Link‐StateRou.ngReading:Sec.ons4.2and4.3.4

COS461:ComputerNetworksSpring2010(MW3:00‐4:20inCOS105)

MichaelFreedmanhJp://www.cs.princeton.edu/courses/archive/spring10/cos461/

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GoalsofToday’sLecture

•  Insidearouter– Controlplane:rou.ngprotocols– Dataplane:packetforwarding

•  Pathselec.on– Minimum‐hopandshortest‐pathrou.ng– Dijkstra’salgorithm

•  Topologychange– Usingbeaconstodetecttopologychanges– Propaga.ngtopologyinforma.on

•  Rou.ngprotocol:OpenShortestPathFirst2

WhatisRou.ng?

•  Afamousquota.onfromRFC791“Anameindicateswhatweseek.Anaddressindicateswhereitis.Arouteindicateshowwegetthere.”‐‐JonPostel

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Rou.ngvs.Forwarding

•  Rou.ng:controlplane– Compu.ngpathsthepacketswillfollow

– Routerstalkingamongstthemselves

– Individualroutercrea,ngaforwardingtable•  Forwarding:dataplane

– Direc.ngadatapackettoanoutgoinglink– Individualrouterusingaforwardingtable

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DataandControlPlanes

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Switching Fabric

Processor

Line card

Line card

Line card

Line card

Line card

Line card

data plane

control plane

RouterPhysicalLayout

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Juniper T series

Cisco 12000

Switch

Linecards

LineCards(InterfaceCards,Adaptors)

•  Interfacing–  Physicallink–  Switchingfabric

•  Packethandling–  Packetforwarding– Decrement.me‐to‐live–  Buffermanagement–  Linkscheduling–  Packetfiltering–  Ratelimi.ng–  Packetmarking– Measurement

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to/from link

to/from switch

lookup

Rec

eive

Transmit

SwitchingFabric

•  Deliverpacketinsidetherouter– Fromincominginterfacetooutgoinginterface

– Asmallnetworkinandofitself

•  Mustoperateveryquickly– Mul.plepacketsgoingtosameoutgoinginterface– Switchschedulingtomatchinputstooutputs

•  Implementa.ontechniques– Bus,crossbar,interconnec.onnetwork,…– Runningatafasterspeed(e.g.,2X)thanlinks– Dividingvariable‐lengthpacketsintofixed‐sizecells

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PacketSwitching

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R1 Link 1

Link 2

Link 3

Link 4

Link 1, ingress Link 1, egress

Link 2, ingress Link 2, egress

Link 3, ingress Link 3, egress

Link 4, ingress Link 4, egress

Choose Egress

Choose Egress

Choose Egress

Choose Egress

“4”

“4”

RouterProcessor

•  So‐called“Loopback”interface–  IPaddressoftheCPUontherouter

•  Interfacetonetworkadministrators– Command‐lineinterfaceforconfigura.on– Transmissionofmeasurementsta.s.cs

•  Handlingofspecialdatapackets– PacketswithIPop.onsenabled– PacketswithexpiredTime‐To‐Livefield

•  Control‐planesoiware–  Implementa.onoftherou.ngprotocols– Crea.onofforwardingtableforthelinecards

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WheredoForwardingTablesComeFrom?

•  Routershaveforwardingtables– MapIPprefixtooutgoinglink(s)

•  Entriescanbesta.callyconfigured– E.g.,“map12.34.158.0/24toSerial0/0.1”

•  But,thisdoesn’tadapt– Tofailures– Tonewequipment– Totheneedtobalanceload

•  Thatiswhererou.ngprotocolscomein

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Compu.ngPathsBetweenRouters

•  Routersneedtoknowtwothings– Whichroutertousetoreachades.na.onprefix– Whichoutgoinginterfacetousetoreachthatrouter

•  Today’sclass:justhowroutersreacheachother– Howuknowshowtoforwardpacketstowardz

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12.34.158.0/24

Interface along the path to z

u z

Router z that can reach destination

Compu.ngtheShortestPaths

assumingyoualreadyknowthetopology

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Shortest‐PathRou.ng

•  Path‐selec.onmodel– Des.na.on‐based– Load‐insensi.ve(e.g.,sta.clinkweights)– Minimumhopcountorsumoflinkweights

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Shortest‐PathProblem•  Given:networktopologywithlinkcosts

–  c(x,y):linkcostfromnodextonodey

–  Infinityifxandyarenotdirectneighbors•  Compute:least‐costpathstoallnodes

–  Fromagivensourceutoallothernodes–  p(v):predecessornodealongpathfromsourcetov

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u

v

p(v)

Dijkstra’sShortest‐PathAlgorithm

•  Itera.vealgorithm– Aierkitera.ons,knowleast‐costpathtoknodes

•  S:nodeswhoseleast‐costpathdefini.velyknown–  Ini.ally,S={u}whereuisthesourcenode– AddonenodetoSineachitera.on

•  D(v):currentcostofpathfromsourcetonodev–  Ini.ally,D(v)=c(u,v)forallnodesvadjacenttou– …andD(v)=∞forallothernodesv

–  Con.nuallyupdateD(v)asshorterpathsarelearned

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Dijsktra’sAlgorithm

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1 Initialization: 2 S = {u} 3 for all nodes v 4 if (v is adjacent to u) 5 D(v) = c(u,v) 6 else D(v) = ∞ 7 8 Loop 9 find w not in S with the smallest D(w) 10 add w to S 11 update D(v) for all v adjacent to w and not in S: 12 D(v) = min{D(v), D(w) + c(w,v)} 13 until all nodes in S

Dijkstra’sAlgorithmExample

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Dijkstra’sAlgorithmExample

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Shortest‐PathTree

•  Shortest‐pathtreefromu •  Forwardingtableatu

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u

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w

x

y

z

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t

v (u,v) w (u,w) x (u,w) y (u,v) z (u,v)

link

s (u,w) t (u,w)

LearningtheTopology

bytherouterstalkamongstthemselves

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Link‐StateRou.ng

•  Eachrouterkeepstrackofitsincidentlinks– Whetherthelinkisupordown–  Thecostonthelink

•  Eachrouterbroadcaststhelinkstate–  Togiveeveryrouteracompleteviewofthegraph

•  EachrouterrunsDijkstra’salgorithm–  Tocomputetheshortestpaths– …andconstructtheforwardingtable

•  Exampleprotocols– OpenShortestPathFirst(OSPF)–  IntermediateSystem–IntermediateSystem(IS‐IS)

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Detec.ngTopologyChanges

•  Beaconing– Periodic“hello”messagesinbothdirec.ons

– Detectafailureaierafewmissed“hellos”

•  Performancetrade‐offs– Detec.onspeed– OverheadonlinkbandwidthandCPU– Likelihoodoffalsedetec.on

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“hello”

Broadcas.ngtheLinkState

•  Flooding– Nodesendslink‐stateinforma.onoutitslinks

– Andthenthenextnodesendsoutallofitslinks– …excepttheonewheretheinforma.onarrived

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X A

C B D (a)

X A

C B D (b)

X A

C B D (c)

X A

C B D (d)

Broadcas.ngtheLinkState

•  Reliableflooding– Ensureallnodesreceivelink‐stateinforma.on

– …andthattheyusethelatestversion•  Challenges

– Packetloss– Out‐of‐orderarrival

•  Solu.ons– Acknowledgmentsandretransmissions– Sequencenumbers– Time‐to‐liveforeachpacket

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WhentoIni.ateFlooding

•  Topologychange– Linkornodefailure– Linkornoderecovery

•  Configura.onchange– Linkcostchange

•  Periodically– Refreshthelink‐stateinforma.on– Typically(say)30minutes– Correctsforpossiblecorrup.onofthedata

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WhentheRoutersDisagree

(duringtransientperiods)

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Convergence

•  Genngconsistentrou.nginforma.ontoallnodes–  E.g.,allnodeshavingthesamelink‐statedatabase

•  Consistentforwardingaierconvergence–  Allnodeshavethesamelink‐statedatabase

–  Allnodesforwardpacketsonshortestpaths–  Thenextrouteronthepathforwardstothenexthop

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TransientDisrup.ons

•  Detec.ondelay– Anodedoesnotdetectafailedlinkimmediately

– …andforwardsdatapacketsintoa“blackhole”– Dependson.meoutfordetec.nglosthellos

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TransientDisrup.ons

•  Inconsistentlink‐statedatabase– Someroutersknowaboutfailurebeforeothers

– Theshortestpathsarenolongerconsistent– Cancausetransientforwardingloops

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ConvergenceDelay

•  Sourcesofconvergencedelay– Detec.onlatency– Floodingoflink‐stateinforma.on– Shortest‐pathcomputa.on– Crea.ngtheforwardingtable

•  Performanceduringconvergenceperiod– LostpacketsduetoblackholesandTTLexpiry– Loopingpacketsconsumingresources– Out‐of‐orderpacketsreachingthedes.na.on

•  VerybadforVoIP,onlinegaming,andvideo31

ReducingConvergenceDelay•  Fasterdetec.on

–  Smallerhello.mers–  Link‐layertechnologiesthatcandetectfailures

•  Fasterflooding–  Floodingimmediately–  Sendinglink‐statepacketswithhigh‐priority

•  Fastercomputa.on–  Fasterprocessorsontherouters–  IncrementalDijkstra’salgorithm

•  Fasterforwarding‐tableupdate– Datastructuressuppor.ngincrementalupdates

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ScalingLink‐StateRou.ng

•  Overheadoflink‐staterou.ng–  Floodinglink‐statepacketsthroughoutthenetwork–  RunningDijkstra’sshortest‐pathalgorithm

•  Introducinghierarchythrough“areas”

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Area 0

Area 1 Area 2

Area 3 Area 4

area border router

Conclusions

•  Rou.ngisadistributedalgorithm– Reacttochangesinthetopology– Computethepathsthroughthenetwork

•  Shortest‐pathlinkstaterou.ng– Floodlinkweightsthroughoutthenetwork– Computeshortestpathsasasumoflinkweights

– Forwardpacketsonnexthopintheshortestpath•  Convergenceprocess

– Changingfromonetopologytoanother– Transientperiodsofinconsistencyacrossrouters

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