

Ndetected error

otherwise

datagram

Parity checking

Link Layer 6-12

single bit paritysect detect single bit errors

0111000110101011 1

paritybit

d data bits

two-dimensional bit paritysect detect and correct single bit errors

d11d21

di1

d1j+1d2j+1

dij+1

d1jd2j

dij

di+11 di+1j+1di+1j

row parity

columnparity

0 1 1 1 0 1

1 0 1 0 1 11 1 1 1 0 0

1 0 1 0 1 0

no errors

0 1 1 1 0 1

1 0 1 0 1 11 0 1 1 0 0

1 0 1 0 1 0parityerror

parityerror

detectedand

correctablesingle-bit

error

Even parity set parity bit so there is an even number of 1rsquos

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

Internet checksum (review)

sendersect treat contents of UDP

segment (including UDP header fields and IP addresses) as sequence of 16-bit integers

sect checksum addition (onersquos complement sum) of segment content

sect checksum value put into UDP checksum field

receiversect compute checksum of received

segmentsect check if computed checksum equals

checksum field valuebull not equal - error detectedbull equal - no error detected But maybe

errors nonetheless More later hellip

Goal detect errors (ie flipped bits) in transmitted segment

Transport Layer 3-13

Cyclic Redundancy Check (CRC)sectmore powerful error-detection codingsectD data bits (given think of these as a binary number)

sectG bit pattern (generator) of r+1 bits (given)

Link Layer 6-14

goal choose r CRC bits R such that ltDRgt exactly divisible by G (mod 2) bull receiver knows G divides ltDRgt by G If non-zero remainder error detectedbull can detect all burst errors less than r+1 bitsbull widely used in practice (Ethernet 80211 WiFi)

r CRC bitsd data bits

D R

ltDRgt = D 2r XOR R

bit pattern

formula for bit pattern

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Parity checking

Link Layer 6-12

single bit paritysect detect single bit errors

0111000110101011 1

paritybit

d data bits

two-dimensional bit paritysect detect and correct single bit errors

d11d21

di1

d1j+1d2j+1

dij+1

d1jd2j

dij

di+11 di+1j+1di+1j

row parity

columnparity

0 1 1 1 0 1

1 0 1 0 1 11 1 1 1 0 0

1 0 1 0 1 0

no errors

0 1 1 1 0 1

1 0 1 0 1 11 0 1 1 0 0

1 0 1 0 1 0parityerror

parityerror

detectedand

correctablesingle-bit

error

Even parity set parity bit so there is an even number of 1rsquos

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

Internet checksum (review)

sendersect treat contents of UDP

segment (including UDP header fields and IP addresses) as sequence of 16-bit integers

sect checksum addition (onersquos complement sum) of segment content

sect checksum value put into UDP checksum field

receiversect compute checksum of received

segmentsect check if computed checksum equals

checksum field valuebull not equal - error detectedbull equal - no error detected But maybe

errors nonetheless More later hellip

Goal detect errors (ie flipped bits) in transmitted segment

Transport Layer 3-13

Cyclic Redundancy Check (CRC)sectmore powerful error-detection codingsectD data bits (given think of these as a binary number)

sectG bit pattern (generator) of r+1 bits (given)

Link Layer 6-14

goal choose r CRC bits R such that ltDRgt exactly divisible by G (mod 2) bull receiver knows G divides ltDRgt by G If non-zero remainder error detectedbull can detect all burst errors less than r+1 bitsbull widely used in practice (Ethernet 80211 WiFi)

r CRC bitsd data bits

D R

ltDRgt = D 2r XOR R

bit pattern

formula for bit pattern

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Internet checksum (review)

sendersect treat contents of UDP

segment (including UDP header fields and IP addresses) as sequence of 16-bit integers

sect checksum addition (onersquos complement sum) of segment content

sect checksum value put into UDP checksum field

receiversect compute checksum of received

segmentsect check if computed checksum equals

checksum field valuebull not equal - error detectedbull equal - no error detected But maybe

errors nonetheless More later hellip

Goal detect errors (ie flipped bits) in transmitted segment

Transport Layer 3-13

Cyclic Redundancy Check (CRC)sectmore powerful error-detection codingsectD data bits (given think of these as a binary number)

sectG bit pattern (generator) of r+1 bits (given)

Link Layer 6-14

goal choose r CRC bits R such that ltDRgt exactly divisible by G (mod 2) bull receiver knows G divides ltDRgt by G If non-zero remainder error detectedbull can detect all burst errors less than r+1 bitsbull widely used in practice (Ethernet 80211 WiFi)

r CRC bitsd data bits

D R

ltDRgt = D 2r XOR R

bit pattern

formula for bit pattern

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Cyclic Redundancy Check (CRC)sectmore powerful error-detection codingsectD data bits (given think of these as a binary number)

sectG bit pattern (generator) of r+1 bits (given)

Link Layer 6-14

goal choose r CRC bits R such that ltDRgt exactly divisible by G (mod 2) bull receiver knows G divides ltDRgt by G If non-zero remainder error detectedbull can detect all burst errors less than r+1 bitsbull widely used in practice (Ethernet 80211 WiFi)

r CRC bitsd data bits

D R

ltDRgt = D 2r XOR R

bit pattern

formula for bit pattern

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Link Layer 6-15

Cyclic Redundancy Check (CRC) exampleWe want

D2r XOR R = nG

Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive

D2rG

R = remainder [ ]

or equivalentlyD2r = nG XOR R

or equivalentlyif we divide D2r by G want remainder R to satisfy

1 0 0 1

1 0 1 0

1 0 10 0 0

1 0 0 1

1 0 0 1

1 0 0 1

0 0 01 1 0

1 1 0 0

1 0 1 0

0 1 1

0 1 1

D

R

1 0 0 1

G0 0 01 0 1 1 1 0

2r

1 0 1

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sectmultiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-16

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Multiple access links protocols

Link Layer 6-17

two types of ldquolinksrdquosect point-to-point

bull point-to-point link between Ethernet switch hostbull PPP for dial-up access

sect broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFC in cable-based access networkbull 80211 wireless LAN 4G4G satellite

shared wire (eg cabled Ethernet) shared radio WiFi shared radio satellite humans at a cocktail party

(shared air acoustical)shared radio 4G5G

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Multiple access protocols

Link Layer 6-18

sect single shared broadcast channel sect two or more simultaneous transmissions by nodes interference bull collision if node receives two or more signals at the same time

sect distributed algorithm that determines how nodes share channel ie determine when node can transmit

sect communication about channel sharing must use channel itself bull no out-of-band channel for coordination

multiple access protocol

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

An ideal multiple access protocol

Link Layer 6-19

given multiple access channel (MAC) of rate R bpsdesiderata

1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average

rate RM3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots

4 simple

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

MAC protocols taxonomy

Link Layer 6-20

three broad classessect channel partitioningbull divide channel into smaller ldquopiecesrdquo (time slots frequency code)bull allocate piece to node for exclusive use

sect random accessbull channel not divided allow collisionsbull ldquorecoverrdquo from collisions

sect ldquotaking turnsrdquobull nodes take turns but nodes with more to send can take longer turns

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Channel partitioning MAC protocols TDMA

Link Layer 6-21

TDMA time division multiple access sect access to channel in ldquoroundsrdquo sect each station gets fixed length slot (length = packet transmission

time) in each round sect unused slots go idle sect example 6-station LAN 134 have packets to send slots 256 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Channel partitioning MAC protocols FDMA

Link Layer 6-22

FDMA frequency division multiple access sect channel spectrum divided into frequency bandssect each station assigned fixed frequency bandsect unused transmission time in frequency bands go idle sect example 6-station LAN 134 have packet to send frequency bands 256 idle

frequ

ency

ban

ds

time

FDM cable

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Random access protocols

Link Layer 6-23

sect when node has packet to sendbull transmit at full channel data rate Rbull no a priori coordination among nodes

sect two or more transmitting nodes ldquocollisionrdquosect random access MAC protocol specifies bull how to detect collisionsbull how to recover from collisions (eg via delayed retransmissions)

sect examples of random access MAC protocolsbull ALOHA slotted ALOHAbull CSMA CSMACD CSMACA

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Slotted ALOHA

Link Layer 6-24

assumptionssect all frames same sizesect time divided into equal size

slots (time to transmit 1 frame)sect nodes start to transmit only

slot beginning sect nodes are synchronizedsect if 2 or more nodes transmit in

slot all nodes detect collision

operationsect when node obtains fresh

frame transmits in next slotbull if no collision node can send

new frame in next slotbull if collision node retransmits

frame in each subsequent slot with probability p until success

randomization ndash why

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Slotted ALOHA

Link Layer 6-25

Prossect single active node can

continuously transmit at full rate of channel

sect highly decentralized only slots in nodes need to be in sync

sect simple

Conssect collisions wasting slotssect idle slotssect nodes may be able to detect collision in

less than time to transmit packetsect clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

C collisionS successE empty

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

efficiency long-run fraction of successful slots (many nodes all with many frames to send)sect suppose N nodes with many frames to send each transmits in slot

with probability pbull prob that given node has success in a slot = p(1-p)N-1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

max efficiency = 1e = 37sect at best channel used for useful transmissions 37 of time

Slotted ALOHA efficiency

Link Layer 6-26

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Pure ALOHA

Link Layer 6-27

sect unslotted Aloha simpler no synchronizationbull when frame first arrives transmit immediately

sect collision probability increases with no synchronizationbull frame sent at t0 collides with other frames sent in [t0-1t0+1]

t0 + 1t0 - 1 t0

will overlapwith end of

irsquos frame

will overlapwith start of

irsquos frame

sect pure Aloha efficiency 18

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

CSMA (carrier sense multiple access)

Link Layer 6-28

simple CSMA listen before transmitbull if channel sensed idle transmit entire framebull if channel sensed busy defer transmission

sect human analogy donʼt interrupt others

CSMACD CSMA with collision detectionbull collisions detected within short timebull colliding transmissions aborted reducing channel wastagebull collision detection easy in wired difficult with wireless

sect human analogy the polite conversationalist

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

CSMA collisions

Link Layer 6-29

sect collisions can still occur with carrier sensing bull propagation delay means two nodes

may not hear each otherrsquos just-started transmission

sect collision entire packet transmission time wastedbull distance amp propagation delay play

role in in determining collision probability

spatial layout of nodes

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

CSMACD

Link Layer 6-30

sect CSMACS reduces the amount of time wasted in collisionsbull transmission aborted on collision

detection

spatial layout of nodes

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Ethernet CSMACD algorithm

Link Layer 6-31

1 NIC receives datagram from network layer creates frame2 If NIC senses channel

if idle start frame transmission if busy wait until channel idle then transmit

3 If NIC transmits entire frame without collision NIC is done with frame

4 If NIC detects another transmission while sending abort send jam signal5 After aborting NIC enters binary (exponential) backoff

bull after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2

bull more collisions longer backoff interval

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

CSMACD efficiency

Link Layer 6-32

sect Tprop = max prop delay between 2 nodes in LANsect ttrans = time to transmit max-size frame

sect efficiency goes to 1 bull as tprop goes to 0bull as ttrans goes to infinity

sect better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

+=

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-33

channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth

allocated even if only 1 active node random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead

ldquotaking turnsrdquo protocolssect look for best of both worlds

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-34

pollingsect master node ldquoinvitesrdquo other nodes

to transmit in turnsect typically used with ldquodumbrdquo

devicessect concernsbull polling overhead bull latencybull single point of failure (master)

master

slaves

poll

data

data

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

ldquoTaking turnsrdquo MAC protocols

Link Layer 6-35

token passingsect control token passed from one

node to next sequentiallysect token messagesect concernsbull token overhead bull latencybull single point of failure

(token)

T

data

(nothingto send)

T

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Cable access network FDM TDM and random access

Link Layer 6-36

cable headend

CMTS

ISP cable modemtermination system

cablemodemsplitter

hellip

hellip

Internet frames TV channels control transmitted downstream at different frequencies

sect multiple downstream (broadcast) FDM channels up to 16 Gbpschannel sect single CMTS transmits into channels

sect multiple upstream channels (up to 1 Gbpschannel)sect multiple access all users contend (random access) for certain upstream

channel time slots others assigned TDM

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Cable access network

Link Layer 6-37

DOCSIS data over cable service interface specificaitonsect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contentionbull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted random access (binary

backoff) in selected slots

Residences with cable modems

Downstream channel i

Upstream channel j

MAP frame forInterval [t1 t2]

t1 t2

Assigned minislots containing cable modemupstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Summary of MAC protocols

Link Layer 6-38

sect channel partitioning by time frequency or codebull Time Division Frequency Division

sect random access (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard in others

(wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

sect taking turnsbull polling from central site token passingbull Bluetooth FDDI token ring

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-39

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

MAC addresses

Link Layer 6-40

sect 32-bit IP address bull network-layer address for interfacebull used for layer 3 (network layer) forwardingbull eg 12811940136

sect MAC (or LAN or physical or Ethernet) addressbull function used ldquolocallyrdquo to get frame from one interface to another

physically-connected interface (same subnet in IP-addressing sense)bull 48-bit MAC address (for most LANs) burned in NIC ROM also

sometimes software settable

hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)

bull eg 1A-2F-BB-76-09-AD

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

MAC addresses

Link Layer 6-41

each interface on LAN sect has unique 48-bit MAC addresssect has a locally unique 32-bit IP address (as wersquove seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired or wireless)

137196724

137196778

137196714

137196788

137196723

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

MAC addresses

Link Layer 6-42

sect MAC address allocation administered by IEEEsect manufacturer buys portion of MAC address space (to

assure uniqueness)sect analogybull MAC address like Social Security Numberbull IP address like postal address

sect MAC flat address portability bull can move interface from one LAN to anotherbull recall IP address not portable depends on IP subnet to which

node is attached

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

ARP address resolution protocol

Link Layer 6-43

ARP table each IP node (host router) on LAN has table

Question how to determine interfacersquos MAC address knowing its IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196778

137196714

137196788

137196723

ARP

ARP

ARPARP

bull IPMAC address mappings for some LAN nodeslt IP address MAC address TTLgt

bull TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

ARP protocol in action

Link Layer 6-44

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A broadcasts ARP query containing Bs IP addrbull destination MAC address = FF-FF-FF-FF-FF-FFbull all nodes on LAN receive ARP query

1Source MAC 71-65-F7-2B-08-53Source IP 137196723Target IP address 137196714hellip

1

Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

ARP protocol in action

Link Layer 6-45

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

B replies to A with ARP response giving its MAC address

2

Target IP address 137196714Target MAC address

58-23-D7-FA-20-B0hellip

2

ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

ARP protocol in action

Link Layer 6-46

58-23-D7-FA-20-B0137196714

B

C

D

TTL

71-65-F7-2B-08-53137196723

A

ARP table in AIP addr MAC addr TTL

example A wants to send datagram to Bbull Bʼs MAC address not in Arsquos ARP table so A uses ARP to find Brsquos MAC address

A receives Brsquos reply adds B entry into its local ARP table

3

137196714

58-23-D7-FA-20-B0 500

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Routing to another subnet addressing

Link Layer 6-47

walkthrough sending a datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame) levels

RA B

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55 222222222222

49-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

sect assume thatbull A knows Brsquos IP addressbull A knows IP address of first hop router R (how)bull A knows Rrsquos MAC address (how)

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Routing to another subnet addressing

Link Layer 6-48

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IP src 111111111111IP dest 222222222222

sect A creates IP datagram with IP source A destination B sect A creates link-layer frame containing A-to-B IP datagrambull Rs MAC address is framersquos destination

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Routing to another subnet addressing

Link Layer 6-49

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

sect frame sent from A to R

IPEthPhy

sect frame received at R datagram removed passed up to IP

MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B

IP src 111111111111IP dest 222222222222

IP src 111111111111IP dest 222222222222

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha

Routing to another subnet addressing

Link Layer 6-50

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

IPEthPhy

Routing to another subnet addressing

Link Layer 6-51

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

IP src 111111111111IP dest 222222222222

MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2Asect transmits link-layer frame

sect R determines outgoing interface passes datagram with IP source A destination B to link layer

sect R creates link-layer frame containing A-to-B IP datagram Frame destination address Bs MAC address

Routing to another subnet addressing

Link Layer 6-52

R

1A-23-F9-CD-06-9B222222222220

111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D

111111111112

11111111111174-29-9C-E8-FF-55

A

22222222222249-BD-D2-C7-56-2A

22222222222188-B2-2F-54-1A-0F

B

IPEthPhy

IPEthPhy

sect B receives frame extracts IP datagram destination B sect B passes datagram up protocol stack to IP

IP src 111111111111IP dest 222222222222

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-53

Ethernet

Link Layer 6-54

ldquodominantrdquo wired LAN technology sect first widely used LAN technologysect simpler cheapsect kept up with speed race 10 Mbps ndash 400 Gbps sect single chip multiple speeds (eg Broadcom BCM5761)

Metcalfersquos Ethernet sketch

httpswwwusptogovlearning-and-resourcesjourneys-innovationaudio-storiesdefying-doubters

Ethernet physical topology

Link Layer 6-55

sect bus popular through mid 90sbull all nodes in same collision domain (can collide with each other)

bus coaxial cable switched

sect switched prevails todaybull active link-layer 2 switch in centerbull each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with

each other)

Ethernet frame structure

Link Layer 6-56

sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

destaddress

sourceaddress data (payload) CRCpreamble

type

preamble sect used to synchronize receiver sender clock ratessect 7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)

Link Layer 6-57

destaddress

sourceaddress data (payload) CRCpreamble

type

sect addresses 6 byte source destination MAC addressesbull if adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to network layer protocolbull otherwise adapter discards frame

sect type indicates higher layer protocol bull mostly IP but others possible eg Novell IPX AppleTalkbull used to demultiplex up at receiver

sect CRC cyclic redundancy check at receiverbull error detected frame is dropped

Ethernet unreliable connectionless

Link Layer 6-58

sectconnectionless no handshaking between sending and receiving NICs

sectunreliable receiving NIC doesnrsquot send ACKs or NAKs to sending NICbull data in dropped frames recovered only if initial sender uses

higher layer rdt (eg TCP) otherwise dropped data lostsectEthernetrsquos MAC protocol unslotted CSMACD with binary

backoff

8023 Ethernet standards link amp physical layers

Link Layer 6-59

bull 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 (twister pair) physical layer

sect many different Ethernet standardsbull common MAC protocol and frame formatbull different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps 40 Gbps

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-60

Ethernet switch

Link Layer 6-61

sect Switch is a link-layer device takes an active rolebull store forward Ethernet framesbull 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

sect transparent hosts unaware of presence of switches

sect plug-and-play self-learningbull switches do not need to be configured

Switch multiple simultaneous transmissions

Link Layer 6-62

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisions

Switch multiple simultaneous transmissions

Link Layer 6-63

switch with six interfaces (123456)

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sect hosts have dedicated direct connection to switch

sect switches buffer packetssect Ethernet protocol used on each

incoming link so bull no collisions full duplexbull each link is its own collision

domainsect switching A-to-Arsquo and B-to-Brsquo can transmit

simultaneously without collisionsbull but A-to-Arsquo and C to Arsquo can not happen

simultaneously

Switch forwarding table

Link Layer 6-64

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5A each switch has a switch table each entrysect (MAC address of host interface to reach

host time stamp)sect looks like a routing table

Q how are entries created maintained in switch table sect something like a routing protocol

Switch self-learning

Link Layer 6-65

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

sectswitch learns which hosts can be reached through which interfaces

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

bull when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentbull records senderlocation pair

in switch table

Switch frame filteringforwarding

Link Layer 6-66

when frame received at switch1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination

then if destination on segment from which frame arrived

then drop frameelse forward frame on interface indicated by entry

else flood forward on all interfaces except arriving interface

A

Arsquo

B

Brsquo C

Crsquo

1 2

345

6

Self-learning forwarding example

Link Layer 6-67

A Arsquo

Source ADest Arsquo

MAC addr interface TTLswitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Arsquo A

Arsquo 4 60

sect frame destination Arsquo location unknown flood

sect destination A location known selectively send

on just one link

Interconnecting switches

Link Layer 6-68

self-learning switches can be connected together

Q sending from A to G - how does S1 know to forward frame destined to G via S4 and S3

sect A self learning (works exactly the same as in single-switch case)

A

B

S1

C D

E

FS2

S4

S3

H

I

G

Self-learning multi-switch example

Link Layer 6-69

Suppose 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

Small institutional network

Link Layer 6-70

to externalnetwork

router

IP subnet

mail server

web server

Switches vs routers

Link Layer 6-71

applicationtransportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

applicationtransportnetwork

linkphysical

frame

frame

framedatagram

6-71

both are store-and-forward sect routers network-layer devices (examine

network-layer headers)sect switches link-layer devices (examine

link-layer headers)

both have forwarding tablessect routers compute tables using routing

algorithms IP addressessect switches learn forwarding table using

flooding learning MAC addresses

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-72

Virtual LANs (VLANs) motivation

Link Layer 6-73

Computer Science EE

Q what happens as LAN sizes scale users change point of attachment

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy issues

Virtual LANs (VLANs) motivation

Link Layer 6-74

administrative issuessect CS user moves office to EE - physically

attached to EE switch but wants to remain logically attached to CS switch

Computer Science EE

single broadcast domainsect scaling all layer-2 broadcast traffic

(ARP DHCP unknown MAC) must cross entire LAN

sect efficiency security privacy efficiency issues

Q what happens as LAN sizes scale users change point of attachment

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-75

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

Virtual Local Area Network (VLAN)

port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

hellip operates as multiple virtual switches

1

82

7

EE (VLAN ports 1-8)

hellip

9

1610

15

hellip

CS (VLAN ports 9-15)

1

82

7 9

1610

15

Port-based VLANs

Link Layer 6-76

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

sect traffic isolation frames tofrom ports 1-8 can only reach ports 1-8bull can also define VLAN based on MAC

addresses of endpoints rather than switch port

sect dynamic membership ports can be dynamically assigned among VLANs

sect forwarding between VLANS done via routing (just as with separate switches)bull in practice vendors sell combined switches

plus routers

1

82

7 9

1610

15

VLANS spanning multiple switches

Link Layer 6-77

hellip

EE (VLAN ports 1-8) CS (VLAN ports 9-15)

hellip

5

82

7

hellip

16

1

6

3

4

Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN

trunk port carries frames between VLANS defined over multiple physical switchessect frames forwarded within VLAN between switches canrsquot be vanilla 8021

frames (must carry VLAN ID info)sect 8021q protocol addsremoved additional header fields for frames

forwarded between trunk ports

8021Q VLAN frame format

Link Layer 6-78

8021 Ethernet framedestaddress

sourceaddress data (payload) CRCpreamble

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 framedestaddress

sourceaddress data (payload) CRCpreamble

type

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-79

label Exp S TTL

20 3 1 5

Multiprotocol label switching (MPLS)

Link Layer 6-80

sect goal high-speed IP forwarding among network of MPLS-capable routers using fixed length label (instead of shortest prefix matching) bull faster lookup using fixed length identifierbull borrowing ideas from Virtual Circuit (VC) approachbull but IP datagram still keeps IP address

remainder of Ethernet frame including IP header with IP source destination addressesMPLS headerEthernet

headerremainder of Ethernet frame including IP

header with IP source destination addresses

MPLS capable routers

Link Layer 6-81

sect aka label-switched routersect forward packets to outgoing interface based only on label

value (donrsquot inspect IP address)bull MPLS forwarding table distinct from IP forwarding tables

sect flexibility MPLS forwarding decisions can differ from those of IPbull use destination and source addresses to route flows to same

destination differently (traffic engineering)bull re-route flows quickly if link fails pre-computed backup paths

MPLS versus IP paths

Link Layer 6-82

R2

D

R3R5

A

R6

R4

sect IP routing path to destination determined by destination address alone

IP router

MPLS versus IP paths

Link Layer 6-83

R2

D

R3R5

A

R6

IP routerR4

sect IP routing path to destination determined by destination address alone

IPMPLS router

IPMPLS entry router (R4) can use different MPLS routes to A based eg on IP source address or other fields

sect MPLS routing path to destination can be based on source anddestination addressbull flavor of generalized forwarding (MPLS 10 years earlier)bull fast reroute precompute backup routes in case of link failure

R1

MPLS signaling

Link Layer 6-84

sect modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing bull eg link bandwidth amount of ldquoreservedrdquo link bandwidth

R2

D

R3R5

A

R6

R4modified link state flooding

RSVP-TE

sect entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

R1

MPLS forwarding tables

Link Layer 6-85

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 112 9 D 0

in out outlabel label dest interface

8 6 A 0

in out outlabel label dest interface

10 A 012 D 08 A 1

R2

D

R3R5

A

R6

R4

R1

0

100

1

0

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-86

Datacenter networks

Link Layer 6-87

10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximitysect e-business (eg Amazon)sect content-servers (eg YouTube Akamai Apple Microsoft)sect search engines data mining (eg Google)

challengessect multiple applications each serving

massive numbers of clients sect reliabilitysect managingbalancing load avoiding

processing networking data bottlenecks Inside a 40-ft Microsoft container Chicago data center

Datacenter networks network elements

Link Layer 6-88

Server rackssect 20- 40 server blades hosts

Top of Rack (TOR) switchsect one per racksect 40-100Gbps Ethernet to blades

Tier-2 switchessect connecting to ~16 TORs below

Tier-1 switchessect connecting to ~16 T-2s below

Border routerssect connections outside datacenter

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks network elements

Link Layer 6-89

Facebook F16 data center network topology

httpsengineeringfbcomdata-center-engineeringf16-minipack (posted 32019)

Datacenter networks multipath

Link Layer 6-90

9 10 11 12 13 14 15 16

two disjoint paths highlighted between racks 1 and 11

sect rich interconnection among switches racksbull increased throughput between racks (multiple routing paths possible)bull increased reliability via redundancy

hellip

hellip

hellip

hellip

hellip

hellip

hellip

hellip

Datacenter networks application-layer routing

Link Layer 6-91

Load balancer

Internet

load balancer application-layer routingsect receives external

client requestssect directs workload

within data centersect returns results to

external client (hiding data center internals from client)

sect link layer bull RoCE remote DMA (RDMA) over Converged Ethernet

sect transport layer bull ECN (explicit congestion notification) used in transport-layer congestion

control (DCTCP DCQCN)bull experimentation with hop-by-hop (backpressure) congestion control

sect routing managementbull SDN widely used withinamong organizationsrsquo datacentersbull place related services data as close as possible (eg in same rack or nearby

rack) to minimize tier-2 tier-1 communication

Datacenter networks protocol innovations

Link Layer 6-92

Link layer LANs roadmap

sect a day in the life of a web request

sect introductionsect error detection correction sect multiple access protocolssect LANsbull addressing ARPbull Ethernetbull switchesbull VLANs

sect link virtualization MPLSsect data center networking

Link Layer 6-93

Synthesis a day in the life of a web request

Link Layer 6-94

sect our journey down the protocol stack is now completebull application transport network link

sect putting-it-all-together synthesisbull goal identify review understand protocols (at all layers) involved in

seemingly simple scenario requesting www pagebull scenario student attaches laptop to campus network requestsreceives

wwwgooglecom

A day in the life scenario

Link Layer 6-95

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

sect arriving mobile client attaches to network hellip

sect requests web page wwwgooglecom

scenario

Sounds simple

A day in the life connecting to the Internet

Link Layer 6-96

router has DHCP server

arriving mobileDHCP client

sect connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

sect DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet

sect Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

sect Ethernet demuxed to IP demuxed UDP demuxed to DHCP

A day in the life connecting to the Internet

Link Layer 6-97

router has DHCP server

arriving mobileDHCP client

DHCPUDP

IPEthPhy

DHCPUDP

IPEthPhy

sect DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP sect 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

sect DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

Link Layer 6-98

router has ARP server

arriving mobileARP client

DNSUDP

IPEthPhy

EthPhy

ARP

sect before sending HTTP request need IP address of wwwgooglecom DNS

DNS

DNS

DNS

sect DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP

sect ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface

sect client now knows MAC address of first hop router so can now send frame containing DNS query

ARP queryARP

ARP reply

A day in the lifehellip using DNS

Link Layer 6-99

DNSUDP

IPEthPhy

Comcast network 68800013

DNS server

DNS

DNS

DNS

DNS

DNS

sect IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

sect IP datagram forwarded from campus network into Comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

sect demuxed to DNSsect DNS replies to client

with IP address of wwwgooglecom

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

A day in the lifehellipTCP connection carrying HTTP

Link Layer 6-100

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTP sect to send HTTP request client first opens TCP socket to web server

sect TCP SYN segment (step 1 in TCP 3-way handshake) inter-domain routed to web server

sect TCP connection established

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

sect web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

A day in the lifehellip HTTP requestreply

Link Layer 6-101

Comcast network 68800013

64233169105Google web server

HTTPTCPIPEthPhy

HTTPTCPIPEthPhy

HTTP sect HTTP request sent into TCP socket

sect IP datagram containing HTTP request routed to wwwgooglecom

sect IP datagram containing HTTP reply routed back to client

sect web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

sect web page finally () displayed

Chapter 6 Summary

Link Layer 6-102

sect principles behind data link layer servicesbull error detection correctionbull sharing a broadcast channel multiple accessbull link layer addressing

sect instantiation implementation of various link layer technologiesbull Ethernetbull switched LANS VLANsbull virtualized networks as a link layer MPLS

sect synthesis a day in the life of a web request

Chapter 6 letrsquos take a breath

Link Layer 6-103

sect journey down protocol stack complete (except PHY)sect solid understanding of networking principles practicesect hellip could stop here hellip but more interesting topicsbull wirelessbull security

Additional Chapter 6 slides

Network Layer 5-104

Pure ALOHA efficiency

Link Layer 6-105

P(success by given node) = P(node transmits) P(no other node transmits in [t0-1t0]P(no other node transmits in [t0-1t0]

= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n= 1(2e) = 18

even worse than slotted Aloha