ComputerNetworks:ArchitectureandProtocols

CS4450

Lecture9SpanningTreeProtocolWhyNetworkLayer?

RachitAgarwal

GoalsforToday’sLecture

• FinishSpanningTreeProtocol

• Whydoweneednetworklayer?

• WhynotjustuseswitchedEthernetacrosstheInternet?

2

RecapofLinkLayersofar

3

• TraditionalLinkLayer:BroadcastEthernet• CSMA/CD

• Randomaccessonabroadcastchannel

• ExponentialBackoff• WhyFrames?

• Toincorporatesentinelbitsforidentifyingframestart/end

• Toincorporatelinklayersourceanddestinationnames

• ToincorporateCRCforcheckingcorrectnessofreceivedframes

• ModernLinkLayer:SwitchedEthernet

• Why?

• ScalabilitylimitsoftraditionalEthernet

• Why?

• Detectingcollisionsonabroadcastchannel

Recap:Linklayer

4

Recap:SwitchedEthernet

• Enablesconcurrentcommunication

• HostAcantalktoC,whileBtalkstoD• Nocollisions->noneedforCSMA,CD

• Noconstraintsonlinklengthsorframesize

5

Recap:Broadcaststorm

6

Recap:AvoidingBroadcastStormProblem

7

Gettingridofloops

using

SpanningTreeProtocol

Recap:SpanningTreedefinition

• Subgraphthatincludesallverticesbutcontainsnocycles• Linksnotinthespanningtreearenotusedinforwardingframes

8

Questions?

AlgorithmhasTwoAspects…

• Pickaroot:• Picktheonewiththesmallestidentifier(MACname/address)

• Computetheshortestpathstotheroot

• Onlykeepthelinksontheshortestpath• Breaktiesinsomeway

• soweonlykeeponeshortestpathfromeachnode

• Oneapproach:Choosethepathvianeighborswitchwiththesmallestidentifier

• Ethernet’sspanningtreeconstructiondoesbothwithasinglealgorithm

10

StepsinSpanningTreeProtocol(assumingalldelays=1)• Messages(Y,d,X)

• ProposingrootY;fromnodeX;advertisingadistancedtoY

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

• Switchesupdatetheirview;eachswitchZ:• Uponreceivingmessage(Y,d,X)fromX,checkY’sid

• IfY’sid<currentroot:setroot=Y• Setnext-hop=X

• Switchescomputetheirdistancefromtheroot;eachswitchZ:

• Shortestdistancetoroot=d+distancefromX=d+1

• IfrootchangedORshortestdistancetotherootchanged:• SwitchZsendsneighborsupdatedmessage(Y,d+1,Z)

11

LetsruntheSpanningTreeProtocolonthisexample

(assumealllinkshave“distance”1)

Round1

Receive Send Next-hop

1 (1,0,1) 1

2 (2,0,2) 2

3 (3,0,3) 3

4 (4,0,4) 4

5 (5,0,5) 5

6 (6,0,6) 6

7 (7,0,7) 7

Round2Receive Send

Nexthop

1(1,0,1) (3,0,3),(5,0,5) 1

2(2,0,2)(3,0,3),(4,0,4),(6,0,6),(7,0,7)

2

3(3,0,3) (1,0,1),(2,0,2) (1,1,3) 1

4(4,0,4) (2,0,2),(7,0,7) (2,1,4) 2

5(5,0,5) (1,0,1),(6,0,6) (1,1,5) 1

6(6,0,6) (2,0,2),(5,0,5) (2,1,6) 2

7(7,0,7) (2,0,2),(4,0,4) (2,1,7) 2

Round3Receive Send Nexthop

1 (1,1,3),(1,1,5) 1

2(1,1,3),(2,1,4),(2,1,6),(2,1,7)

(1,2,2) 3

3(1,1,3) 1

4(2,1,4) (2,1,7) 2

5(1,1,5) (2,1,6) 1

6(2,1,6) (1,1,5) (1,2,5) 5

7(2,1,7) (2,1,4) 2

Round4Receive Send Nexthop

1 1

2(1,2,2) (1,2,6) 3

3 (1,2,2) 1

4 (1,2,2) (1,3,4) 2

5 (1,2,6) 1

6(1,2,6) (1,2,2) 5

7 (1,2,2) (1,3,7) 2

Round5Receive Send

Nexthop

1 1

2 (1,3,4),(1,3,7) 3

3 1

4(1,3,4) (1,3,7) 2

5 1

6 1

7(1,3,7) (1,3,4) 2

AfterRound5:WehaveourSpanningTree

• 3-1• 5-1• 6-1• 2-3• 4-2• 7-2

18

Questions?

SpanningTreeProtocol(++Incorporatingdistances)• Messages(Y,d,X)

• ProposingrootY;fromnodeX;advertisingadistancedtoY

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

• Switchesupdatetheirview;eachswitchZ:• Uponreceivingmessage(Y,d,X)fromX,checkY’sid

• IfY’sid<currentroot:setroot=Y• Setnext-hop=X

• Switchescomputetheirdistancefromtheroot;eachswitchZ:

• Shortestdistancetoroot=d+distanceTo(X)

• IfrootchangedORshortestdistancetotherootchanged:• switchZsendsneighborsupdatedmessage(Y,d+distanceTo(X),Z)

20

• Protocolmustreacttofailures

• Failureoftherootnode• Failureofswitchesandlinks

• Rootnodesendsperiodicannouncementmessages

• Fewpossibleimplementations,butthisissimpletounderstand

• Otherswitchescontinueforwardingmessages

• Detectingfailuresthroughtimeout(softstate)

• Ifnowordfromroot,timeoutandsenda(Y,0,Y)messagetoallneighbors(inthegraph)!

• Ifmultiplemessageswithanewrootreceived,sendmessage(Y,d,X)totheneighborsendingthemessage

21

SpanningTreeProtocol++(incorporatingfailures)

Example:Supposelink2-4fails

• 4willsend(4,0,4)toallitsneighbors• 4willstopreceivingannouncementmessagesfromtheroot

• Why?

• Atsomepoint,7willrespondwith(1,3,7)

• 4willnowupdateto(1,4,4)andsendupdatemessage

• Newspanningtree!

22

Questions?

24

TheendofLinkLayer….

Andthebeginningofnetworklayer:-D

Builtontopofreliabledelivery

Builtontopofbest-effortforwarding

Builtontopofbest-effortrouting

Builtontopofphysicalbittransfer

• Whynotjustusespanningtreesacrosstheentirenetwork?

• Easytodesignroutingalgorithmsfor(spanning)trees

• Nodescan“flood”packettoallothernodes

Whydoweneedanetworklayer?

• Sendspackettoeverynodeinthenetwork

• Step1:IgnorethelinksnotbelongingtotheSpanningTree

• Step2:Originatingnodesends“flood”packetouteverylink(onspanningtree)

• Step3:Sendincomingpacketouttoalllinksotherthantheonethatsentthepacket

FloodingonaSpanningTree

FloodingExample

Source

Deshnahon

1

3

2

7

6

5

4

FloodingExample

Source

Destination

1

3

2

7

6

5

4

Eventuallyallnodesarecovered

Onecopyofpacketdeliveredtodesdnadon

1

2

4

7

5

6

• There’sonlyonepathfromsourcetodestination

• Howdoyoufindthatpath?Ideas?

• Easytodesignroutingalgorithmsfortrees

• Nodescan“flood”packettoallothernodes

• Amazingproperties:

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

• Assumingnofailures

RoutingviaFloodingonSpanningTree…

Threefundamentalissues!

Source

Destination

1

3

2

7

6

5

4

1

2

4

7

5

6

Issue1:Eachhosthastodounnecessarypacketprocessing!(todecidewhetherthepacketisdesdnedtothehost)

Source

Destination

1

3

2

7

6

5

4

Threefundamentalissues!

Issue2:Higherlatency!(Thepacketsunnecessarilytraversemuchlongerpaths)

Source

Destination

1

3

2

7

6

5

4

Threefundamentalissues!

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

Questions?

• Networklayerperforms“routing”ofpacketstoalleviatetheseissues

• Usesroutingtables

• Letsunderstandroutingtablesfirst

Whydoweneedanetworklayer?

WhatpathwillapackettakefromCornelltoMIT?

• Routingtablesallowfindingpathfromsourcetodestination

RoutingPacketsviaRoutingTables

Cornell

Harvard

MIT

Switch#1

Switch#2

Switch#3

• Findingpathforapacketfromsourcetodestination

RoutingPacketsviaRoutingTables

Cornell

Harvard

MIT

Switch#1

Switch#2

Switch#3

HowtospecifywhetherthepacketshouldtakePath1orPath2?

EachSwitchstoresatableindicadngthenexthopforcorrespondingdesdnadonofapacket(calledaroudngtable)

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

RoutingTable

Cornell

Harvard

MIT

Switch#1

Switch#2

Switch#3L1

L2 L3

L4

L5 L6

DESTINATION NEXTHOP

CORNELL L1

MIT L3

HARVARD L4

DESTINATION NEXTHOP

CORNELL L2

MIT L5

HARVARD L5

DESTINATION NEXTHOP

CORNELL L5

MIT L6

HARVARD L3

Seesomethinginteresdng?

• Letsfocusononedestination-MIT

RoutingTable:Therightwaytothinkaboutthem

Cornell

Harvard

MIT

Switch#1

Switch#2

Switch#3L1

L2 L3

L4

L5 L6

DESTINATION NEXTHOP

CORNELL L1

MIT L3

HARVARD L4

DESTINATION NEXTHOP

CORNELL L2

MIT L5

HARVARD L5

DESTINATION NEXTHOP

CORNELL L5

MIT L6

HARVARD L3

Roudngtableentriesforapardculardesdnadonforma(directed)spanningtreewiththatdesdnadonastheroot!!!!

• Letsfocusononedestination-MIT

RoutingTable:Therightwaytothinkaboutthem

Cornell

Harvard

MIT

Switch#1

Switch#2

Switch#3L1

L2 L3

L4

L5 L6

DESTINATION NEXTHOP

CORNELL L1

MIT L3

HARVARD L4

DESTINATION NEXTHOP

CORNELL L2

MIT L5

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

Cornell

Harvard

MIT

Switch#1

Switch#2

Switch#3L1

L2 L3

L4

L5L6

DESTINATION NEXTHOP

CORNELL L1

MIT L2

HARVARD L4

DESTINATION NEXTHOP

CORNELL L2

HARVARD L5

DESTINATION NEXTHOP

CORNELL L5

MIT L6

HARVARD L3

NoforwardingdecisionforMIT!

DeadEnd!PacketneverreachesMIT

Example:RoutingwithLoops

Cornell

Harvard

MIT

Switch#1

Switch#2

Switch#3L1

L2 L3

L4

L5L6

DESTINATION NEXTHOP

CORNELL L1

MIT L3

HARVARD L4

DESTINATION NEXTHOP

CORNELL L2

MIT L2

HARVARD L5

DESTINATION NEXTHOP

CORNELL L5

MIT L5

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!

• Trytothinkaloopavoidancedesignforfiveminutes

CreatingValidRoutingState

• Removeenoughlinkstocreateatreecontainingallnodes

• Soundsfamiliar?Spanningtrees!

• Ifthetopologyhasnoloops,thenjustmakesurenotsendingpacketsbackfromwheretheycame

• Thatcausesanimmediateloop

• Therefore,ifnoloopsintopologyandnoformationofimmediateloopsensuresvalidrouting

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

#1:CreateTreeOutofTopology

• Aglobalviewofthenetworkmakescomputingpathswithoutloopseasy

• Manygraphalgorithmsforcomputingloop-freepaths

• Fore.g.,Dijkstra’sAlgorithm

• Gettingtheglobalviewofnetworkischallenging!

#2:ObtainaGlobalView

• Oftengettingaglobalviewofthenetworkisinfeasible• 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!