chapter 4 network layerqianzh/comp562-2009/notes/chapter4_2009.pdf · chapter 4: network layer 4. 1...

Chapter 4Network Layer

Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.

A note on the use of these ppt slides:The notes used in this course are substantially based on powerpoint slides developed and copyrighted by J.F. Kurose and K.W. Ross, 1996-2007

Network Layer 4-1

Chapter 4: Network Layer

4.1Introduction4.2Virtualcircuitanddatagramnetworks4.3Whatsinsidearouter4.4IP:InternetProtocol

DatagramformatIPv4addressingICMPIPv6

4.5RoutingalgorithmsLinkstateDistanceVectorHierarchicalrouting

4.6RoutingintheInternetRIPOSPFBGP

Network Layer 4-2

Two Key Network-Layer Functions

Forwarding:movepacketsfromroutersinputtoappropriaterouteroutput

Routing: determineroutetakenbypacketsfromsourcetodestRoutingalgorithms

Analogy:

Routing: processofplanningtripfromsourcetodest

Forwarding: processofgettingthroughsingleinterchange

Network Layer 4-3

Interplay between Routing and Forwarding

1

23

0111

value in arrivingpackets header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

4-4Network Layer

Connection Setup

The3rd importantfunctioninsome networkarchitectures:

ATM,framerelay,X.25

Beforedatagramsflow,twoendhostsandinterveningroutersestablishvirtualconnection

RoutersgetinvolvedNetworkvs.transportlayerconnectionservice:

Network: betweentwohosts(mayalsoinvolveinterveningroutersincaseofVCs)Transport: betweentwoprocesses

Network Layer 4-5

Network Service Model

Q: Whatservicemodel forchannel transportingpacketsfromsendertoreceiver?Guaranteedbandwidth?Preservationofinterpackettiming(nojitter)?Lossfreedelivery?Inorderdelivery?Congestionfeedbacktosender?

service abstraction? ??virtual circuit

or datagram?

Themostimportantabstractionprovidedbynetworklayer:

Network Layer 4-6






Network Layer 4-7

Virtual Circuits

Callsetup,teardownforeachcallbefore datacanflowEachpacketcarriesVCidentifier(notdestinationhostaddress)Every routeronsourcedestpathmaintainsstate foreachpassingconnectionLink,routerresources(bandwidth,buffers)maybeallocatedto

sourcetodestpathbehavesmuchliketelephonecircuitPerformancewiseNetworkactionsalongsourcetodestpath

VC(dedicatedresources=predictableservice)

Network Layer 4-8

Virtual Circuits: Signaling Protocols

Usedtosetup,maintainteardownVCUsedinATM,framerelay,X.25NotusedintodaysInternet

applicationtransportnetworkdata linkphysical


1. Initiate call 2. incoming call3. Accept call4. Call connected

5. Data flow begins 6. Receive data

Network Layer 4-9

Datagram NetworksNocallsetupatnetworklayerRouters:nostateaboutendtoendconnections

NonetworklevelconceptofconnectionPacketsforwardedusingdestinationhostaddress

Packetsbetweensamesourcedestpairmaytakedifferentpaths

4-10



1. Send data 2. Receive data

Network Layer

Datagram or VC Network: Why?

Internet(datagram)Dataexchangeamongcomputers

Elastic service,nostricttimingreq.

Smart endsystems(computers)

Canadapt,performcontrol,errorrecoverySimpleinsidenetwork,complexityatedge

ManylinktypesDifferentcharacteristicsUniformservicedifficult

ATM(VC)EvolvedfromtelephonyHumanconversation:

Stricttiming,reliabilityrequirementsNeedforguaranteedservice

Dumb endsystemsTelephonesComplexityinsidenetwork

Network Layer 4-11






Network Layer 4-12

Router Architecture Overview

Twokeyrouterfunctions:Runroutingalgorithms/protocol(RIP,OSPF,BGP)Forwardingdatagramsfromincomingtooutgoinglink

Network Layer 4-13






Network Layer 4-14

The Internet Network LayerHost,routernetworklayerfunctions:

forwardingtable

Routing protocolsPath selectionRIP, OSPF, BGP

IP protocolAddressing conventionsDatagram formatPacket handling conventions

ICMP protocolError reportingRouter signaling

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

Network Layer 4-15






Network Layer 4-16

IP Datagram Format

Network Layer 4-17

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifierheader

checksumtime to

live

32 bit source IP address

head.len

type ofservice

flgs fragmentoffsetupperlayer

32 bit destination IP address

Options (if any)

IP protocol versionnumber

header length(bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

type of data (QoS)

E.g. timestamp,record routetaken, specifylist of routers to visit.

How much overhead with TCP?20 bytes of TCP20 bytes of IP= 40 bytes + app layer overhead






Network Layer 4-18

IP Addressing: IntroductionIPaddress: 32bitidentifierforhost,routerinterface

Interface: connectionbetweenhost/routerandphysicallink

RouterstypicallyhavemultipleinterfacesHosttypicallyhasoneinterface(mayhavemultipletoday)IPaddressesassociatedwitheachinterface,not

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

withhostorrouter223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

Network Layer 4-19

SubnetsIPaddress:

Subnetpart(highorderbits)Hostpart(loworderbits)

Whatsasubnet?DeviceinterfaceswithsamesubnetpartofIPaddressCanphysicallyreacheachotherwithoutinterveningrouter

network consisting of 3 subnets

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

subnet

(for IP addresses starting with 223, first 24 bits are network address)

Network Layer 4-20

IP AddressesGivennotionofnetwork,letsreexamineIPaddresses:

class-full addressing:

class1.0.0.0 to127.255.255.255A 0network host

128.0.0.0 to191.255.255.255

B 10 network host

192.0.0.0 to223.255.255.255C 110 network host

224.0.0.0 to239.255.255.255D 1110 multicast address

32 bits

Network Layer 4-21

IP Addressing: CIDRClassfuladdressing:

Inefficientuseofaddressspace,addressspaceexhaustionE.g.,classBnetallocatedenoughaddressesfor65Khosts,even ifonly2Khostsinthatnetwork

CIDR: ClasslessInterDomainRoutingSubnetportionofaddressofarbitrarylengthAddressformat:a.b.c.d/x,wherexis#bitsinsubnetportionofaddress

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23Network Layer 4-22

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or destination in this networkhave 10.0.0/24 address for source, destination (as usual)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers

Network Layer 4-23


Implementation: NATroutermust:

Outgoingdatagrams: replace (sourceIPaddress,port#)ofeveryoutgoingdatagramto(NATIPaddress,newport#)...remoteclients/serverswillrespondusing(NATIPaddress,newport#)asdestinationaddr.

Remember(inNATtranslationtable)every(sourceIPaddress,port#)to(NATIPaddress,newport#)translationpair

Incomingdatagrams: replace (NATIPaddress,newport#)indestfieldsofeveryincomingdatagramwithcorresponding(sourceIPaddress,port#)storedinNATtable

Network Layer 4-24


10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

110.0.0.4

138.76.29.7

1: host 10.0.0.1 sends datagram to 128.119.40.186, 80

NAT translation tableWAN side addr LAN side addr138.76.29.7, 5001 10.0.0.1, 3345

S: 128.119.40.186, 80 D: 10.0.0.1, 3345 4

S: 138.76.29.7, 5001D: 128.119.40.186, 802

2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table

S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3

3: Reply arrivesdest. address:138.76.29.7, 5001

4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345

Network Layer 4-25


Outsidenodecannotinitiatethecommunication

Reservedaddresses:10.0.0.0 10.255.255.255/8172.16.0.0 172.31.255.255/12192.168.0.0 192.168.255.255/16

Network Layer 4-26


16bitportnumberfield:60,000simultaneousconnectionswithasingleLANsideaddress!

NATiscontroversial:Routersshouldonlyprocessuptolayer3Violatesendtoendargument

NATpossibilitymustbetakenintoaccountbyappdesigners,eg,P2Papplications

AddressshortageshouldinsteadbesolvedbyIPv6

Network Layer 4-27






Network Layer 4-28

ICMP: Internet Control Message Protocol

Usedbyhosts&routerstocommunicatenetworklevelinformation

Errorreporting:unreachablehost,network,port,protocolEchorequest/reply(usedbyping)

Networklayerabove IP:ICMPmsgscarriedinIPdatagrams

ICMPmessage: type,codeplusfirst8bytesofIPdatagram

Type Code Description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion

control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

causingerror

Network Layer 4-29

ICMP: Brief Summary

ICMPisthecontrolsiblingofIPICMPisusedbyIPandusesIPasnetworklayerprotocolICMPisusedforping,traceroute,andpathMTUdiscovery

Ping:UsesICMPEchorequest/replymessagesPathMTUDiscovery

SendalargeIPdatagramwithNofragment bitsetReducesizeuntilsuccess(NoICMPmessagereceived)

Network Layer 4-30






Network Layer 4-31

IPv6Initialmotivation: 32bitaddressspacesoontobecompletelyallocatedAdditionalmotivation:Headerformathelpsspeedprocessing/forwardingHeaderchangestofacilitateQoS

IPv6datagramformat:Fixedlength40byteheaderNofragmentationallowed

Network Layer 4-32

Transition From IPv4 To IPv6

NotallrouterscanbeupgradedsimultaneousnoflagdaysHowwillthenetworkoperatewithmixedIPv4andIPv6routers?

Tunneling: IPv6carriedaspayloadinIPv4datagramamongIPv4routers

Network Layer 4-33

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6IPv4 IPv4

Network Layer 4-34

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Network Layer

A B E F

IPv6 IPv6 IPv6 IPv6

C D

IPv4 IPv4

Flow: XSrc: ADest: F

data


data


data

Src:BDest: E


data

Src:BDest: E

A-to-B:IPv6

E-to-F:IPv6B-to-C:IPv6 inside

IPv4

B-to-C:IPv6 inside

IPv4

Physical view:

4-35






Network Layer 4-36

Graph Abstraction

u

yx

wv

z2

21

3

1

1

2

53

5

Graph: G = (N,E)

N = set of routers = { u, v, w, x, y, z }

E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }

Remark: Graph abstraction is useful in other network contexts

Example: P2P, where N is set of peers and E is set of TCP connections

Network Layer 4-37

Graph Abstraction: Costs

u

yx

wv

z2

21

3

1

1

2

53

5 c(x,x) = cost of link (x,x)

- e.g., c(w,z) = 5

Cost could always be 1, or inversely related to bandwidth,or inversely related to congestion

Cost of path (x1, x2, x3,, xp) = c(x1,x2) + c(x2,x3) + + c(xp-1,xp)

Question: Whats the least-cost path between u and z ?

Routing algorithm: algorithm that finds least-cost path

Network Layer 4-38






Network Layer 4-39

A Link-State Routing Algorithm

DijkstrasalgorithmNettopology,linkcostsknowntoallnodes

AccomplishedvialinkstatebroadcastAllnodeshavesameinfo

Computesleastcostpathsfromonenode(source)toallothernodes

Givesforwardingtable forthatnode

Iterative:afterkiterations,knowleastcostpathtokdest.s

Notation:c(x,y): linkcostfromnodextoy;= ifnotdirectneighborsD(v): currentvalueofcostofpathfromsourcetodest.vp(v): predecessornodealongpathfromsourcetovN': setofnodeswhoseleastcostpathdefinitivelyknown

Network Layer 4-40

Dijkstras Algorithm

1 Initialization:2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = 7 8 Loop9 find w not in N' such that D(w) is a minimum 10 add w to N' 11 update D(v) for all v adjacent to w and not in N' : 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N'

Network Layer 4-41

Dijkstras Algorithm: Example

Step012345

start NA

D(B),p(B)2,A

D(C),p(C)5,A

D(D),p(D)1,A

D(E),p(E)

D(F),p(F)

2,A2,A 4,D

3,E3,E

2,D 4,E4,E4,E

ADEBCFADEBC

ADEBADE

AD

A

ED

CB

F2

21

3

1

1

2

53

5

Network Layer 4-42






Network Layer 4-43

Distance Vector Algorithm

BellmanFordEquation(dynamicprogramming)Definedx(y):=costofleastcostpathfromxtoy

Then

dx(y)=min{c(x,v)+dv(y)}

whereministakenoverallneighborsvofx

v

Network Layer 4-44

Bellman-Ford Example

u

yx

wv

z2

21

3

1

1

2

53

5Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3

du(z) = min { c(u,v) + dv(z),c(u,x) + dx(z),c(u,w) + dw(z) }

= min {2 + 5,1 + 3,5 + 3} = 4

B-F equation says:

Node that achieves minimum is nexthop in shortest path forwarding table

Network Layer 4-45

Distance Vector Algorithm

Dx(y) =estimateofleastcostfromxtoyNodexknowscosttoeachneighborv:c(x,v)NodexmaintainsdistancevectorDx =[Dx(y):y N]Nodexalsomaintainsitsneighbors distancevectorsForeachneighborv,xmaintainsDv =[Dv(y):y N]

Network Layer 4-46

Distance Vector Algorithm (4)

Basicidea:EachnodeperiodicallysendsitsowndistancevectorestimatetoneighborsWhenanodexreceivesnewDVestimatefromneighbor,itupdatesitsownDVusingBFequation:

Dx(y) minv{c(x,v) + Dv(y)} for each node y N

Underminor,naturalconditions,theestimateDx(y)convergetotheactualleastcostdx(y)

Network Layer 4-47

Distance Vector Algorithm (5)

Iterative,asynchronous:eachlocaliterationcausedby:LocallinkcostchangeDVupdatemessagefromneighbor

Distributed:EachnodenotifiesneighborsonlywhenitsDVchanges

Neighborsthennotifytheir

wait for (change in local link cost or msg from neighbor)

recompute estimates

if DV to any dest has changed, notify neighbors

Each node:

neighborsifnecessary

Network Layer 4-48

x y zxyz

0 2 7

from

cost to

from

x y zxyz

cost to

x y z

7 1 0

cost to

2 0 1

Network Layer 4-49

from

xyz

node x table

node y table

node z table

x z12

7

y

time

x y zxyz

0 2 7

from

cost to

from

x y z0

from

cost to

xyz

x y zxyz

cost to

x y z

7 1 0

cost to

2 0 1

2 0 17 1 0

x

from yz

time

x z12

7

y

node x table

node y table

node z table

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2

Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)}

= min{2+1 , 7+0} = 3

32

Network Layer 4-50

x y zxyz

0 2 7

from

cost to

from

x y z0 2 3

from

cost to

xyz

x y zxyz

cost tox y zcost to

0 2 7x

from y

Network Layer 4-51

from

z

x y zxyzf

rom 0 2 7

cost tox y z

7 1 0

cost to

2 0 1

2 0 17 1 0

2 0 13 1 0

2 0 17 1 0

xyz

time

x z12

7

y

node x table

node y table

node z table



= min{2+1 , 7+0} = 3

x y zxyz

0 2 7

from

cost to

from

x y zxyz

0 2 3

from

cost tox y z0 2 3

from

cost to

xyz

x y zxyz

cost tox y z0 2 7

from

cost to

Network Layer 4-52

from

xyz

x y zcost to

0 2 3xyzf

rom

x y zxyz

0 2 3

from

cost tox y z

z

0 2 7

from

cost to

xy

x y z

7 1 0

cost to

2 0 1

2 0 17 1 0

2 0 17 1 0

2 0 13 1 0

2 0 13 1 0

2 0 1

3 1 02 0 1

3 1 0

xyz

time

x z12

7

y

node x table

node y table

node z table



= min{2+1 , 7+0} = 3

Distance Vector: Link Cost Changes

Link cost changes:Node detects local link cost change Updates routing info, recalculates distance vectorIf DV changes, notify neighbors

goodnews travelsfast

x z14

50

y1

At time t0, y detects the link-cost change, updates its DV, and informs its neighbors.

At time t1, z receives the update from y and updates its table. It computes a new least cost to x and sends its neighbors its DV.

At time t2, y receives zs update and updates its distance table. ys least costs do not change and hence y does not send any message to z.

Network Layer 4-53

badnews travelsslow

Network Layer

xyz

cost tox y z0 4 5

from 4 0 1

5 1 0

node x table

node y table

node z table

xyz

cost tox y z0 4 5

from 4 0 1

5 1 0

xyz

cost tox y z0 4 5

from 4 0 1

5 1 0

Initial routing table

count to infinityproblem

x z14

50

y

4-54

x z14

50

y60

xyz

cost tox y z0 51 50

from 4 0 1

5 1 0

node x table

node y table

node z table

xyz

cost tox y z0 4 5

from 6 0 1

5 1 0

xyz

cost tox y z0 4 5

from 4 0 1

5 1 0

xyz

cost tox y z0 51 50

from 6 0 1

5 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 6 0 1

5 1 0

xyz

cost tox y z0 51 50

from 6 0 1

7 1 0Cost of link xy changes to 60

badnews travelsslow

xyz

cost tox y z0 51 50

from 6 0 1

7 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 8 0 1

7 1 0

xyz

cost tox y z0 51 50

from 6 0 1

7 1 0

Algorithm converges in 44 steps.

xyz

cost tox y z0 51 50

from 8 0 1

7 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 8 0 1

7 1 0

xyz

cost tox y z0 51 50

from 8 0 1

9 1 0

Network Layer 4-55

badnews travelsslow

xyz

cost tox y z0 51 50

from 48 0 1

49 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 50 0 1

49 1 0

xyz

cost tox y z0 51 50

from 48 0 1

49 1 0

xyz

cost tox y z0 51 50

from 50 0 1

49 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 50 0 1

49 1 0

xyz

cost tox y z0 51 50

from 50 0 1

50 1 0

xyz

cost tox y z0 51 50

from 50 0 1

50 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 51 0 1

50 1 0

xyz

cost tox y z0 51 50

from 50 0 1

50 1 0

xyz

cost tox y z0 51 50

from 51 0 1

50 1 0

node x table

node y table

node z table

xyz

cost tox y z0 51 50

from 51 0 1

50 1 0

xyz

cost tox y z0 51 50

from 51 0 1

50 1 0

Algorithm converges in 44 steps.

x z14

50

y60

Cost of link xy changes to 60 Network Layer 4-56

Distance Vector: Link Cost Changes

Network Layer 4-57

Linkcostchanges:GoodnewstravelsfastBadnewstravelsslowcounttoinfinity problem!44iterationsbeforealgorithmstabilizes:seetext

Poisonedreverse:IfZroutesthroughYtogettoX:

ZtellsYits(Zs)distancetoXisinfinite(soYwontroutetoXviaZ)

Willthiscompletelysolvecounttoinfinityproblem?

x z14

50

y60

Realproblemisthatythinksitsshortestpathtoxisthroughz,whilezthinksitsshortestpathtoxisthroughy.Theypingpongbackandforthwiththisinformation.

Comparison of LS and DV algorithms

MessagecomplexityLS:

Robustness: whathappensifroutermalfunctions?

LS:Nodecanadvertiseincorrectlink costEachnodecomputesonlyitsown table

DV:DVnodecanadvertiseincorrectpath costEachnodestableusedbyothers

Errorpropagatethrunetwork

withnnodes,Elinks,O(nE)msgssentDV:exchangebetweenneighborsonly

Convergencetimevaries

SpeedofConvergenceLS: O(n2)algorithmrequiresO(nE)msgs

MayhaveoscillationsDV:convergencetimevaries

MayberoutingloopsCounttoinfinityproblem

Network Layer 4-58






Network Layer 4-59

Hierarchical Routing

Aggregateroutersintoregions, autonomoussystems (AS)

RoutersinsameASrunsameroutingprotocol

IntraAS routing protocolRoutersindifferentAScanrundifferentintraASroutingprotocol

Network Layer 4-60

SpecialroutersinASRunintraASroutingprotocolwithallotherroutersinASAlso responsibleforroutingtodestinationsoutsideAS

RuninterASroutingprotocolwithothergatewayrouters

gateway routers

Interconnected ASes

ForwardingtableisconfiguredbybothintraandinterASroutingalgorithm

IntraASsetsentriesforinternaldestsInterAS&IntraAssetsentriesforexternaldests

Network Layer 4-61

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

3c

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

Inter-AS TasksSupposerouterinAS1receivesdatagramforwhichdestisoutsideofAS1

Routershouldforwardpackettowardsoneofthegatewayrouters,butwhichone?

AS1needs:1. Tolearnwhichdests

arereachablethroughAS2andwhichthroughAS3

2. TopropagatethisreachabilityinfotoallroutersinAS1

JobofinterASrouting!

Network Layer 4-62

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

3c

Example: Choosing Among Multiple ASes

NowsupposeAS1learnsfromtheinterASprotocolthatsubnetx isreachablefromAS3and fromAS2Toconfigureforwardingtable,router1dmustdeterminetowardswhichgatewayitshouldforwardpacketsfordestxThisisalsothejoboninterASroutingprotocol!Hotpotatorouting: sendpackettowardsclosestoftworouters

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determinecosts of least-cost

paths to eachof the gateways

Hot potato routing:Choose the gateway

that has the smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway. Enter (x,I) in

forwarding table

Network Layer 4-63






Network Layer 4-64

Routing in the InternetTheGlobalInternetconsistsofAutonomousSystems(AS)interconnectedwitheachother:

StubAS:smallcorporation:oneconnectiontootherASsMultihomedAS:largecorporation(notransit):multipleconnectionstootherASsTransitAS:provider,hookingmanyASstogether

Twolevelrouting:IntraAS: (withinAS)administratorresponsibleforchoiceofroutingalgorithmwithinnetworkInterAS: (betweenASes)uniquestandardforinterASrouting:BGP

Network Layer 4-65

Intra-AS Routing

AlsoknownasInteriorGatewayProtocols(IGP)

MostcommonIntraASroutingprotocols:

RIP:RoutingInformationProtocol

OSPF:OpenShortestPathFirst

IGRP:InteriorGatewayRoutingProtocol(Ciscoproprietary)

Network Layer 4-66






Network Layer 4-67

RIP (Routing Information Protocol)

Distancevector algorithmIncludedinBSDUNIXDistributionin1982Distancemetric:#ofhops(max=15hops)

DC

BA

u vw

x

yz

destination hopsu 1v 2w 2x 3y 3z 2

From router A to subsets:

Network Layer 4-68

RIP Advertisements

Distancevectors:exchangedamongneighborsevery30secviaResponseMessage(alsocalledadvertisement)Eachadvertisement:listofupto25destinationnetswithinAS

Network Layer 4-69

RIP: Link Failure and RecoveryIfnoadvertisementheardafter180sec>neighbor/linkdeclareddeadRoutesvianeighborinvalidatedNewadvertisementssenttoneighborsNeighborsinturnsendoutnewadvertisements(iftableschanged)LinkfailureinfoquicklypropagatestoentirenetPoisonreverse usedtopreventpingpongloops(infinitedistance=16hops)

Network Layer 4-70






Network Layer 4-71

OSPF (Open Shortest Path First)

Open:publiclyavailableUsesLinkStatealgorithm

LSpacketdisseminationTopologymapateachnodeRoutecomputationusingDijkstrasalgorithm

OSPFadvertisementcarriesoneentryperneighborrouterAdvertisementsdisseminatedtoentire AS(viaflooding)

CarriedinOSPFmessagesdirectlyoverIP(ratherthanTCPorUDP

Network Layer 4-72

OSPF Advanced Features (not in RIP)

Security: allOSPFmessagesauthenticated(topreventmaliciousintrusion)Multiplesamecostpathsallowed(onlyonepathinRIP)Foreachlink,multiplecostmetricsfordifferentTOS(e.g.,satellitelinkcostsetlow forbesteffort;highforrealtime)Integrateduni andmulticast support:

MulticastOSPF(MOSPF)usessametopologydatabaseasOSPF

Hierarchical OSPFinlargedomains

Network Layer 4-73

Hierarchical OSPF

Network Layer 4-74






Network Layer 4-75

Inter-AS Routing in the Internet: BGP

Figure 4.5.2-new2: BGP use for inter-domain routing

AS2 (OSPF

intra-AS routing)

AS1 (RIP intra-AS

routing) BGP

AS3 (OSPF intra-AS

routing)

BGP

R1 R2

R3

R4 R5

Network Layer 4-76

Internet Inter-AS Routing: BGP

BGP(BorderGatewayProtocol): the defactostandardBGPprovideseachASameansto:1. ObtainsubnetreachabilityinformationfromneighboringASs.

2. PropagatereachabilityinformationtoallASinternalrouters.

3. Determinegood routestosubnetsbasedonreachabilityinformationandpolicy.

AllowssubnettoadvertiseitsexistencetorestofInternet:Iamhere

Network Layer 4-77

BGP BasicsPairsofrouters(BGPpeers)exchangeroutinginfooversemipermanentTCPconnections:BGPsessions

BGPsessionsneednotcorrespondtophysicallinks

WhenAS2advertisesaprefixtoAS1,AS2ispromising itwillforwardanydatagramsdestinedtothatprefixtowardstheprefix

AS2canaggregateprefixesinitsadvertisement

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP sessionNetwork Layer 4-78

Distributing Reachability InfoWitheBGPsessionbetween3aand1c,AS3sendsprefixreachabilityinfotoAS11ccanthenuseiBGPdodistributethisnewprefixreachinfoto allroutersinAS11bcanthenreadvertisenewreachabilityinfotoAS2over1bto2aeBGPsessionWhenrouterlearnsofnewprefix,createsentryforprefixinitsforwardingtable

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP sessionNetwork Layer 4-79

Path Attributes & BGP Routes

Whenadvertisingaprefix,advertincludesBGPattributes.prefix+attributes=route

Twoimportantattributes:ASPATH: containsASsthroughwhichprefixadvertisementhaspassed:AS67AS17NEXTHOP: IndicatesspecificinternalASroutertonexthopAS.(TheremaybemultiplelinksfromcurrentAStonexthopAS.)

Whengatewayrouterreceivesrouteadvertisement,usesimportpolicy toaccept/decline

Network Layer 4-80

BGP Route Selection

Routermaylearnaboutmorethan1routetosomeprefix.Routermustselectroute

Eliminationrules:1. Localpreferencevalueattribute:policydecision2. ShortestASPATH3. ClosestNEXTHOProuter:hotpotatorouting4. Additionalcriteria

Network Layer 4-81

BGP Routing Policy

Network Layer 4-82

A,B,CareprovidernetworksX,W,Yarecustomer(ofprovidernetworks)Xisdualhomed: attachedtotwonetworks

XdoesnotwanttoroutefromBviaXtoC..soXwillnotadvertisetoBaroutetoC

BGP Routing Policy (2)

Network Layer 4-83

AadvertisestoBthepathAWBadvertisestoXthepathBAWShouldBadvertisetoCthepathBAW?

Noway!Bgetsnorevenue forroutingCBAWsinceneitherWnorCareBscustomersBwantstoforceCtoroutetowviaABwantstorouteonlyto/fromitscustomers!

Why different Intra- and Inter-AS routing ?

Policy:InterAS:adminwantscontroloverhowitstrafficrouted,whoroutesthroughitsnet.IntraAS:singleadmin,sonopolicydecisionsneeded

Scale:Hierarchicalroutingsavestablesize,reducedupdatetraffic

Performance:IntraAS:canfocusonperformanceInterAS:policymaydominateoverperformance

Network Layer 4-84

Network Layer: SummaryWhatwevecovered:NetworklayerservicesRoutingprinciples:linkstateanddistancevectorHierarchicalroutingIPInternetroutingprotocolsRIP,OSPF,BGPWhatsinsidearouter?IPv6

Network Layer 4-85

chapter 4 network layerqianzh/comp562-2009/notes/chapter4_2009.pdf · chapter 4: network layer 4. 1...

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