ComputerNetworks:ArchitectureandProtocols

CS4450

Lecture10

SpanningTreeProtocol

FundamentalsofRou:ng

Spring2018

RachitAgarwal

MyTuesdayeveningandWednesday….

• Wallowingintheshameoffailure

• Ileftyouconfusedattheendoflastlecture…• IfeltlikeIhavefailed,yetagain,asateacher…• Ifeltlikemyclassmusthateme,yetagain…

• Today’sgoals:• Redeemmyesteem,oratleast,tryit…

• Seeifmystudentscanloveme(again?)

2

• Firstattempt

• I(almost)emptiedmyqueues:-)

• Answeredalltheemails

• Updatedthewebsite(socketslides/code,PS2,…)• Thingsshould(hopefully)begooduntilthespringbreak!

• ProblemSet2solutionsreleasedonPiazza

• Quizsolutionswillbereleasedbythisweekend

3

MyTuesdayeveningandWednesday….

GoalsforToday’sLecture

• Bringusbackintoourloveforcomputernetworks(andme?)…

• QuickReview:SpanningTreeProtocol(+Failures)

• Whydoweneedroutinglayer?

• Whynotjustusespanningtreeprotocol?

• StartonFundamentalsofRouting

4

Recap:SpanningTreeProtocol(failuresonlaterslides)

5

• Messages(Y,d,X):ForrootY;FromnodeX;advertisingadistancedtoY

• InitiallyeachswitchXannounces(X,0,X)toitsneighbors

• Switchesupdatetheirview• Uponreceivingmessage(Y,d,Y)fromZ,checkY’sid

• IfY’sid<currentroot:setroot=Y

• Switchescomputetheirdistancefromtheroot

• Add1totheshortestdistancereceivedfromaneighbor

• IfrootchangedORshortestdistancetotherootchanged,sendallneighborsupdatedmessage(Y,d+1,X)

GroupExercise:

LetsruntheSpanningTreeProtocolonthisexample

Round1

Receive Send

1 (1,0,1)

2 (2,0,2)

3 (3,0,3)

4 (4,0,4)

5 (5,0,5)

6 (6,0,6)

7 (7,0,7)

Round2Receive Send

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

(6,0,6)

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

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

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

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

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

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

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

Round3Receive Send

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

(1,1,6)

2(2,0,2)(1,1,3),(2,1,4),(1,1,6),(2,1,7)

(1,2,2)

3(1,1,3)

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

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

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

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

Round4Receive Send

1(1,0,1)

2(1,2,2)

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

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

5(1,1,5)

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

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

Round5Receive Send

1(1,0,1)

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

3(1,1,3)

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

5(1,1,5)

6(1,1,6)

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

AfterRound5:WehaveourSpanningTree

• 3-1• 5-1• 6-1• 2-3• 4-2• 7-2

12

Questions?

• Protocolmustreacttofailures

• Failureoftherootnode• Failureofswitchesandlinks

• Rootnodesendsperiodicannouncementmessages

• Fewpossibleimplementations,butthisissimpletounderstand

• Otherswitchescontinueforwardingmessages

• Detectingfailuresthroughtimeout(softstate)

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

• Ifmultiplemessageswithanewrootreceived,sendmessage(Y,d,X)

totheneighborsendingthemessage

14

SpanningTreeProtocol++(incorporatingfailures)

Supposelink2-4fails

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

• Why?

• Atsomepoint,7willrespondwith(1,3,7)

• 4willnowupdateto(1,4,4)andsendupdatemessage

• Newspanningtree!

15

Questions?

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TheendofLinkLayer….

Andthebeginningofnetworklayer:-D

Builtontopof

reliabledelivery

Builtontopofbest-

effortforwarding

Builtontopof

best-effortrouting

Builtontopof

physicalbittransfer

• There’sonlyonepathfromsourcetodestination

• Howdoyoufindthatpath?Ideas?

• Easytodesignroutingalgorithmsfortrees

• Nodescan“flood”packettoallothernodes

Whydoweneedanetworklayer?

• Sendspackettoeverynodeinthenetwork

• Step1:IgnorethelinksnotbelongingtotheSpanningTree

• Step2:Originatingnodesends“flood”packetouteverylink(onspanningtree)

• Step3:Sendincomingpacketouttoalllinksotherthantheonethatsentthepacket

FloodingonaSpanningTree

FloodingExample

Source

Destination

1

3

2

7

6

5

4

FloodingExample

Source

Destination

1

3

2

7

6

5

4

Eventually all nodes are covered

One copy of packet delivered to destination

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

Issue 1: Each host has to do unnecessary packet processing! (to decide whether the packet is destined to the host)

Source

Destination

1

3

2

7

6

5

4

Threefundamentalissues!

Issue 2: Higher latency! (The packets unnecessarily traverse much longer paths)

Source

Destination

1

3

2

7

6

5

4

Threefundamentalissues!

Issue 2: Lower bandwidth availability! (2-6 and 3-1 packets unnecessarily have to share bandwidth)

Questions?

• Networklayerperforms“routing”ofpacketstoalleviatetheseissues

• Usesroutingtables

• Letsunderstandroutingtablesfirst• Wewillseeroutingtablesarenothingbut…

• Guess?• Acollectionof(carefullyconstructed)spanningtrees

• Oneperdestination

Whydoweneedanetworklayer?

What path will a packet take from Cornell to MIT?

• Routingtablesallowfindingpathfromsourcetodestination

RoutingPacketsviaRoutingTables

Cornell

Harvard

MIT

Switch #1

Switch #2

Switch #3

• Findingpathforapacketfromsourcetodestination

RoutingPacketsviaRoutingTables

Cornell

Harvard

MIT

Switch #1

Switch #2

Switch #3

How to specify whether the packet should take Path 1 or Path 2?

Each Switch stores a table indicating the next hop for corresponding destination of a packet (called a routing table)

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

RoutingTable

Cornell

Harvard

MIT

Switch#1

Switch#2

Switch#3L1

L2 L3

L4

L5 L6

DESTINATION NEXT HOPCORNELL L1

MIT L3HARVARD L4

DESTINATION NEXT HOPCORNELL L2

MIT L5HARVARD L5

DESTINATION NEXT HOPCORNELL L5

MIT L6HARVARD L3

• 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

L5 L6

DESTINATION NEXT HOPCORNELL L1

MIT L2HARVARD L4

DESTINATION NEXT HOP

CORNELL L2

HARVARD L5

DESTINATION NEXT HOPCORNELL L5

MIT L6HARVARD L3

No forwarding decision for MIT!

Dead End!Packet never reaches MIT

Example:RoutingwithLoops

Cornell

Harvard

MIT

Switch #1

Switch #2

Switch #3L1

L2 L3

L4

L5 L6

DESTINATION NEXT HOPCORNELL L1

MIT L3HARVARD L4

DESTINATION NEXT HOPCORNELL L2

MIT L2HARVARD L5

DESTINATION NEXT HOPCORNELL L5

MIT L5HARVARD L3

• SupposepacketwantstogofromCornelltoMITusinggivenstate:

Loop!Packet never reaches MIT

• 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

Leaves Spanning Tree: Valid

Example2:

Example2:

Is this valid?

Example3:

Example3:

Is this valid?

• 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(actuallydatagrams;nextlecture)

• Routing(usingavarietyofprotocols;severallectures)

Nextlecture!