ComputerNetworks:ArchitectureandProtocols

CS4450

Lecture9WhyNetworkLayer?

FundamentalsofRou:ng

RachitAgarwal

Announcements

• Iamback!

• Allemailsanswered,caughtuponsleep,readytoroll!

• Livecodingsessiononthe27th.Pleasebringyourlaptops.

• Prelimwillbe"inclass”on03/26.Nomakeup.

• ProblemSet2solutionsareposted(Piazza)

• Project1posted(coursewebpage)

• Emails:Pleasepleaseplease

• ccyourTAs(Katie,Qizhe)

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GoalsforToday’sLecture

• Weareatastagewhereyouhaveastrongfoundationinnetworks

• Sharingnetworks,architecturalprinciples,designgoals• And,youmaynotrealize,butyouunderstandmanynewtradeoffs

• Whydoweneednetworklayer?

• WhynotjustuseswitchedEthernetacrosstheInternet?

• Whatdoroutingtableslooklike?

• Therightwaytothinkaboutroutingtables….

• Correctnessdefinitionforroutingtables

• Ourfirstnetworklayerprotocol:LinkState

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RecapofLinkLayersofar

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• TraditionalLinkLayer:BroadcastEthernet

• CSMA/CD

• Randomaccessonabroadcastchannel

• ExponentialBackoff

• WhyFrames?

• Toincorporatesentinelbitsforidentifyingframestart/end

• Toincorporatelinklayersourceanddestinationnames

• ToincorporateCRCforcheckingcorrectnessofreceivedframes

• ModernLinkLayer:SwitchedEthernet

• Why?ScalabilitylimitsoftraditionalEthernet

• Why?Detectingcollisionsonabroadcastchannel

Recap:Linklayer

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Recap:SpanningTreedefinition

• Subgraphthatincludesallverticesbutcontainsnocycles• Linksnotinthespanningtreearenotusedinforwardingframes

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Recap:SpanningTreeProtocol

• Messages(Y,d,X)

• ProposingrootY;fromnodeX;advertisingadistancedtoY

• Initiallyeachswitchproposesitselfastheroot• thatis,switchXannounces(X,0,X)toitsneighbors

• AteachswitchZ:WHENEVERamessage(Y,d,X)isreceivedfromX:

• IFY’sid<currentroot• THENsetroot=Y;next-hop=X

• IFShortestdistancetoroot>d+distance_from_X

• THENsetshortest-distance-to-root=d+distance_from_X

• IFrootchangedORshortestdistancetotherootchanged:• Sendallneighborsmessage(Y,shortest-distance-to-root,Z)

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TheendofLinkLayer….

Andthebeginningofnetworklayer:-D

Builtontopofreliabledelivery

Builtontopofbest-effortforwarding

Builtontopofbest-effortrouting

Builtontopofphysicalbittransfer

Questions?

• Whynotjustusespanningtreesacrosstheentirenetwork?

• Easytodesignroutingalgorithmsfor(spanning)trees

• Step1:Sourcenode“floods”itspacketonitsspanningtreelinks

• Step2:Wheneveranodereceivesapacket:

• Forwardsincomingpacketouttoalllinksotherthantheonethatsentthepacket

Whydoweneedanetworklayer?

• Sendspackettoeverynodeinthenetwork

• Step1:IgnorethelinksnotbelongingtotheSpanningTree

• Step2:Sourcesends“flood”packetouteverylink(onspanningtree)

• Step3:Sendincomingpacketouttoalllinksotherthantheonethatsentthepacket

FloodingonaSpanningTree

FloodingExample

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FloodingExample

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Eventuallyallnodesarecovered

Onecopyofpacketdeliveredtodes:na:on

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• Easytodesignroutingalgorithmsfor(spanning)trees

• Step1:Sourcenode“floods”itspacketonitsspanningtreelinks

• Step2:Wheneveranodereceivesapacket:

• Forwardsincomingpacketouttoalllinksotherthantheonethatsentthepacket

• Amazingproperties:

• Noroutingtablesneeded!• Nopacketswilleverloop.• Atleast(andexactly)onepacketmustreachthedestination

• Assumingnofailures

RoutingviaFloodingonSpanningTree…

Threefundamentalissues!

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Issue1:Eachhosthastodounnecessarypacketprocessing!(todecidewhetherthepacketisdes:nedtothehost)

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Threefundamentalissues!

Issue2:Higherlatency!(Thepacketsunnecessarilytraversemuchlongerpaths)

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Threefundamentalissues!

Issue3:Wastedbandwidth!(2-6and3-1packetsunnecessarilyhavetosharebandwidth)

Questions?

• Networklayerperforms“routing”ofpacketstoalleviatetheseissues

• Usesroutingtables

• Letsunderstandroutingtablesfirst

Whydoweneedanetworklayer?

WhatpathwillapackettakefromCornelltoMIT?

• Routingtablesallowfindingpathfromsourcetodestination

RoutingPacketsviaRoutingTables

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Switch#1

Switch#2

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• Findingpathforapacketfromsourcetodestination

RoutingPacketsviaRoutingTables

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Switch#1

Switch#2

Switch#3

HowtospecifywhetherthepacketshouldtakePath1orPath2?

EachSwitchstoresatableindica:ngthenexthopforcorrespondingdes:na:onofapacket(calledarou:ngtable)

• SupposepacketfollowsPath1:Cornell-S#1-S#3-MIT

RoutingTable

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DESTINATION NEXTHOP

CORNELL L1

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DESTINATION NEXTHOP

CORNELL L2

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HARVARD L5

DESTINATION NEXTHOP

CORNELL L5

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HARVARD L3

Seesomethinginteres:ng?

• Letsfocusononedestination-MIT

RoutingTable:Therightwaytothinkaboutthem

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DESTINATION NEXTHOP

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HARVARD L3

Rou:ngtableentriesforapar:culardes:na:onforma(directed)spanningtreewiththatdes:na:onastheroot!!!!

• Letsfocusononedestination-MIT

RoutingTable:Therightwaytothinkaboutthem

Cornell

Harvard

MIT

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Switch#2

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L2 L3

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DESTINATION NEXTHOP

CORNELL L1

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HARVARD L4

DESTINATION NEXTHOP

CORNELL L2

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HARVARD L5

DESTINATION NEXTHOP

CORNELL L5

MIT L6

HARVARD L3

• Routingtablesarenothingbut….• Acollectionof(directed)spanningtree• Oneforeachdestination

• RoutingProtocols• “n”spanningtreeprotocolsrunninginparallel

RoutingTable:Therightwaytothinkaboutthem

• Globalroutingstateisvalidif:• italwaysresultsindeliverpacketstotheirdestinations

• GoalofRoutingProtocols• Computeavalidstate

• Buthowtotellifaroutingstateisvalid?…• Thinkaboutit,whatcouldmakeroutingincorrect?

“ValidRoutingTables”(routingstate)

• Globalroutingstatevalidifandonlyif:• Therearenodeadends(otherthandestination)• Therearenoloops

• Adeadendiswhenthereisnooutgoinglink• Apacketarrives,but..

• theroutingtabledoesnothaveanoutgoinglink• Andthatnodeisnotthedestination

• Aloopiswhenapacketcyclesaroundthesamesetofnodesforever

ValidityofaRoutingState

• SupposepacketwantstogofromCornelltoMITusinggivenstate:

Example:RoutingwithDeadEnds

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DESTINATION NEXTHOP

CORNELL L1

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DESTINATION NEXTHOP

CORNELL L2

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DESTINATION NEXTHOP

CORNELL L5

MIT L6

HARVARD L3

NoforwardingdecisionforMIT!

DeadEnd!PacketneverreachesMIT

Example:RoutingwithLoops

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Harvard

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DESTINATION NEXTHOP

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CORNELL L2

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DESTINATION NEXTHOP

CORNELL L5

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HARVARD L3

• SupposepacketwantstogofromCornelltoMITusinggivenstate:

Loop!PacketneverreachesMIT

• Howcanweverifygivenroutingstateisvalid?

• Howcanweproducevalidroutingstate?

TwoQuestions

• Checkvalidityofroutingstateforonedestinationatatime…

• Foreachnode:• Marktheoutgoinglinkwitharrowfortherequireddestination

• Therecanonlybeoneateachnode

• Eliminatealllinkswithnoarrows

• Lookwhat’sleft.Stateisvalidifandonlyif• Remaininggraphisaspanningtreewithdestinationassink

• Whyisthistrue?

• Tree->Noloops• Spanning(tree)->Nodeadends

CheckingValidityofaRoutingState

Example1

Example1:PickDestination

Example1:PutArrowsonOutgoingPorts

Example1:RemoveunusedLinks

LeavesSpanningTree:Valid

Example2:

Example2:

Isthisvalid?

Example3:

Example3:

Isthisvalid?

• Simpletocheckvalidityofroutingstateforaparticulardestination

• Deadends:nodeswithoutarrows

• Loops:obvious,disconnectedfromdestinationandrestofthegraph

CheckingValidityofaRoutingState

• Howcanweverifygivenroutingstateisvalid?

• Howcanweproducevalidroutingstate?

TwoQuestions

• Easytoavoiddeadends

• Avoidingloopsishard

• Thekeydifferencebetweenroutingprotocolsishowtheyavoidloops!

CreatingValidRoutingState

• CreateTree,routeontree• E.g.,Spanningtreeprotocol(switchedEthernet)• Good:easy,no(persistent)loops,nodeadends• Not-so-good:unnecessaryprocessing,highlatency,lowbandwidth

• Obtainaglobalview:• E.g.,Linkstate

• Distributedroutecomputation:

• E.g.,Distancevector• E.g.,BorderGatewayProtocol

Fourflavorsofprotocols

RoutingMetrics

• Routinggoals:computepathswithminimumX

• X=numberof“hops”(nodesinthemiddle)

• X=latency• X=weight• X=failureprobability• …

• Generallyassumeeverylinkhas“cost”associatedwithit

• Wewanttominimizethecostoftheentirepath

• WewillfocusonasubsetofpropertiesX,where:

• Costofapath=sumofcostsofindividuallinks/nodesonthepath• E.g.,numberofhopsandlatency

#1:CreateaTree

• Removeenoughlinkstocreateatreecontainingallnodes

• Soundsfamiliar?Spanningtrees!

• Ifthetopologyhasnoloops,thenjustmakesurenotsendingpacketsbackfromwheretheycame

• Thatcausesanimmediateloop

• Therefore,ifnoloopsintopologyandnoformationofimmediateloopsensuresvalidrouting

• However…threechallenges• Unnecessaryhostresourcesusedtoprocesspackets• Highlatency• Lowbandwidth(utilization)

#1:CreateTreeOutofTopology

#2:Globalview

TwoAspectsofGlobalViewMethod

• Protocol:Whatwefocusontoday

• Wheretocreateglobalview

• Howtocreateglobalview• Disseminatingroutecomputation(ifnecessary)

• Whentorunroutecomputation

• Algorithm:computingloop-freepathsongraph

• Straightforwardtocomputelowestcostpaths

• UsingDijkstra’salgorithm(pleasestudy;algorithmscourse)

• Wewon’tspendtimeonthis

Wheretocreateglobalview?

• Oneoption:Centralserver• Collectsaglobalview• Computestheroutingtableforeachnode

• “Installs”routingtablesateachnode• Software-definedNetworks:laterincourse

• Secondoption:Ateachrouter• Eachroutercollectsaglobalview• ComputesitsownroutingtableusingLink-stateprotocol

• Link-stateroutingprotocol• OSPFisaspecificimplementationoflink-stateprotocol

• IETFRFC2328(IPv4)or5340(IPv6)

OverviewofLink-StateRouting

• Everyrouterknowsitslocal“linkstate”• Knowsstateoflinkstoneighbors• Up/down,andassociatedcost

• Arouterfloodsitslinkstatetoallotherrouters• Usesaspecialpacket—LinkStateAnnouncements(LSA)

• Announcementisdeliveredtoallnodes(nextslide)

• Hence,everyrouterlearnstheentirenetworkgraph

• Runsroutecomputationlocally

• Computingleastcostpathsfromthemtoallothernodes

• E.g.,usingDijkstra’salgorithm

HowdoesFloodingWork?

• “Linkstateannouncement”(LSA)arrivesonalinkatarouter

• Thatrouter:• Remembersthepacket

• Forwardsthepacketoutallotherlinks• Doesnotsenditouttheincominglink

• Why?

• Ifapreviouslyreceivedannouncementarrivesagain…

• Routerdropsit(noneedtoforwardagain)

Link-StateRouting

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EachNodeThenhasaGlobalView

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WhentoInitiateFloodingofannouncements?

• Topologychange• Linkfailures• Linkrecovery

• Configurationchange• Linkcostchange(whywouldonechangelinkcost?)

• Periodically• Refreshthelink-stateinformation

• Typically(say)30minutes

• Correctsforpossiblecorruptionofdata

MakingFloodsReliable

• ReliableFlooding• Ensureallnodesreceivesamelinkstateannouncements

• Noannouncementsdropped

• Ensureallnodesusethelatestversion

• Supposewecanimplementreliableflooding.Howcanitstillfail?

• Canyoueverhaveloopswithlink-staterouting?

• Again:Canyoueverhaveloopswithlink-staterouting?

• Aglobalviewofthenetworkmakescomputingpathswithoutloopseasy

• Manygraphalgorithmsforcomputingloop-freepaths

• Fore.g.,Dijkstra’sAlgorithm

• But,but,but…..

• Gettinganaccurateglobalviewofnetworkischallenging!• Especiallyina“timely”manner

• Asthenetworkischanging

FundamentalChallengewithobtainingaGlobalView

AreLoopsStillPossible?

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A and D think this is the path to C

E-C link fails, but D doesn’t know yet

E thinks that this the path to C

E reaches C via D, D reaches C via E Loop!

TransientDisruptions

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• Inconsistentlink-stateviews• Someroutersknowaboutfailurebeforeothers

• Theshortestpathsarenolongerconsistent• Cancausetransientforwardingloops

• Transientloopsarestillaproblem!

Convergence

• Allroutershaveconsistentroutinginformation

• E.g.,allnodeshavingthesamelink-statedatabase

• Forwardingisconsistentafterconvergence• Allnodeshavethesamelink-statedatabase

• Allnodesforwardpacketsonsamepaths

• Butwhilestillconverging,badthingscanhappen

TimetoReachConvergence

• Sourcesofconvergencedelay?• Timetodetectfailure

• Timetofloodlink-stateinformation(~longestRTT)

• Timetorecomputeforwardingtables

• Performanceproblemsduringconvergenceperiod?

• Deadends• Loopingpackets• Andsomemorewe’llseelater….

LinkStateisConceptuallySimple

• Everyonefloodslinksinformation

• Everyonethenknowsgraphofthenetwork

• Everyoneindependentlycomputespathsonthegraph

• Allthecomplexityisinthedetails

#3:DistributedRouteComputation

• Distributedalgorithmstocomputefeasibleroute

• ApproachA:Findingoptimalrouteformaximizing/minimizingametric

• ApproachB:Findingfeasiblerouteviaexchangingpathsamongswitches

#3:DistributedRouteComputation

WelcometotheNetworkLayer!

• THEfunctionality:deliveringthedata

• THEprotocol:InternetProtocol(IP)• Toachieveitsfunctionality(deliveringthedata),IPprotocolhasthreeresponsibilities

• Addressing(nextlecture)

• Encapsulatingdataintopackets(nextlecture)

• Routing(usingavarietyofprotocols;severallectures)

Nextlecture!