networking ii: the link and network layers. 2 announcements prelim ii will be thursday, november 20...

Networking II:The Link and Network Layers

2

Announcements• Prelim II will be Thursday, November 20th, in class

• Homework 5 available later today, November 4th

• Vote today

3

Review: OSI Levels• Physical Layer

– electrical details of bits on the wire

• Data Link Layer– sending “frames” of bits and error detection

• Network Layer– routing packets to the destination

• Transport Layer– reliable transmission of messages, disassembly/assembly, ordering,

retransmission of lost packets

• Session Layer– really part of transport, typically Not implemented

• Presentation Layer– data representation in the message

• Application– high-level protocols (mail, ftp, etc.)

4

Review: OSI Levels

Presentation

Transport

Network

Data Link

Physical

Application

Presentation

Transport

Network

Data Link

Physical

ApplicationNode A Node B

Network

Session Session

5

What is purpose of this layer?• Invoke Physical Layer

– Physically encode bits on the wire

• Link = pipe to send information– E.g. point to point or broadcast

• Can be built out of:– Twisted pair, coaxial cable, optical fiber, radio waves, etc

• Links should only be able to send data– Could corrupt, lose, reorder, duplicate, (fail in other ways)

6

Broadcast Networks Details

• Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!)

– Put header on front of packet: [ Destination | Packet ]– Everyone gets packet, discards if not the target– In Ethernet, this check is done in hardware

• No OS interrupt if not for particular destination– This is layering: we’re going to build complex network protocols by layering on

top of the packet

Header(Dest:2)

Body(Data)

MessageID:1

(ignore)

ID:2(receive)

ID:4(ignore)

ID:3(sender)

7

Point-to-point networks

• Why have a shared broadcast medium? Why not simplify and only have point-to-point links + routers/switches?

– Didn’t used to be cost-effective– Now, easy to make high-speed switches and routers that can forward

packets from a sender to a receiver.• Point-to-point network: a network in which every physical wire

is connected to only two computers• Switch: a bridge that transforms a shared-bus configuration

into a point-to-point network.• Router: a device that acts as a junction between two networks

to transfer data packets among them.

Router

Intern

et

Switch

8

Point-to-Point Networks Discussion• Advantages:

– Higher link performance• Can drive point-to-point link faster than broadcast link since less capacitance/less

echoes (from impedance mismatches)– Greater aggregate bandwidth than broadcast link

• Can have multiple senders at once– Can add capacity incrementally

• Add more links/switches to get more capacity– Better fault tolerance (as in the Internet)– Lower Latency

• No arbitration to send, although need buffer in the switch• Disadvantages:

– More expensive than having everyone share broadcast link– However, technology costs now much cheaper

• Examples– ATM (asynchronous transfer mode)

• The first commercial point-to-point LAN• Inspiration taken from telephone network

– Switched Ethernet• Same packet format and signaling as broadcast Ethernet, but only two machines

on each ethernet.

9

How to connect routers/machines?• WAN/Router Connections

– Commercial:• T1 (1.5 Mbps), T3 (44 Mbps)

• OC1 (51 Mbps), OC3 (155 Mbps)

• ISDN (64 Kbps)

• Frame Relay (1-100 Mbps, usually 1.5 Mbps)

• ATM (some Gbps)

– To your home:• DSL

• Cable

• Local Area:– Ethernet: IEEE 802.3 (10 Mbps, 100 Mbps, 1 Gbps)– Wireless: IEEE 802.11 b/g/a (11 Mbps, 22 Mbps, 54 Mbps)

10

Link level Issues• Encoding: map bits to analog signals• Framing: Group bits into frames (packets)• Arbitration: multiple senders, one resource• Addressing: multiple receivers, one wire

11

Encoding• Map 1s and 0s to electric signals• Simple scheme: Non-Return to Zero (NRZ)

– 0 = low voltage, 1 = high voltage

• Problems:– How to tell an error? When jammed? When is bus idle?– When to sample? Clock recovery is difficult.

• Idea: Recover clock using encoding transitions

1 0 1 1 0

12

Manchester Encoding• Used by Ethernet• Idea: Map 0 to low-to-high transition, 1 to high-to-low

• Plusses: can detect dead-link, can recover clock• Bad: reduce bandwidth, i.e. bit rate = ½ baud rate

– If wire can do X transition per second?

0 1 1 0

13

Framing• Why send packets?

– Error control

• How do you know when to stop reading?– Sentinel approach: send start and end sequence– For example, if sentinel is 11111– 11111 00101001111100 11111 10101001 11111 010011 11111

– What if sentinel appears in the data?• map sentinel to something else, receiver maps it back

– Bit stuffing

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Example: HDLC• High-Level Data Link Control (HLDC)

– Data link layer protocol developed by the ISO

• Same sentinel for begin and end: 0111 1110• packet format:

• Bit stuffing– Sender: If 5 1s then insert a 0

– Receiver: if 5 1s followed by a 0, remove 0

• Else read next bit

• Packet size now depends on the contents

0111 1110 header data CRC 0111 1110

0111 1110 0111 1101 0

0111 1101 0 0111 1110

15

Broadcast Network Arbitration• Arbitration: Act of negotiating use of shared medium

– What if two senders try to broadcast at same time?– Concurrent activity but can’t use shared memory to coordinate!

• Aloha network (70’s): packet radio within Hawaii– Blind broadcast, with checksum at end of

packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard.

• Need checksum anyway – in case airplane flies overhead

– Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits.

– If two senders try to send at same time, both get garbled, both simply re-send later.

– Problem: Stability: what if load increases?• More collisions less gets through more resent more load… More

collisions…• Unfortunately: some sender may have started in clear, get scrambled

without finishing

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Arbitration• One medium, multiple senders

– What did we do for CPU, memory, readers/writers?

– New Problem: No centralized control

• Approaches– TDMA: Time Division Multiple Access

• Divide time into slots, round robin among senders• If you exceed the capacity do not admit more (busy signal)

– FDMA: Frequency Division Multiple Access (AMPS)• Divide spectrum into channels, give each sender a channel• If no more channels available, give a busy signal

– Good for continuous streams: fixed delay, constant data rate

– Bad for bursty Internet traffic: idle slots

17

Ethernet• Developed in 1976, Metcalfe and Boggs at Xerox

• Uses CSMA/CD: – Carrier Sense Multiple Access with Collision Detection

• Easy way to connect LANs

Metcalfe’s Ethernet sketch

18

CSMA/CD• Carrier Sense:

– Listen before you speak

• Multiple Access:– Multiple hosts can access the network

• Collision Detection:– Can make out if someone else started speaking

Older Ethernet Frame

19

CSMA

Wait until

carrier free

20

CSMA/CD

Garbled signals

If the sender detects a collision, it will stop and then retry!

What is the problem?

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CSMA/CD

Packet?

Sense Carrier

Discard Packet

Send Detect Collision

Jam channel b=CalcBackoff();

wait(b);attempts++;

No

Yes

attempts < 16

attempts == 16

22

Ethernet’s CSMA/CD (more)Jam Signal: make sure all other transmitters are aware of collision; 48 bits;

Exponential Backoff: • Goal: adapt retransmission attempts to estimated current load

– heavy load: random wait will be longer

• Adaptive and Random– First time, pick random wait time with some initial mean. If collide

again, pick random value from bigger mean wait time. Etc.– Randomness is important to decouple colliding senders– Scheme figures out how many people are trying to send!

• Example– first collision: choose K from {0,1}; delay is K x 512 bit transmission times

– after second collision: choose K from {0,1,2,3}…

– after ten or more collisions, choose K from {0,1,2,3,4,…,1023}

23

Packet Size

If packets are too small, the collision goes unnoticed Limit packet size Limit network diameter Use CRC to check frame integrity truncated packets are filtered out

24

Ethernet Problems• What if there is a malicious user?

– Might not use exponential backoff– Might listen promiscuously to packets

• Integrating Fast and Gigabit Ethernet

25

Addressing & ARP

• ARP is used to discover physical addresses• ARP = Address Resolution Protocol

“What is the physical address of the host named 128.84.96.89”

128.84.96.90128.84.96.89

128.84.96.91

“I’m at 1a:34:2c:9a:de:cc”

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Addressing & RARP

• RARP is used to discover virtual addresses• RARP = Reverse Address Resolution Protocol

“I just got here. My physical address is 1a:34:2c:9a:de:cc. What’s my name ?”

128.84.96.90RARP Server

???

128.84.96.91

“Your name is 128.84.96.89”

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Repeaters and Bridges• Both connect LAN segments

• Usually do not originate data

• Repeaters (Hubs): physical layer devices– forward packets on all LAN segments– Useful for increasing range– Increases contention

• Bridges: link layer devices– Forward packets only if meant on that segment– Isolates congestion– More expensive

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Backbone Bridge

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Summary• Data Link Layer

– layer two of the seven-layer OSI model • Layer two of the five-layer TCP/IP reference model as well.

– Responds to service requests from the network layer and issues service requests to the physical layer.

• Broadcast vs Point-to-point– Point-to-point is often higher performance, but traditionally higher cost as well– Switched Ethernet is common now

• Data Link Layer Issues– Encoding: map bits to analog signals

• Manchester encoding

– Framing: Group bits into frames (packets)• Bit stuffing

– Arbitration: multiple senders, one resource• Ethernet uses CSMA/CD (carrier sense multiple access/collision detection)

– Addressing: multiple receivers, one wire• ARP (address resolution protocol)

The Network Layer

31

Review: OSI Levels• Physical Layer

– electrical details of bits on the wire

• Data Link Layer– sending “frames” of bits and error detection

• Network Layer– routing packets to the destination

• Transport Layer– reliable transmission of messages, disassembly/assembly, ordering,

retransmission of lost packets

• Session Layer– really part of transport, typically Not implemented

• Presentation Layer– data representation in the message

• Application– high-level protocols (mail, ftp, etc.)

32

Review: OSI Levels

Presentation

Transport

Network

Data Link

Physical

Application

Presentation

Transport

Network

Data Link

Physical

ApplicationNode A Node B

Network

Session Session

33

Review: OSI Levels

Presentation

Transport

Network

Data Link

Physical

Application

Presentation

Transport

Network

Data Link

Physical

ApplicationNode A Node B

Network

Session Session

Network

Data Link

Physical

Router

34

Purpose of Network layer

• Given a packet, send it across the network to destination• 2 key issues:

– Portability: • connect different technologies

– Scalability• To the Internet scale

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

35

What does it involve?Two important functions:• routing: determine path from source to dest. • forwarding: move packets from router’s input to output

T1T3

Sts-1

T3

T1

36

Network service model

Q: What service model for “channel” transporting packets from sender to receiver?

• guaranteed bandwidth?• preservation of inter-packet

timing (no jitter)?• loss-free delivery?• in-order delivery?• congestion feedback to

sender?

???

virtual circuitor

datagram?

The most important abstraction provided

by network layer:

serv

ice a

bst

ract

ion

Which things can be “faked” at the transport layer?

37

Two connection models

• Connectionless (or “datagram”):– each packet contains enough information that routers can decide how

to get it to its final destination

• Connection-oriented (or “virtual circuit”)– first set up a connection between two nodes– label it (called a virtual circuit identifier (VCI))– all packets carry label

BAb b

C

BA1 1

C

1

38

Virtual circuits: signaling protocols• used to setup, maintain teardown VC• setup gives opportunity to reserve resources

– used in ATM (Asynchronous Transfer Mode), frame-relay, X.25 (or OSI protocol suite)

• not used in today’s Internet

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

1. Initiate call 2. incoming call

3. Accept call4. Call connected5. Data flow begins 6. Receive data

39

Virtual circuit switching• Forming a circuit:

– send a connection request from A to B. • Contains VCI + address of B

• VCI is the Virtual Circuit Identifier

– rule: VCI must be unique on the link its used on– switch creates an entry mapping input messages with VCI to output

port– switch picks a new VCI unique between it and next switch

a b

25

21

c12

1

40

(Input link,VCI) (output link, new VCI)(1, 2) (?, ?)(1, 5) (?, ?)

Virtual circuit forwarding• For each VCI switch has a table which maps input link to

output link and gives the new VCI to use– if a’s messages come into switch 1 on link 2 and go out on link 3 then

the table will be:

a b

25

21

c12

1

Switch 1

Switch 2

Switch 3

41

Virtual Circuits: Discussion• Plusses: easy to associate resources with VC

– Easy to provide QoS guarantees (bandwidth, delay)– Very little state in packet

• Minuses:– Not good in case of crashes

• Requires explicit connect and teardown phases

– What if teardown does not get to all routers?– What if one switch crashes?

• Will have to teardown and rebuild route

42

Datagram networks• no call setup at network layer• routers: no state about end-to-end connections

– no network-level concept of “connection”

• packets typically routed using destination host ID– packets between same source-dest pair may take different paths

• Best effort: data corruption, packet drops, route loops

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

1. Send data 2. Receive data

43

Datagrams: ForwardingHow does packet get to the destination?• switch creates a “forwarding table”, mapping destinations to output port

(ignores input ports)• when a packet with a destination address in the table arrives, it pushes it

out on the appropriate output port• when a packet with a destination address not in the table arrives, it must

find out more routing information (next problem)

a b

c1d

22

00S1

S2

S3

1

01

44

Datagrams• Plusses:

– No round trip connection setup time– No explicit route teardown– No resource reservation each flow could get max bandwidth– Easily handles switch failures; routes around it

• Minuses– Difficult to provide resource guarantees– Higher per packet overhead

• Internet uses datagrams: IP (Internet Protocol)

45

Datagrams Forwarding• How to build forwarding tables?

– Manually enter it• What if nodes crashed

• What about scale?

• The graph-theoretic routing problem– Given a graph, with vertices (switches), edges (links), and edge costs

(cost of sending on that link)– Find the least cost path between any two nodes

• Path cost = (cost of edges in path)

46

Simple Routing Algorithm• Choose a central node

– All nodes send their (nbr, cost) information to this node– Central node uses info to learn entire topology of the network– It then computes shortest paths between all pairs of nodes

• Using All Pair Shortest Path Algorithm

– Sends the new matrix to every node

• Nice, simple, elegant!• What is the problem?

– Scalability: centralization hurts scalability– Central node is “crushed” with traffic

47

Link State Routing• Basic idea:

– Every node propagates its (nbr, cost) information– This information at all nodes is enough to construct topology– Can use a graph algorithm to find the shortest routes

• Mechanisms required:– Reliable flooding of link information– Method to calculate shortest route (Dijkstra’s algorithm)

• Example link state update packet:– [node id, (nbr, cost) list, seq. no., ttl]

• Seq. no. to identify latest updates, ttl specifies when to stop msg.

48

Reliable floodingreceive(pkt) If already have a copy of LSP from pkt.ID

or if pkt’s sequence number <= copy’sdiscard pkt

elsedecrement pkt.TTLreplace copy with pktforward pkt to all links besides the one that we received it on

# done every 10 minutes or sogen_LSP()

increment node’s sequence # by onerecompute cost vectorsend created LSP to all neighbors

49

Discussion: Link-State Routing

• Plusses: – Simple, determines the optimal route most of the time– Used by OSPF (Open Shortest Path First)

• Minuses:– Might have oscillations

– Avoid using load as cost metric, reduce herding effect

A

D

C

B1 1+e

e0

e

1 1

0 0

A

D

C

B2+e 0

001+e1

A

D

C

B0 2+e

1+e10 0

A

D

C

B2+e 0

e01+e1

Initially start withalmost equal routes

… everyone goes with least loaded

… recomputeLeast loaded =>

Most loaded

… recompute

50

Is our routing algo scalable?• Route table size grows with size of network

– Because our address structure is flat!

• Solution: have a hierarchical structure– Used by OSPF– Divide the network into areas, each area has unique number

• Nodes carry their area number in the address 1.A, 2.B, etc.

– Nodes know complete topology in their area– Area border routers (ABR) know how to get to any other area

51

Hierarchical Addressing

1.a3.b

2.b11.b2

2

00S1

S2

S3

1

01

2.a

3

3.a

2

Zone 3

Zone 2

Forwarding table for switch 1Destination switch port

2. ?3. ? 1.b ? 1.a ?

52

IP has 2-layer addressing• Each IP address is 32 bits

– Network part: which network the host is on?

– Host part: identifies the host. • All hosts on same network have the same network part

• 3 classes of addresses: A, B and C

18.26.0.1

network 32-bits host

0 net host

1 7 24 bits

1 0 net host

2 14 16 bits

110 net host

3 21 8 bits

53

IP addressing• The different classes:

• Problems: inefficient, address space exhaustion– cornell.edu is a class B network (can address 64K hosts)– mit.edu is a class A network (can address 4M hosts)

0network host

10 network host

110 network host

1110 multicast address

A

B

C

D

class1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to239.255.255.255

32 bits

Unicast

Multicast

1111 reservedE 240.0.0.0 to255.255.255.255Reserved

54

IP addressing: CIDR• Classless InterDomain Routing

– network portion of address of arbitrary length– address format: a.b.c.d/x, where x is # bits in network portion

– Examples:• Class A: /8

• Class B: /16

• Class C: /24

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23

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Internet Protocol Datagram

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

Internet checksum

time tolive

32 bit source IP address

IP protocol versionNumber

header length

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, pecifylist of routers to visit.

56

Datagram Portability• IP Goal: To create one logical network from multiple physical

networks– All intermediate routers should understand IP– IP header information sufficient to carry the packet to destination– Goal: Run over anything!

• Problem:– Physical networks have different MTUs (maximum transfer units)– “max. transmission unit”: 1500 for Ethernet, 48 for ATM

• Solution 1:– Fit everything in the MTU (!)

57

IP Fragmentation & Reassembly• Solution 2: (the one used)

– If packet size > MTU of network, then fragment into pieces• Each fragment is less than MTU size

• Each has IP headers + frag bit set + frag id + offset

– Packets may get refragmented on the way to destination– Reassembly only done at the destination– What is a good initial packet size?

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

58

Summary• Virtual Circuit

– Plusses: easy to associate resources with VC• Easy to provide QoS guarantees (bandwidth, delay)• Very little state in packet

– Minuses:• Not good in case of crashes

• Datagrams– Plusses:

• Easily handles switch failures; routes around it • No round trip connection setup time• No explicit route teardown• No resource reservation each flow could get max bandwidth

– Minuses• Difficult to provide resource guarantees• Higher per packet overhead

– Forwarding• Link-state routing: OSPF• Hierarchical addressing: IP and OSPF