lecture10-SPT-routing-fundamentals...Announcements • Quiz 1 solutions and grades posted • Average — 26.7/30; median — 30/30; std. dev. — 6.1 • Live coding session on the

ComputerNetworks:ArchitectureandProtocols

CS4450

Lecture10FundamentalsofRou4ng

RachitAgarwal

Announcements

• Quiz1solutionsandgradesposted• Average—26.7/30;median—30/30;std.dev.—6.1

• Livecodingsessiononthe02/27.Pleasebringyourlaptops.

• Prelimwillbe"inclass”on03/26.Nomakeup.

2

GoalsforToday’sLecture

• Recap:WhyNetworkLayer?

• Recap:TherightwaytothinkaboutRoutingTables…

• OurfirstNetworkLayerProtocol:LinkState

• OursecondNetworkLayerProtocol:Distance-vector

3

Recapfromlastlecture

• Whynotjustusespanningtreesacrosstheentirenetwork?

• Easytodesignroutingalgorithmsfor(spanning)trees

• Step1:Sourcenode“floods”itspacketonitsspanningtreelinks

• Step2:Wheneveranodereceivesapacket:

• Forwardsincomingpacketouttoalllinksotherthantheonethatsentthepacket

Recap:Whydoweneednetworklayer?

Recap:RoutingviaFlooding—Anexample

Source

DesVnaVon

1

3

2

7

6

5

4

Eventuallyallnodesarecovered

Onecopyofpacketdeliveredtodes4na4on

1

2

4

7

5

6

• Whynotjustusespanningtreesacrosstheentirenetwork?

• Easytodesignroutingalgorithmsfor(spanning)trees

• Step1:Sourcenode“floods”itspacketonitsspanningtreelinks

• Step2:Wheneveranodereceivesapacket:

• Forwardsincomingpacketouttoalllinksotherthantheonethatsentthepacket

• Threefundamentallimitations:• Unnecessaryprocessingatendhosts• Highlatency• LowBandwidthutilization

Recap:Whydoweneednetworklayer?

Networklayerusesrou4ngtablestoalleviatetheseissues

WelcometotheNetworkLayer!

• THEfunctionality:deliveringthedata

• THEprotocol:InternetProtocol(IP)• Toachieveitsfunctionality,IPprotocolhasthreeresponsibilities

• Addressing

• Encapsulatingdataintopackets

• Routing(usingavarietyofprotocols;severallectures)

Rou4ngtableentriesforapar4culardes4na4onforma(directed)spanningtreewiththatdes4na4onastheroot!!!!

• Letsfocusononedestination-MIT

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

Recap:TherightwaytothinkaboutRoutingTables

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

• RoutingProtocols• “n”spanningtreeprotocolsrunninginparallel

Recap:TherightwaytothinkaboutRoutingTables

• Globalroutingstatevalidifandonlyif:• Therearenodeadends(otherthandestination)• Therearenoloops

• Easytocheckvalidityofroutingstate

Recap:ValidityofaRoutingState

• 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

Recap:Threeflavorsofprotocols

Questions?

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

S1

S2

S3

S7S5

S6

S4

Host A

Host B

Host C

Host DHost E

EachNodeThenhasaGlobalView

S1

S2

S3

S7S5

S6

S4

Host A

Host B

Host C

Host DHost E

WhentoInitiateFloodingofannouncements?

• Topologychange• Linkfailures• Linkrecovery

• Configurationchange• Linkcostchange(whywouldonechangelinkcost?)

• Periodically• Refreshthelink-stateinformation

• Typically(say)30minutes

• Correctsforpossiblecorruptionofdata

AreLoopsStillPossible?

55

5 35

51

1

A

D E

F

CB

A and D think this is the path to C

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

55

5 35

5

1

A

D E

F

CB

E thinks that this the path to C

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

• Canyoueverhaveloopswithlink-staterouting?

• Again:Canyoueverhaveloopswithlink-staterouting?

TransientDisruptions

55

5 35

5

1

A

D E

F

CB

55

5 35

5

1

A

D E

F

CB

• Inconsistentlink-stateviews• Someroutersknowaboutfailurebeforeothers

• Theshortestpathsarenolongerconsistent• Cancausetransientforwardingloops

• Transientloopsarestillaproblem!

ConvergenceinLink-StateRouting

• Eventually,allroutershaveconsistentroutinginformation

• E.g.,allnodeshavingthesamelink-statedatabase

• Here,eventuallymeans“ifnothingchangesafterawhile”

• 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

• Eachnodecomputestheoutgoinglinks(foreachdestination)basedon:

• Locallinkcosts• Informationadvertisedbyneighbors

• Algorithmsdifferinwhattheseexchangescontain

• Distance-vector:justthedistance(andnexthop)toeachdestination• Pathvector:theentirepathtoeachdestination

• Wewillfocusondistance-vectorfornow

DistributedComputationofRoutes

Recall:RoutingTables=CollectionofSpanningTrees

30

• Canweusethespanningtreeprotocol(withmodifications)?

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

• InitiallyeachswitchXannounces(X,0,X)toitsneighbors

Distancevector:acollectionof“n”STPinparallel

LetsruntheProtocolonthisexample

(destination=1)

Round1

Receive Send

1 (1,0,1)

2

3

4

5

6

7

Round2Receive Send

1(1,0,1)

2

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

4

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

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

7

Round3Receive Send

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

(1,1,6)

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

3(1,1,3)

4

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

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

7

Round4Receive Send

1(1,0,1)

2(1,2,2)

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

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

5(1,1,5)

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

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)

• Thesameprotocol/algorithmappliestoalldestinations

• Eachnodeannouncesdistancetoeachdestination• Iam4hopsawayfromnodeA

• Iam6hopsawayfromnodeB

• Iam3hopsawayfromnodeC

• …

• Nodesareexchangingavectorofdistances

WhynotSpanningTreeProtocol?WhyDistance“Vector”?

TowardsDistanceVectorProtocol(withnofailures)

38



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

• IfY’sid<currentroot:setrootdestination=Y

• Switchescomputetheirshortestdistancefromtherootdestination

• Ifcurrent_distance_to_Y>d+costoflinktoZ:• updatecurrent_distance_to_Y=d+costoflinktoZ

• IfrootchangedORshortestdistancetotherootdestinationchanged,sendallneighborsupdatedmessage(Y,d+c,X)

GroupExercise:


2

1

3

2 1

7

Round1

Receive Send

1 (1,0,1)

2 (2,0,2)

3 (3,0,3)

2

1

3

2 1

7

Round2

Receive Send

1(1,0,1)

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

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

2(2,0,2)

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

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

3(3,0,3)

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

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

2

1

3

2 1

7

Round3

Receive Send

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

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

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

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

(3,3,2)

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

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

(2,3,3)

2

1

3

2 1

7

Round4

Receive Send

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

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

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

(2,3,3)

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

(3,3,2)

2

1

3

2 1

7

TowardsDistance-vectorprotocolwithnext-hops(nofailures)

44



• SwitchZupdatesitsview• Uponreceivingmessage(Y,d,X)fromX,checkY’sid

• IfY’sid<currentroot:setrootdestination=Y

• SwitchZcomputesitsshortestdistancefromtherootdestination

• Ifcurrent_distance_to_Y>d+costoflinktoX:• updatecurrent_distance_to_Y=d+costoflinktoX• updatenext_hop_to_destination=X

• IfrootchangedORshortestdistancetotherootdestinationchanged,sendallneighborsupdatedmessage(Y,d+cost-of-link-to-X,Z)

GroupExercise:


(thistimewithnext-hops)

2

1

3

2 1

7

Round1

Receive Send Next-hops

1 (1,0,1) [-]

2 (2,0,2) [-]

3 (3,0,3) [-]

2

1

3

2 1

7

Round2


1(1,0,1)

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

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

[-,2,3]

2(2,0,2)

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

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

[1,-,3]

3(3,0,3)

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

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

[1,2,-]

2

1

3

2 1

7

Round3


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

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

[-,2,3]

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

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

(3,3,2)[1,-,1]

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

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

(2,3,3)[1,1,-]

2

1

3

2 1

7

Round4


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

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

[-,2,3]

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

(2,3,3)[1,-,1]

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

(3,3,2)[1,1,-]

2

1

3

2 1

7

DistanceVectorProtocol

50


• InitiallyeachswitchXinitializesitsroutingtableto(X,0,-)anddistanceinfinitytoallotherdestinations

• Switchesannouncetheirentiredistancevectors(routingtablew/0nexthops)

• Uponreceivingaroutingtablefromanode(sayX),eachnodedoes:

• ForeachdestinationYintheannouncement(distance(X,Y)=d):

• Ifcurrent_distance_to_Y>d+costoflinktoX:• updatecurrent_distance_to_Y=d+costoflinktoX• updatenext_hop_to_destination=X

• Ifshortestdistancetoanydestinationchanged,sendallneighborsyourdistancevectors

• Protocol:• Exchangingthatroutinginformationwithneighbors

• Whatandwhenforexchanges

• RIPisaprotocolthatimplementsDV(IETFRFC2080)

• Algorithm:

• Howtousetheinformationfromyourneighborstoupdateyour

ownroutingtables?

TwoAspectstoThisApproach

GroupExercise:

LetsruntheProtocolagainonthisexample

(thistimewithdistancevectors)

2

1

3

2 1

7

Round1

2

1

3

2 1

7

distance next-hop1 0 -2 infinity3 infinity

distance next-hop1 infinity2 0 -3 infinity

distance next-hop1 infinity2 infinity3 0 -

Round2

2

1

3

2 1

7

distance next-hop1 0 -2 2 23 1 3

distance next-hop1 2 12 0 -3 7 3

distance next-hop1 1 12 7 23 0 -

Round3

2

1

3

2 1

7




Round4

2

1

3

2 1

7




• Algorithm:

• NodesuseBellman-Fordtocomputedistances

• Protocol• Nodesexchangedistancevectors• Updatetheirownroutingtables• Andexchangeagain…• Details:whentoexchange,whattoexchange,etc….

FromAlgorithmtoProtocol

• Whendoyousendmessages?

• Whenanyofyourdistancesd(u,v)change

• Periodically,toensureconsistencybetweenneighbors

• Whatinformationdoyousend?

• Couldsendentirevector• Orjustupdatedentries

• Doyousendeveryonethesameinformation?

• Nextlecture

OtherAspectsofProtocol

Nextlecture!HappyFebruaryBreak!!

Documents

lecture10-SPT-routing-fundamentals...Announcements • Quiz 1 solutions and grades posted • Average — 26.7/30; median — 30/30; std. dev. — 6.1 • Live coding session on the