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[Docs][txt|pdf][draft-ietf-mpls-arch][Diff1][Diff2][Errata]Updatedby:6178PROPOSEDSTANDARDErrataExistNetwor

Wor

ingGroupE.RosenRequestforComments:3031CiscoSystems,Inc.Category:StandardsTrac

A.ViswanathanForce10Networ s,Inc.R.CallonJuniperNetwor s,Inc.January2001

MultiprotocolLabelSwitchingArchitecture

StatusofthisMemo

ThisdocumentspecifiesanInternetstandardstrac protocolfortheInternetcommunity,andrequestsdiscussionandsuggestionsforimprovements.Pleaserefertothecurrenteditionofthe"InternetOfficialProtocolStandards"(STD1)forthestandardizationstateandstatusofthisprotocol.Distributionofthismemoisunlimited.

CopyrightNotice

Copyright(C)TheInternetSociety(2001).AllRightsReserved.

Abstract

ThisdocumentspecifiesthearchitectureforMultiprotocolLabelSwitching(MPLS).

TableofContents

1Specification......................................32IntroductiontoMPLS...............................3

2.1Overview...........................................42.2Terminology........................................62.3AcronymsandAbbreviations.........................92.4Ac

nowledgments....................................93MPLSBasics........................................93.1Labels.............................................93.2UpstreamandDownstreamLSRs.......................103.3LabeledPac

et.....................................113.4LabelAssignmentandDistribution..................113.5AttributesofaLabelBinding......................113.6LabelDistributionProtocols.......................113.7UnsolicitedDownstreamvs.Downstream-on-Demand....123.8LabelRetentionMode...............................12

3.9TheLabelStac

....................................133.10TheNextHopLabelForwardingEntry(NHLFE)........133.11IncomingLabelMap(ILM)...........................14

Rosen,etal.StandardsTrac

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3.12FEC-to-NHLFEMap(FTN).............................143.13LabelSwapping.....................................153.14ScopeandUniquenessofLabels.....................153.15LabelSwitchedPath(LSP),LSPIngress,LSPEgress.163.16PenultimateHopPopping............................183.17LSPNextHop.......................................203.18InvalidIncomingLabels............................203.19LSPControl:OrderedversusIndependent............203.20Aggregation........................................213.21RouteSelection....................................233.22Lac

ofOutgoingLabel.............................243.23Time-to-Live(TTL).................................243.24LoopControl.......................................253.25LabelEncodings....................................263.25.1MPLS-specificHardwareand/orSoftware.............263.25.2ATMSwitchesasLSRs...............................263.25.3InteroperabilityamongEncodingTechniques.........283.26LabelMerging......................................283.26.1Non-mergingLSRs...................................293.26.2LabelsforMergingandNon-MergingLSRs............303.26.3MergeoverATM.....................................313.26.3.1MethodsofEliminatingCellInterleave.............313.26.3.2Interoperation:VCMerge,VPMerge,andNon-Merge..31

3.27TunnelsandHierarchy..............................323.27.1Hop-by-HopRoutedTunnel...........................323.27.2ExplicitlyRoutedTunnel...........................333.27.3LSPTunnels........................................333.27.4Hierarchy:LSPTunnelswithinLSPs.................333.27.5LabelDistributionPeeringandHierarchy...........343.28LabelDistributionProtocolTransport..............353.29WhyMorethanoneLabelDistributionProtocol?.....363.29.1BGPandLDP........................................363.29.2LabelsforRSVPFlowspecs..........................363.29.3LabelsforExplicitlyRoutedLSPs..................363.30Multicast..........................................374SomeApplicationsofMPLS..........................37

4.1MPLSandHopbyHopRoutedTraffic.................374.1.1LabelsforAddressPrefixes........................374.1.2DistributingLabelsforAddressPrefixes...........374.1.2.1LabelDistributionPeersforanAddressPrefix.....374.1.2.2DistributingLabels................................384.1.3UsingtheHopbyHoppathastheLSP...............394.1.4LSPEgressandLSPProxyEgress....................394.1.5TheImplicitNULLLabel............................404.1.6Option:Egress-TargetedLabelAssignment...........404.2MPLSandExplicitlyRoutedLSPs....................424.2.1ExplicitlyRoutedLSPTunnels......................424.3LabelStac

sandImplicitPeering..................43



4.4MPLSandMulti-PathRouting........................444.5LSPTreesasMultipoint-to-PointEntities..........444.6LSPTunnelingbetweenBGPBorderRouters...........45

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4.7OtherUsesofHop-by-HopRoutedLSPTunnels........474.8MPLSandMulticast.................................475LabelDistributionProcedures(Hop-by-Hop).........475.1TheProceduresforAdvertisingandUsinglabels....485.1.1DownstreamLSR:DistributionProcedure.............485.1.1.1PushUnconditional..................................495.1.1.2PushConditional....................................495.1.1.3PulledUnconditional................................495.1.1.4PulledConditional..................................505.1.2UpstreamLSR:RequestProcedure....................515.1.2.1RequestNever.......................................515.1.2.2RequestWhenNeeded..................................515.1.2.3RequestOnRequest...................................515.1.3UpstreamLSR:NotAvailableProcedure...............525.1.3.1RequestRetry.......................................525.1.3.2RequestNoRetry.....................................525.1.4UpstreamLSR:ReleaseProcedure....................525.1.4.1ReleaseOnChange....................................525.1.4.2NoReleaseOnChange..................................535.1.5UpstreamLSR:labelUseProcedure...................535.1.5.1UseImmediate.......................................535.1.5.2UseIfLoopNotDetected...............................535.1.6DownstreamLSR:WithdrawProcedure.................535.2MPLSSchemes:SupportedCombinationsofProcedures.54

5.2.1SchemesforLSRsthatSupportLabelMerging........555.2.2SchemesforLSRsthatdonotSupportLabelMerging.565.2.3InteroperabilityConsiderations....................576SecurityConsiderations............................587IntellectualProperty..............................588Authors'Addresses.................................599References.........................................5910FullCopyrightStatement...........................61

1.Specification

The eywords"MUST","MUSTNOT","REQUIRED","SHALL","SHALLNOT","SHOULD","SHOULDNOT","RECOMMENDED","MAY",and"OPTIONAL"inthis

documentaretobeinterpretedasdescribedinRFC2119.

2.IntroductiontoMPLS

ThisdocumentspecifiesthearchitectureforMultiprotocolLabelSwitching(MPLS).

NotethattheuseofMPLSformulticastisleftforfurtherstudy.


[Page3]

RFC3031MPLSArchitectureJanuary2001

2.1.Overview

Asapac etofaconnectionlessnetwor layerprotocoltravelsfromoneroutertothenext,eachrouterma

esanindependentforwardingdecisionforthatpac et.Thatis,eachrouteranalyzesthepac et'sheader,andeachrouterrunsanetwor layerroutingalgorithm.Eachrouterindependentlychoosesanexthopforthepac et,basedonits

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analysisofthepac et'sheaderandtheresultsofrunningtheroutingalgorithm.

Pac etheaderscontainconsiderablymoreinformationthanisneededsimplytochoosethenexthop.Choosingthenexthopcanthereforebethoughtofasthecompositionoftwofunctions.Thefirstfunctionpartitionstheentiresetofpossiblepac etsintoasetof"ForwardingEquivalenceClasses(FECs)".ThesecondmapseachFECtoanexthop.Insofarastheforwardingdecisionisconcerned,differentpac

etswhichgetmappedintothesameFECareindistinguishable.Allpac etswhichbelongtoaparticularFECandwhichtravelfromaparticularnodewillfollowthesamepath(orifcertain indsofmulti-pathroutingareinuse,theywillallfollowoneofasetofpathsassociatedwiththeFEC).

InconventionalIPforwarding,aparticularrouterwilltypicallyconsidertwopac etstobeinthesameFECifthereissomeaddressprefixXinthatrouter'sroutingtablessuchthatXisthe"longestmatch"foreachpac et'sdestinationaddress.Asthepac ettraversesthenetwor ,eachhopinturnreexaminesthepac etandassignsittoaFEC.

InMPLS,theassignmentofaparticularpac ettoaparticularFECisdonejustonce,asthepac etentersthenetwor .TheFECtowhich

thepac

etisassignedisencodedasashortfixedlengthvalue

nownasa"label".Whenapac etisforwardedtoitsnexthop,thelabelissentalongwithit;thatis,thepac etsare"labeled"beforetheyareforwarded.

Atsubsequenthops,thereisnofurtheranalysisofthepac et'snetwor layerheader.Rather,thelabelisusedasanindexintoatablewhichspecifiesthenexthop,andanewlabel.Theoldlabelisreplacedwiththenewlabel,andthepac etisforwardedtoitsnexthop.

IntheMPLSforwardingparadigm,onceapac etisassignedtoaFEC,nofurtherheaderanalysisisdonebysubsequentrouters;all

forwardingisdrivenbythelabels.Thishasanumberofadvantagesoverconventionalnetwor layerforwarding.



-MPLSforwardingcanbedonebyswitcheswhicharecapableof

doinglabelloo

upandreplacement,butareeithernotcapableofanalyzingthenetwor layerheaders,orarenotcapableofanalyzingthenetwor

layerheadersatadequatespeed.

-Sinceapac etisassignedtoaFECwhenitentersthenetwor ,theingressroutermayuse,indeterminingtheassignment,anyinformationithasaboutthepac

et,evenifthatinformationcannotbegleanedfromthenetwor layerheader.Forexample,pac etsarrivingondifferentportsmaybeassignedtodifferentFECs.Conventionalforwarding,ontheotherhand,

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canonlyconsiderinformationwhichtravelswiththepac etinthepac

etheader.

-Apac etthatentersthenetwor ataparticularroutercanbelabeleddifferentlythanthesamepac etenteringthenetworatadifferentrouter,andasaresultforwardingdecisionsthatdependontheingressroutercanbeeasilymade.Thiscannotbedonewithconventionalforwarding,sincetheidentityofapac et'singressrouterdoesnottravelwiththepac et.

-Theconsiderationsthatdeterminehowapac etisassignedtoaFECcanbecomeevermoreandmorecomplicated,withoutanyimpactatallontheroutersthatmerelyforwardlabeledpac

ets.

-Sometimesitisdesirabletoforceapac

ettofollowaparticularroutewhichisexplicitlychosenatorbeforethetimethepac etentersthenetwor ,ratherthanbeingchosenbythenormaldynamicroutingalgorithmasthepac ettravelsthroughthenetwor .Thismaybedoneasamatterofpolicy,ortosupporttrafficengineering.Inconventionalforwarding,thisrequiresthepac ettocarryanencodingofitsroutealongwithit("sourcerouting").InMPLS,alabelcanbeusedtorepresenttheroute,sothattheidentityoftheexplicit

routeneednotbecarriedwiththepac

et.

Someroutersanalyzeapac et'snetwor layerheadernotmerelytochoosethepac et'snexthop,butalsotodetermineapac et's"precedence"or"classofservice".Theymaythenapplydifferentdiscardthresholdsorschedulingdisciplinestodifferentpac ets.MPLSallows(butdoesnotrequire)theprecedenceorclassofservicetobefullyorpartiallyinferredfromthelabel.Inthiscase,onemaysaythatthelabelrepresentsthecombinationofaFECandaprecedenceorclassofservice.



MPLSstandsfor"Multiprotocol"LabelSwitching,multiprotocolbecauseitstechniquesareapplicabletoANYnetwor layerprotocol.Inthisdocument,however,wefocusontheuseofIPasthenetworlayerprotocol.

ArouterwhichsupportsMPLSis

nownasa"LabelSwitchingRouter",orLSR.

2.2.Terminology

Thissectiongivesageneralconceptualoverviewofthetermsusedinthisdocument.Someofthesetermsaremorepreciselydefinedinlatersectionsofthedocument.

DLCIalabelusedinFrameRelaynetwor sto

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identifyframerelaycircuits

forwardingequivalenceclassagroupofIPpac etswhichareforwardedinthesamemanner(e.g.,overthesamepath,withthesameforwardingtreatment)

framemergelabelmerging,whenitisappliedtooperationoverframebasedmedia,sothatthepotentialproblemofcellinterleaveisnotanissue.

labelashortfixedlengthphysicallycontiguousidentifierwhichisusedtoidentifyaFEC,usuallyoflocalsignificance.

labelmergingthereplacementofmultipleincominglabelsforaparticularFECwithasingleoutgoinglabel

labelswapthebasicforwardingoperationconsistingofloo ingupanincoming

labeltodeterminetheoutgoinglabel,encapsulation,port,andotherdatahandlinginformation.

labelswappingaforwardingparadigmallowingstreamlinedforwardingofdatabyusinglabelstoidentifyclassesofdatapac etswhicharetreatedindistinguishablywhenforwarding.

Rosen,etal.StandardsTrac [Page6]


labelswitchedhopthehopbetweentwoMPLSnodes,onwhichforwardingisdoneusinglabels.

labelswitchedpathThepaththroughoneormoreLSRsatonelevelofthehierarchyfollowedbyapac etsinaparticularFEC.

labelswitchingrouteranMPLSnodewhichiscapableofforwardingnativeL3pac ets

layer2theprotocollayerunderlayer3(whichthereforeofferstheservicesusedbylayer3).Forwarding,whendonebytheswappingofshortfixedlengthlabels,occursatlayer2regardlessofwhetherthelabelbeingexaminedisanATMVPI/VCI,aframerelayDLCI,oranMPLSlabel.

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layer3theprotocollayeratwhichIPanditsassociatedroutingprotocolsoperatelin layersynonymouswithlayer2

loopdetectionamethodofdealingwithloopsinwhichloopsareallowedtobesetup,anddatamaybetransmittedovertheloop,buttheloopislaterdetected

looppreventionamethodofdealingwithloopsinwhichdataisnevertransmittedoveraloop

labelstac anorderedsetoflabels

mergepointanodeatwhichlabelmergingisdone

MPLSdomainacontiguoussetofnodeswhichoperateMPLSroutingandforwardingandwhicharealsoinoneRoutingorAdministrativeDomain

MPLSedgenodeanMPLSnodethatconnectsanMPLSdomainwithanodewhichisoutsideofthedomain,eitherbecauseitdoesnot

runMPLS,and/orbecauseitisinadifferentdomain.NotethatifanLSRhasaneighboringhostwhichisnotrunningMPLS,thatthatLSRisanMPLSedgenode.

Rosen,etal.StandardsTrac [Page7]RFC3031MPLSArchitectureJanuary2001

MPLSegressnodeanMPLSedgenodeinitsroleinhandlingtrafficasitleavesanMPLSdomain

MPLSingressnodeanMPLSedgenodeinitsroleinhandlingtrafficasitentersanMPLSdomain

MPLSlabelalabelwhichiscarriedinapac etheader,andwhichrepresentsthepac et'sFEC

MPLSnodeanodewhichisrunningMPLS.AnMPLS

nodewillbeawareofMPLScontrolprotocols,willoperateoneormoreL3routingprotocols,andwillbecapableofforwardingpac etsbasedonlabels.AnMPLSnodemayoptionallybealsocapableofforwardingnativeL3pac ets.

MultiProtocolLabelSwitchinganIETFwor inggroupandtheeffortassociatedwiththewor inggroup

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networ

layersynonymouswithlayer3

stac synonymouswithlabelstac

switchedpathsynonymouswithlabelswitchedpath

virtualcircuitacircuitusedbyaconnection-orientedlayer2technologysuchasATMorFrameRelay,requiringthemaintenanceofstateinformationinlayer2switches.

VCmergelabelmergingwheretheMPLSlabeliscarriedintheATMVCIfield(orcombinedVPI/VCIfield),soastoallowmultipleVCstomergeintoonesingleVC

VPmergelabelmergingwheretheMPLSlabeliscarrieddintheATMVPIfield,soastoallowmultipleVPstobemergedintoonesingleVP.InthiscasetwocellswouldhavethesameVCIvalueonlyiftheyoriginatedfromthesamenode.Thisallowscellsfromdifferentsourcesto

bedistinguishedviatheVCI.


VPI/VCIalabelusedinATMnetwor stoidentifycircuits

2.3.AcronymsandAbbreviations

ATMAsynchronousTransferModeBGPBorderGatewayProtocolDLCIDataLin

CircuitIdentifierFECForwardingEquivalenceClassFTNFECtoNHLFEMapIGPInteriorGatewayProtocolILMIncomingLabelMapIPInternetProtocolLDPLabelDistributionProtocolL2Layer2L3Layer3LSPLabelSwitchedPathLSRLabelSwitchingRouter

MPLSMultiProtocolLabelSwitchingNHLFENextHopLabelForwardingEntrySVCSwitchedVirtualCircuitSVPSwitchedVirtualPathTTLTime-To-LiveVCVirtualCircuitVCIVirtualCircuitIdentifierVPVirtualPathVPIVirtualPathIdentifier

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2.4.Ac nowledgments

Theideasandtextinthisdocumenthavebeencollectedfromanumberofsourcesandcommentsreceived.Wewouldli etothan RicBoivie,PaulDoolan,NancyFeldman,Ya ovRe hter,VijaySrinivasan,andGeorgeSwallowfortheirinputsandideas.

3.MPLSBasics

Inthissection,weintroducesomeofthebasicconceptsofMPLSanddescribethegeneralapproachtobeused.

3.1.Labels

Alabelisashort,fixedlength,locallysignificantidentifierwhichisusedtoidentifyaFEC.Thelabelwhichisputonaparticularpac etrepresentstheForwardingEquivalenceClasstowhichthatpac etisassigned.



Mostcommonly,apac etisassignedtoaFECbased(completelyorpartially)onitsnetwor layerdestinationaddress.However,thelabelisneveranencodingofthataddress.

IfRuandRdareLSRs,theymayagreethatwhenRutransmitsapac ettoRd,Ruwilllabelwithpac etwithlabelvalueLifandonlyifthepac etisamemberofaparticularFECF.Thatis,theycanagreetoa"binding"betweenlabelLandFECFforpac etsmoving

fromRutoRd.Asaresultofsuchanagreement,LbecomesRu's"outgoinglabel"representingFECF,andLbecomesRd's"incominglabel"representingFECF.

NotethatLdoesnotnecessarilyrepresentFECFforanypac etsotherthanthosewhicharebeingsentfromRutoRd.LisanarbitraryvaluewhosebindingtoFislocaltoRuandRd.

Whenwespea aboveofpac ets"beingsent"fromRutoRd,wedonotimplyeitherthatthepac etoriginatedatRuorthatitsdestinationisRd.Rather,wemeantoincludepac etswhichare"transitpac

ets"atoneorbothoftheLSRs.

SometimesitmaybedifficultorevenimpossibleforRdtotell,ofanarrivingpac etcarryinglabelL,thatthelabelLwasplacedinthepac

etbyRu,ratherthanbysomeotherLSR.(ThiswilltypicallybethecasewhenRuandRdarenotdirectneighbors.)Insuchcases,Rdmustma esurethatthebindingfromlabeltoFECisone-to-one.Thatis,RdMUSTNOTagreewithRu1tobindLtoFECF1,whilealsoagreeingwithsomeotherLSRRu2tobindLtoadifferentFECF2,UNLESSRdcanalwaystell,whenitreceivesapac etwithincominglabelL,whetherthelabelwasputonthepac etbyRu1orwhetheritwasputonbyRu2.

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ItistheresponsibilityofeachLSRtoensurethatitcanuniquelyinterpretitsincominglabels.

3.2.UpstreamandDownstreamLSRs

SupposeRuandRdhaveagreedtobindlabelLtoFECF,forpac etssentfromRutoRd.Thenwithrespecttothisbinding,Ruisthe"upstreamLSR",andRdisthe"downstreamLSR".

TosaythatonenodeisupstreamandoneisdownstreamwithrespecttoagivenbindingmeansonlythataparticularlabelrepresentsaparticularFECinpac etstravellingfromtheupstreamnodetothedownstreamnode.ThisisNOTmeanttoimplythatpac

etsinthatFECwouldactuallyberoutedfromtheupstreamnodetothedownstreamnode.


3.3.LabeledPac et

A"labeledpac et"isapac etintowhichalabelhasbeenencoded.Insomecases,thelabelresidesinanencapsulationheaderwhichexistsspecificallyforthispurpose.Inothercases,thelabelmayresideinanexistingdatalin ornetwor layerheader,aslongasthereisafieldwhichisavailableforthatpurpose.Theparticularencodingtechniquetobeusedmustbeagreedtobyboththeentitywhichencodesthelabelandtheentitywhichdecodesthelabel.

3.4.LabelAssignmentandDistribution

IntheMPLSarchitecture,thedecisiontobindaparticularlabelLtoaparticularFECFismadebytheLSRwhichisDOWNSTREAMwithrespecttothatbinding.ThedownstreamLSRtheninformstheupstreamLSRofthebinding.Thuslabelsare"downstream-assigned",andlabelbindingsaredistributedinthe"downstreamtoupstream"direction.

IfanLSRhasbeendesignedsothatitcanonlyloo

uplabelsthatfallintoacertainnumericrange,thenitmerelyneedstoensurethatitonlybindslabelsthatareinthatrange.

3.5.AttributesofaLabelBinding

AparticularbindingoflabelLtoFECF,distributedbyRdtoRu,mayhaveassociated"attributes".IfRu,actingasadownstreamLSR,alsodistributesabindingofalabeltoFECF,thenundercertainconditions,itmayberequiredtoalsodistributethecorrespondingattributethatitreceivedfromRd.

3.6.LabelDistributionProtocols

AlabeldistributionprotocolisasetofproceduresbywhichoneLSRinformsanotherofthelabel/FECbindingsithasmade.TwoLSRs

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whichusealabeldistributionprotocoltoexchangelabel/FECbindinginformationare

nownas"labeldistributionpeers"withrespecttothebindinginformationtheyexchange.IftwoLSRsarelabeldistributionpeers,wewillspea oftherebeinga"labeldistributionadjacency"betweenthem.

(N.B.:twoLSRsmaybelabeldistributionpeerswithrespecttosomesetofbindings,butnotwithrespecttosomeothersetofbindings.)

Thelabeldistributionprotocolalsoencompassesanynegotiationsinwhichtwolabeldistributionpeersneedtoengageinordertolearnofeachother'sMPLScapabilities.


THEARCHITECTUREDOESNOTASSUMETHATTHEREISONLYASINGLELABELDISTRIBUTIONPROTOCOL.Infact,anumberofdifferentlabeldistributionprotocolsarebeingstandardized.Existingprotocols

havebeenextendedsothatlabeldistributioncanbepiggybac

edonthem(see,e.g.,[MPLS-BGP],[MPLS-RSVP-TUNNELS]).Newprotocolshavealsobeendefinedfortheexplicitpurposeofdistributinglabels(see,e.g.,[MPLS-LDP],[MPLS-CR-LDP].

Inthisdocument,wetrytousetheacronym"LDP"toreferspecificallytotheprotocoldefinedin[MPLS-LDP];whenspea ingoflabeldistributionprotocolsingeneral,wetrytoavoidtheacronym.

3.7.UnsolicitedDownstreamvs.Downstream-on-Demand

TheMPLSarchitectureallowsanLSRtoexplicitlyrequest,fromitsnexthopforaparticularFEC,alabelbindingforthatFEC.Thisis

nownas"downstream-on-demand"labeldistribution.

TheMPLSarchitecturealsoallowsanLSRtodistributebindingstoLSRsthathavenotexplicitlyrequestedthem.Thisis

nownas"unsoliciteddownstream"labeldistribution.

ItisexpectedthatsomeMPLSimplementationswillprovideonlydownstream-on-demandlabeldistribution,andsomewillprovideonlyunsoliciteddownstreamlabeldistribution,andsomewillprovideboth.Whichisprovidedmaydependonthecharacteristicsoftheinterfaceswhicharesupportedbyaparticularimplementation.However,bothoftheselabeldistributiontechniquesmaybeusedinthesamenetwor atthesametime.Onanygivenlabeldistribution

adjacency,theupstreamLSRandthedownstreamLSRmustagreeonwhichtechniqueistobeused.

3.8.LabelRetentionMode

AnLSRRumayreceive(orhavereceived)alabelbindingforaparticularFECfromanLSRRd,eventhoughRdisnotRu'snexthop(orisnolongerRu'snexthop)forthatFEC.

Ruthenhasthechoiceofwhetherto eeptrac ofsuchbindings,or

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whethertodiscardsuchbindings.IfRu eepstrac ofsuchbindings,thenitmayimmediatelybeginusingthebindingagainifRdeventuallybecomesitsnexthopfortheFECinquestion.IfRudiscardssuchbindings,thenifRdlaterbecomesthenexthop,thebindingwillhavetobereacquired.


IfanLSRsupports"LiberalLabelRetentionMode",itmaintainsthebindingsbetweenalabelandaFECwhicharereceivedfromLSRswhicharenotitsnexthopforthatFEC.IfanLSRsupports"ConservativeLabelRetentionMode",itdiscardssuchbindings.

Liberallabelretentionmodeallowsforquic eradaptationtoroutingchanges,butconservativelabelretentionmodethoughrequiresanLSR

tomaintainmanyfewerlabels.

3.9.TheLabelStac

Sofar,wehavespo enasifalabeledpac etcarriesonlyasinglelabel.Asweshallsee,itisusefultohaveamoregeneralmodelinwhichalabeledpac etcarriesanumberoflabels,organizedasalast-in,first-outstac .Werefertothisasa"labelstac ".

Although,asweshallsee,MPLSsupportsahierarchy,theprocessingofalabeledpac etiscompletelyindependentofthelevelofhierarchy.Theprocessingisalwaysbasedonthetoplabel,withoutregardforthepossibilitythatsomenumberofotherlabelsmayhave

been"aboveit"inthepast,orthatsomenumberofotherlabelsmaybebelowitatpresent.

Anunlabeledpac

etcanbethoughtofasapac

etwhoselabelstac

isempty(i.e.,whoselabelstac hasdepth0).

Ifapac et'slabelstac isofdepthm,werefertothelabelatthebottomofthestac

asthelevel1label,tothelabelaboveit(ifsuchexists)asthelevel2label,andtothelabelatthetopofthestac asthelevelmlabel.

Theutilityofthelabelstac

willbecomeclearwhenweintroducethenotionofLSPTunnelandtheMPLSHierarchy(section3.27).

3.10.TheNextHopLabelForwardingEntry(NHLFE)

The"NextHopLabelForwardingEntry"(NHLFE)isusedwhenforwardingalabeledpac et.Itcontainsthefollowinginformation:

1.thepac

et'snexthop

2.theoperationtoperformonthepac et'slabelstac ;thisisoneofthefollowingoperations:

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a)replacethelabelatthetopofthelabelstac

withaspecifiednewlabel

b)popthelabelstac


c)replacethelabelatthetopofthelabelstac

withaspecifiednewlabel,andthenpushoneormorespecifiednewlabelsontothelabelstac

.

Itmayalsocontain:

d)thedatalin encapsulationtousewhentransmittingthepac et

e)thewaytoencodethelabelstac whentransmittingthepac et

f)anyotherinformationneededinordertoproperlydisposeof

thepac

et.

NotethatatagivenLSR,thepac et's"nexthop"mightbethatLSRitself.Inthiscase,theLSRwouldneedtopopthetoplevellabel,andthen"forward"theresultingpac ettoitself.Itwouldthenma eanotherforwardingdecision,basedonwhatremainsafterthelabelstac edispopped.Thismaystillbealabeledpac et,oritmaybethenativeIPpac et.

ThisimpliesthatinsomecasestheLSRmayneedtooperateontheIPheaderinordertoforwardthepac et.

Ifthepac et's"nexthop"isthecurrentLSR,thenthelabelstac

operationMUSTbeto"popthestac

".

3.11.IncomingLabelMap(ILM)

The"IncomingLabelMap"(ILM)mapseachincominglabeltoasetofNHLFEs.Itisusedwhenforwardingpac etsthatarriveaslabeledpac ets.

IftheILMmapsaparticularlabeltoasetofNHLFEsthatcontainsmorethanoneelement,exactlyoneelementofthesetmustbechosenbeforethepac etisforwarded.Theproceduresforchoosinganelementfromthesetarebeyondthescopeofthisdocument.HavingtheILMmapalabeltoasetcontainingmorethanoneNHLFEmaybe

usefulif,e.g.,itisdesiredtodoloadbalancingovermultipleequal-costpaths.

3.12.FEC-to-NHLFEMap(FTN)

The"FEC-to-NHLFE"(FTN)mapseachFECtoasetofNHLFEs.Itisusedwhenforwardingpac

etsthatarriveunlabeled,butwhicharetobelabeledbeforebeingforwarded.

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IftheFTNmapsaparticularlabeltoasetofNHLFEsthatcontainsmorethanoneelement,exactlyoneelementofthesetmustbechosenbeforethepac

etisforwarded.Theproceduresforchoosinganelementfromthesetarebeyondthescopeofthisdocument.HavingtheFTNmapalabeltoasetcontainingmorethanoneNHLFEmaybeusefulif,e.g.,itisdesiredtodoloadbalancingovermultipleequal-costpaths.

3.13.LabelSwapping

Labelswappingistheuseofthefollowingprocedurestoforwardapac et.

Inordertoforwardalabeledpac et,aLSRexaminesthelabelatthetopofthelabelstac .ItusestheILMtomapthislabeltoan

NHLFE.UsingtheinformationintheNHLFE,itdetermineswheretoforwardthepac et,andperformsanoperationonthepac et'slabelstac .Itthenencodesthenewlabelstac intothepac et,andforwardstheresult.

Inordertoforwardanunlabeledpac et,aLSRanalyzesthenetworlayerheader,todeterminethepac et'sFEC.ItthenusestheFTNtomapthistoanNHLFE.UsingtheinformationintheNHLFE,itdetermineswheretoforwardthepac et,andperformsanoperationonthepac et'slabelstac .(Poppingthelabelstac would,ofcourse,beillegalinthiscase.)Itthenencodesthenewlabelstac intothepac et,andforwardstheresult.

ITISIMPORTANTTONOTETHATWHENLABELSWAPPINGISINUSE,THENEXTHOPISALWAYSTAKENFROMTHENHLFE;THISMAYINSOMECASESBEDIFFERENTFROMWHATTHENEXTHOPWOULDBEIFMPLSWERENOTINUSE.

3.14.ScopeandUniquenessofLabels

AgivenLSRRdmaybindlabelL1toFECF,anddistributethatbindingtolabeldistributionpeerRu1.RdmayalsobindlabelL2toFECF,anddistributethatbindingtolabeldistributionpeerRu2.WhetherornotL1==L2isnotdeterminedbythearchitecture;thisisalocalmatter.

AgivenLSRRdmaybindlabelLtoFECF1,anddistributethat

bindingtolabeldistributionpeerRu1.RdmayalsobindlabelLtoFECF2,anddistributethatbindingtolabeldistributionpeerRu2.IF(ANDONLYIF)RDCANTELL,WHENITRECEIVESAPACKETWHOSETOPLABELISL,WHETHERTHELABELWASPUTTHEREBYRU1ORBYRU2,THENTHEARCHITECTUREDOESNOTREQUIRETHATF1==F2.Insuchcases,wemaysaythatRdisusingadifferent"labelspace"forthelabelsitdistributestoRu1thanforthelabelsitdistributestoRu2.

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Ingeneral,RdcanonlytellwhetheritwasRu1orRu2thatputtheparticularlabelvalueLatthetopofthelabelstac ifthefollowingconditionshold:

-Ru1andRu2aretheonlylabeldistributionpeerstowhichRddistributedabindingoflabelvalueL,and

-Ru1andRu2areeachdirectlyconnectedtoRdviaapoint-to-pointinterface.

Whentheseconditionshold,anLSRmayuselabelsthathave"perinterface"scope,i.e.,whichareonlyuniqueperinterface.WemaysaythattheLSRisusinga"per-interfacelabelspace".Whentheseconditionsdonothold,thelabelsmustbeuniqueovertheLSRwhichhasassignedthem,andwemaysaythattheLSRisusinga"per-platformlabelspace."

IfaparticularLSRRdisattachedtoaparticularLSRRuovertwopoint-to-pointinterfaces,thenRdmaydistributetoRuabindingof

labelLtoFECF1,aswellasabindingoflabelLtoFECF2,F1!=F2,ifandonlyifeachbindingisvalidonlyforpac etswhichRusendstoRdoveraparticularoneoftheinterfaces.Inallothercases,RdMUSTNOTdistributetoRubindingsofthesamelabelvaluetotwodifferentFECs.

Thisprohibitionholdsevenifthebindingsareregardedasbeingatdifferent"levelsofhierarchy".InMPLS,thereisnonotionofhavingadifferentlabelspacefordifferentlevelsofthehierarchy;wheninterpretingalabel,thelevelofthelabelisirrelevant.

ThequestionarisesastowhetheritispossibleforanLSRtousemultipleper-platformlabelspaces,ortousemultipleper-interface

labelspacesforthesameinterface.Thisisnotprohibitedbythearchitecture.However,insuchcasestheLSRmusthavesomemeans,notspecifiedbythearchitecture,ofdetermining,foraparticularincominglabel,whichlabelspacethatlabelbelongsto.Forexample,[MPLS-SHIM]specifiesthatadifferentlabelspaceisusedforunicastpac etsthanformulticastpac ets,andusesadatalinlayercodepointtodistinguishthetwolabelspaces.

3.15.LabelSwitchedPath(LSP),LSPIngress,LSPEgress

A"LabelSwitchedPath(LSP)oflevelm"foraparticularpac etPisasequenceofrouters,

withthefollowingproperties:



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1.R1,the"LSPIngress",isanLSRwhichpushesalabelontoP'slabelstac ,resultinginalabelstac ofdepthm;

2.Foralli,10).

Inotherwords,wecanspea ofthelevelmLSPforPac etPasthesequenceofrouters:

1.whichbeginswithanLSR(an"LSPIngress")thatpushesonalevelmlabel,

2.allofwhoseintermediateLSRsma etheirforwardingdecisionbylabelSwitchingonalevelmlabel,

3.whichends(atan"LSPEgress")whenaforwardingdecisionismadebylabelSwitchingonalevelm- label,where >0,orwhenaforwardingdecisionismadeby"ordinary",non-MPLSforwardingprocedures.

Aconsequence(orperhapsapresupposition)ofthisisthatwheneveranLSRpushesalabelontoanalreadylabeledpac et,itneedstoma

esurethatthenewlabelcorrespondstoaFECwhoseLSPEgressistheLSRthatassignedthelabelwhichisnowsecondinthestac .



WewillcallasequenceofLSRsthe"LSPforaparticularFECF"ifitisanLSPoflevelmforaparticularpac etPwhenP'slevelmlabelisalabelcorrespondingtoFECF.

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ConsiderthesetofnodeswhichmaybeLSPingressnodesforFECF.ThenthereisanLSPforFECFwhichbeginswitheachofthosenodes.IfanumberofthoseLSPshavethesameLSPegress,thenonecanconsiderthesetofsuchLSPstobeatree,whoserootistheLSPegress.(Sincedatatravelsalongthistreetowardstheroot,thismaybecalledamultipoint-to-pointtree.)Wecanthusspea ofthe"LSPtree"foraparticularFECF.

3.16.PenultimateHopPopping

Notethataccordingtothedefinitionsofsection3.15,ifisalevelmLSPforpac etP,PmaybetransmittedfromR[n-1]toRnwithalabelstac

ofdepthm-1.Thatis,thelabelstac

maybepoppedatthepenultimateLSRoftheLSP,ratherthanattheLSPEgress.

Fromanarchitecturalperspective,thisisperfectlyappropriate.Thepurposeofthelevelmlabelistogetthepac ettoRn.OnceR[n-1]hasdecidedtosendthepac ettoRn,thelabelnolongerhasanyfunction,andneednolongerbecarried.

Thereisalsoapracticaladvantagetodoingpenultimatehoppopping.Ifonedoesnotdothis,thenwhentheLSPegressreceivesapac et,

itfirstloo

supthetoplabel,anddeterminesasaresultofthatloo upthatitisindeedtheLSPegress.Thenitmustpopthestac ,andexaminewhatremainsofthepac et.Ifthereisanotherlabelonthestac ,theegresswillloo thisupandforwardthepac etbasedonthisloo up.(Inthiscase,theegressforthepac et'slevelmLSPisalsoanintermediatenodeforitslevelm-1LSP.)Ifthereisnootherlabelonthestac ,thenthepac etisforwardedaccordingtoitsnetwor layerdestinationaddress.NotethatthiswouldrequiretheegresstodoTWOloo ups,eithertwolabelloo upsoralabelloo upfollowedbyanaddressloo up.

If,ontheotherhand,penultimatehoppoppingisused,thenwhenthepenultimatehoploo supthelabel,itdetermines:

-thatitisthepenultimatehop,and

-whothenexthopis.

Thepenultimatenodethenpopsthestac ,andforwardsthepac etbasedontheinformationgainedbyloo ingupthelabelthatwaspreviouslyatthetopofthestac

.WhentheLSPegressreceivesthe


[Page18]


pac et,thelabelwhichisnowatthetopofthestac willbethelabelwhichitneedstoloo upinordertoma eitsownforwardingdecision.Or,ifthepac etwasonlycarryingasinglelabel,theLSPegresswillsimplyseethenetwor

layerpac

et,whichisjustwhatitneedstoseeinordertoma eitsforwardingdecision.

Thistechniqueallowstheegresstodoasingleloo up,andalso

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requiresonlyasingleloo upbythepenultimatenode.

Thecreationoftheforwarding"fastpath"inalabelswitchingproductmaybegreatlyaidedifitis nownthatonlyasingleloo upiseverrequired:

-thecodemaybesimplifiedifitcanassumethatonlyasingleloo

upiseverneeded

-thecodecanbebasedona"timebudget"thatassumesthatonlyasingleloo upiseverneeded.

Infact,whenpenultimatehoppoppingisdone,theLSPEgressneednotevenbeanLSR.

However,somehardwareswitchingenginesmaynotbeabletopopthelabelstac ,sothiscannotbeuniversallyrequired.Theremayalsobesomesituationsinwhichpenultimatehoppoppingisnotdesirable.Thereforethepenultimatenodepopsthelabelstac onlyifthisisspecificallyrequestedbytheegressnode,ORifthenextnodeintheLSPdoesnotsupportMPLS.(IfthenextnodeintheLSPdoessupportMPLS,butdoesnotma esucharequest,thepenultimatenodehasnowayof nowingthatitinfactisthepenultimatenode.)

AnLSRwhichiscapableofpoppingthelabelstac

atallMUSTdopenultimatehoppoppingwhensorequestedbyitsdownstreamlabeldistributionpeer.

InitiallabeldistributionprotocolnegotiationsMUSTalloweachLSRtodeterminewhetheritsneighboringLSRSarecapableofpoppingthelabelstac .ALSRMUSTNOTrequestalabeldistributionpeertopopthelabelstac unlessitiscapableofdoingso.

Itmaybeas edwhethertheegressnodecanalwaysinterpretthetoplabelofareceivedpac etproperlyifpenultimatehoppoppingisused.Aslongastheuniquenessandscopingrulesofsection3.14areobeyed,itisalwayspossibletointerpretthetoplabelofa

receivedpac

etunambiguously.



3.17.LSPNextHop

TheLSPNextHopforaparticularlabeledpac etinaparticularLSRistheLSRwhichisthenexthop,asselectedbytheNHLFEentryusedforforwardingthatpac et.

TheLSPNextHopforaparticularFECisthenexthopasselectedbytheNHLFEentryindexedbyalabelwhichcorrespondstothatFEC.

NotethattheLSPNextHopmaydifferfromthenexthopwhichwouldbechosenbythenetwor layerroutingalgorithm.Wewillusethe

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term"L3nexthop"whenwerefertothelatter.

3.18.InvalidIncomingLabels

WhatshouldanLSRdoifitreceivesalabeledpac etwithaparticularincominglabel,buthasnobindingforthatlabel?Itistemptingtothin thatthelabelscanjustberemoved,andthepac etforwardedasanunlabeledIPpac

et.However,insomecases,doingsocouldcausealoop.IftheupstreamLSRthin sthelabelisboundtoanexplicitroute,andthedownstreamLSRdoesn'tthin

thelabelisboundtoanything,andifthehopbyhoproutingoftheunlabeledIPpac

etbringsthepac

etbac

totheupstreamLSR,thenaloopisformed.

ItisalsopossiblethatthelabelwasintendedtorepresentaroutewhichcannotbeinferredfromtheIPheader.

Therefore,whenalabeledpac etisreceivedwithaninvalidincominglabel,itMUSTbediscarded,UNLESSitisdeterminedbysomemeans(notwithinthescopeofthecurrentdocument)thatforwardingitunlabeledcannotcauseanyharm.

3.19.LSPControl:OrderedversusIndependent

SomeFECscorrespondtoaddressprefixeswhicharedistributedviaadynamicroutingalgorithm.ThesetupoftheLSPsfortheseFECscanbedoneinoneoftwoways:IndependentLSPControlorOrderedLSPControl.

InIndependentLSPControl,eachLSR,uponnotingthatitrecognizesaparticularFEC,ma esanindependentdecisiontobindalabeltothatFECandtodistributethatbindingtoitslabeldistributionpeers.ThiscorrespondstothewaythatconventionalIPdatagramroutingwor s;eachnodema esanindependentdecisionastohowtotreateachpac et,andreliesontheroutingalgorithmtoconvergerapidlysoastoensurethateachdatagramiscorrectlydelivered.



InOrderedLSPControl,anLSRonlybindsalabeltoaparticularFECifitistheegressLSRforthatFEC,orifithasalreadyreceivedalabelbindingforthatFECfromitsnexthopforthatFEC.

IfonewantstoensurethattrafficinaparticularFECfollowsa

pathwithsomespecifiedsetofproperties(e.g.,thatthetrafficdoesnottraverseanynodetwice,thataspecifiedamountofresourcesareavailabletothetraffic,thatthetrafficfollowsanexplicitlyspecifiedpath,etc.)orderedcontrolmustbeused.Withindependentcontrol,someLSRsmaybeginlabelswitchingatrafficintheFECbeforetheLSPiscompletelysetup,andthussometrafficintheFECmayfollowapathwhichdoesnothavethespecifiedsetofproperties.OrderedcontrolalsoneedstobeusediftherecognitionoftheFECisaconsequenceofthesettingupofthecorrespondingLSP.

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OrderedLSPsetupmaybeinitiatedeitherbytheingressortheegress.

Orderedcontrolandindependentcontrolarefullyinteroperable.However,unlessallLSRsinanLSPareusingorderedcontrol,theoveralleffectonnetwor behaviorislargelythatofindependentcontrol,sinceonecannotbesurethatanLSPisnotuseduntilitisfullysetup.

Thisarchitectureallowsthechoicebetweenindependentcontrolandorderedcontroltobealocalmatter.Sincethetwomethodsinterwor ,agivenLSRneedsupportonlyoneortheother.Generallyspea

ing,thechoiceofindependentversusorderedcontroldoesnotappeartohaveanyeffectonthelabeldistributionmechanismswhichneedtobedefined.

3.20.Aggregation

OnewayofpartitioningtrafficintoFECsistocreateaseparateFECforeachaddressprefixwhichappearsintheroutingtable.However,withinaparticularMPLSdomain,thismayresultinasetofFECssuchthatalltrafficinallthoseFECsfollowsthesameroute.Forexample,asetofdistinctaddressprefixesmightallhavethesame

egressnode,andlabelswappingmightbeusedonlytogetthethetraffictotheegressnode.Inthiscase,withintheMPLSdomain,theunionofthoseFECsisitselfaFEC.Thiscreatesachoice:shouldadistinctlabelbeboundtoeachcomponentFEC,orshouldasinglelabelbeboundtotheunion,andthatlabelappliedtoalltrafficintheunion?

TheprocedureofbindingasinglelabeltoaunionofFECswhichisitselfaFEC(withinsomedomain),andofapplyingthatlabeltoall



trafficintheunion,is nownas"aggregation".TheMPLSarchitectureallowsaggregation.Aggregationmayreducethenumberoflabelswhichareneededtohandleaparticularsetofpac ets,andmayalsoreducetheamountoflabeldistributioncontroltrafficneeded.

GivenasetofFECswhichare"aggregatable"intoasingleFEC,itispossibleto(a)aggregatethemintoasingleFEC,(b)aggregatethemintoasetofFECs,or(c)notaggregatethematall.Thuswecan

spea

ofthe"granularity"ofaggregation,with(a)beingthe"coarsestgranularity",and(c)beingthe"finestgranularity".

Whenordercontrolisused,eachLSRshouldadopt,foragivensetofFECs,thegranularityusedbyitsnexthopforthoseFECs.

Whenindependentcontrolisused,itispossiblethattherewillbetwoadjacentLSRs,RuandRd,whichaggregatesomesetofFECsdifferently.

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IfRuhasfinergranularitythanRd,thisdoesnotcauseaproblem.RudistributesmorelabelsforthatsetofFECsthanRddoes.ThismeansthatwhenRuneedstoforwardlabeledpac etsinthoseFECstoRd,itmayneedtomapnlabelsintomlabels,wheren>m.Asanoption,Rumaywithdrawthesetofnlabelsthatithasdistributed,andthendistributeasetofmlabels,correspondingtoRd'slevelofgranularity.Thisisnotnecessarytoensurecorrectoperation,butitdoesresultinareductionofthenumberoflabelsdistributedbyRu,andRuisnotgaininganyparticularadvantagebydistributingthelargernumberoflabels.Thedecisionwhethertodothisornotisalocalmatter.

IfRuhascoarsergranularitythanRd(i.e.,RdhasdistributednlabelsforthesetofFECs,whileRuhasdistributedm,wheren>m),ithastwochoices:

-ItmayadoptRd'sfinerlevelofgranularity.Thiswouldrequireittowithdrawthemlabelsithasdistributed,anddistributenlabels.Thisisthepreferredoption.

-ItmaysimplymapitsmlabelsintoasubsetofRd'snlabels,ifitcandeterminethatthiswillproducethesamerouting.Forexample,supposethatRuappliesasinglelabeltoalltrafficthatneedstopassthroughacertainegressLSR,

whereasRdbindsanumberofdifferentlabelstosuchtraffic,dependingontheindividualdestinationaddressesofthepac ets.IfRu nowstheaddressoftheegressrouter,andifRdhasboundalabeltotheFECwhichisidentifiedbythataddress,thenRucansimplyapplythatlabel.


Inanyevent,everyLSRneedsto

now(byconfiguration)whatgranularitytouseforlabelsthatitassigns.Whereorderedcontrolisused,thisrequireseachnodeto nowthegranularityonlyforFECswhichleavetheMPLSnetwor

atthatnode.Forindependentcontrol,bestresultsmaybeobtainedbyensuringthatallLSRsareconsistentlyconfiguredto nowthegranularityforeachFEC.However,inmanycasesthismaybedonebyusingasinglelevelofgranularitywhichappliestoallFECs(suchas"onelabelperIPprefixintheforwardingtable",or"onelabelperegressnode").

3.21.RouteSelection

RouteselectionreferstothemethodusedforselectingtheLSPfora

particularFEC.TheproposedMPLSprotocolarchitecturesupportstwooptionsforRouteSelection:(1)hopbyhoprouting,and(2)explicitrouting.

HopbyhoproutingallowseachnodetoindependentlychoosethenexthopforeachFEC.ThisistheusualmodetodayinexistingIPnetwor

s.A"hopbyhoproutedLSP"isanLSPwhoserouteisselectedusinghopbyhoprouting.

InanexplicitlyroutedLSP,eachLSRdoesnotindependentlychoose

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thenexthop;rather,asingleLSR,generallytheLSPingressortheLSPegress,specifiesseveral(orall)oftheLSRsintheLSP.IfasingleLSRspecifiestheentireLSP,theLSPis"strictly"explicitlyrouted.IfasingleLSRspecifiesonlysomeoftheLSP,theLSPis"loosely"explicitlyrouted.

ThesequenceofLSRsfollowedbyanexplicitlyroutedLSPmaybechosenbyconfiguration,ormaybeselecteddynamicallybyasinglenode(forexample,theegressnodemayma euseofthetopologicalinformationlearnedfromalin

statedatabaseinordertocomputetheentirepathforthetreeendingatthategressnode).

Explicitroutingmaybeusefulforanumberofpurposes,suchaspolicyroutingortrafficengineering.InMPLS,theexplicitrouteneedstobespecifiedatthetimethatlabelsareassigned,buttheexplicitroutedoesnothavetobespecifiedwitheachIPpac

et.Thisma esMPLSexplicitroutingmuchmoreefficientthanthealternativeofIPsourcerouting.

Theproceduresforma inguseofexplicitroutes,eitherstrictorloose,arebeyondthescopeofthisdocument.


3.22.Lac ofOutgoingLabel

Whenalabeledpac etistravelingalonganLSP,itmayoccasionallyhappenthatitreachesanLSRatwhichtheILMdoesnotmapthe

pac

et'sincominglabelintoanNHLFE,eventhoughtheincominglabelisitselfvalid.Thiscanhappenduetotransientconditions,orduetoanerrorattheLSRwhichshouldbethepac et'snexthop.

Itistemptinginsuchcasestostripoffthelabelstac andattempttoforwardthepac etfurtherviaconventionalforwarding,basedonitsnetwor layerheader.However,ingeneralthisisnotasafeprocedure:

-Ifthepac ethasbeenfollowinganexplicitlyroutedLSP,thiscouldresultinaloop.

-Thepac et'snetwor headermaynotcontainenoughinformation

toenablethisparticularLSRtoforwarditcorrectly.

Unlessitcanbedetermined(throughsomemeansoutsidethescopeofthisdocument)thatneitherofthesesituationsobtains,theonlysafeprocedureistodiscardthepac et.

3.23.Time-to-Live(TTL)

InconventionalIPforwarding,eachpac etcarriesa"TimeToLive"(TTL)valueinitsheader.Wheneverapac etpassesthrougha

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router,itsTTLgetsdecrementedby1;iftheTTLreaches0beforethepac

ethasreacheditsdestination,thepac

etgetsdiscarded.

Thisprovidessomelevelofprotectionagainstforwardingloopsthatmayexistduetomisconfigurations,orduetofailureorslowconvergenceoftheroutingalgorithm.TTLissometimesusedforotherfunctionsaswell,suchasmulticastscoping,andsupportingthe"traceroute"command.ThisimpliesthattherearetwoTTL-relatedissuesthatMPLSneedstodealwith:(i)TTLasawaytosuppressloops;(ii)TTLasawaytoaccomplishotherfunctions,suchaslimitingthescopeofapac et.

Whenapac ettravelsalonganLSP,itSHOULDemergewiththesameTTLvaluethatitwouldhavehadifithadtraversedthesamesequenceofrouterswithouthavingbeenlabelswitched.Ifthepac

ettravelsalongahierarchyofLSPs,thetotalnumberofLSR-hopstraversedSHOULDbereflectedinitsTTLvaluewhenitemergesfromthehierarchyofLSPs.



ThewaythatTTLishandledmayvarydependinguponwhethertheMPLSlabelvaluesarecarriedinanMPLS-specific"shim"header[MPLS-SHIM],oriftheMPLSlabelsarecarriedinanL2header,suchasanATMheader[MPLS-ATM]oraframerelayheader[MPLS-FRMRLY].

Ifthelabelvaluesareencodedina"shim"thatsitsbetweenthedatalin andnetwor layerheaders,thenthisshimMUSThaveaTTLfieldthatSHOULDbeinitiallyloadedfromthenetwor layerheader

TTLfield,SHOULDbedecrementedateachLSR-hop,andSHOULDbecopiedintothenetwor layerheaderTTLfieldwhenthepac etemergesfromitsLSP.

Ifthelabelvaluesareencodedinadatalin layerheader(e.g.,theVPI/VCIfieldinATM'sAAL5header),andthelabeledpac etsareforwardedbyanL2switch(e.g.,anATMswitch),andthedatalinlayer(li

eATM)doesnotitselfhaveaTTLfield,thenitwillnotbepossibletodecrementapac et'sTTLateachLSR-hop.AnLSPsegmentwhichconsistsofasequenceofLSRsthatcannotdecrementapac et'sTTLwillbecalleda"non-TTLLSPsegment".

Whenapac etemergesfromanon-TTLLSPsegment,itSHOULDhowever

begivenaTTLthatreflectsthenumberofLSR-hopsittraversed.Intheunicastcase,thiscanbeachievedbypropagatingameaningfulLSPlengthtoingressnodes,enablingtheingresstodecrementtheTTLvaluebeforeforwardingpac etsintoanon-TTLLSPsegment.

Sometimesitcanbedetermined,uponingresstoanon-TTLLSPsegment,thataparticularpac

et'sTTLwillexpirebeforethepac

etreachestheegressofthatnon-TTLLSPsegment.Inthiscase,theLSRattheingresstothenon-TTLLSPsegmentmustnotlabelswitchthepac et.Thismeansthatspecialproceduresmustbedevelopedto

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supporttraceroutefunctionality,forexample,traceroutepac etsmaybeforwardedusingconventionalhopbyhopforwarding.

3.24.LoopControl

Onanon-TTLLSPsegment,bydefinition,TTLcannotbeusedtoprotectagainstforwardingloops.TheimportanceofloopcontrolmaydependontheparticularhardwarebeingusedtoprovidetheLSRfunctionsalongthenon-TTLLSPsegment.

Suppose,forinstance,thatATMswitchinghardwareisbeingusedtoprovideMPLSswitchingfunctions,withthelabelbeingcarriedintheVPI/VCIfield.SinceATMswitchinghardwarecannotdecrementTTL,thereisnoprotectionagainstloops.IftheATMhardwareiscapableofprovidingfairaccesstothebufferpoolforincomingcellscarryingdifferentVPI/VCIvalues,thisloopingmaynothaveanydeleteriouseffectonothertraffic.IftheATMhardwarecannot


providefairbufferaccessofthissort,however,theneventransientloopsmaycauseseveredegradationoftheLSR'stotalperformance.

Eveniffairbufferaccesscanbeprovided,itisstillworthwhiletohavesomemeansofdetectingloopsthatlast"longerthanpossible".Inaddition,evenwhereTTLand/orper-VCfairqueuingprovidesameansforsurvivingloops,itstillmaybedesirablewherepracticaltoavoidsettingupLSPswhichloop.AllLSRsthatmayattachtonon-TTLLSPsegmentswillthereforeberequiredtosupportacommontechniqueforloopdetection;however,useoftheloopdetectiontechniqueisoptional.Theloopdetectiontechniqueisspecifiedin[MPLS-ATM]and[MPLS-LDP].

3.25.LabelEncodings

Inordertotransmitalabelstac

alongwiththepac

etwhoselabelstac itis,itisnecessarytodefineaconcreteencodingofthelabelstac .Thearchitecturesupportsseveraldifferentencodingtechniques;thechoiceofencodingtechniquedependsontheparticular

indofdevicebeingusedtoforwardlabeledpac

ets.

3.25.1.MPLS-specificHardwareand/orSoftware

IfoneisusingMPLS-specifichardwareand/orsoftwaretoforwardlabeledpac ets,themostobviouswaytoencodethelabelstac isto

defineanewprotocoltobeusedasa"shim"betweenthedatalin

layerandnetwor layerheaders.Thisshimwouldreallybejustanencapsulationofthenetwor

layerpac

et;itwouldbe"protocol-independent"suchthatitcouldbeusedtoencapsulateanynetworlayer.Hencewewillrefertoitasthe"genericMPLSencapsulation".

ThegenericMPLSencapsulationwouldinturnbeencapsulatedinadatalin layerprotocol.

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TheMPLSgenericencapsulationisspecifiedin[MPLS-SHIM].

3.25.2.ATMSwitchesasLSRs

ItwillbenotedthatMPLSforwardingproceduresaresimilartothoseoflegacy"labelswapping"switchessuchasATMswitches.ATMswitchesusetheinputportandtheincomingVPI/VCIvalueastheindexintoa"cross-connect"table,fromwhichtheyobtainanoutputportandanoutgoingVPI/VCIvalue.Thereforeifoneormorelabelscanbeencodeddirectlyintothefieldswhichareaccessedbytheselegacyswitches,thenthelegacyswitchescan,withsuitablesoftwareupgrades,beusedasLSRs.Wewillrefertosuchdevicesas"ATM-LSRs".


TherearethreeobviouswaystoencodelabelsintheATMcellheader(presumingtheuseofAAL5):

1.SVCEncoding

UsetheVPI/VCIfieldtoencodethelabelwhichisatthetopofthelabelstac .Thistechniquecanbeusedinanynetwor .Withthisencodingtechnique,eachLSPisrealizedasanATMSVC,andthelabeldistributionprotocolbecomestheATM"signaling"protocol.Withthisencodingtechnique,theATM-LSRscannotperform"push"or"pop"operationsonthelabelstac .

2.SVPEncoding

UsetheVPIfieldtoencodethelabelwhichisatthetopof

thelabelstac

,andtheVCIfieldtoencodethesecondlabelonthestac ,ifoneispresent.Thistechniquesomeadvantagesoverthepreviousone,inthatitpermitstheuseofATM"VP-switching".Thatis,theLSPsarerealizedasATMSVPs,withthelabeldistributionprotocolservingastheATMsignalingprotocol.

However,thistechniquecannotalwaysbeused.Ifthenetwor

includesanATMVirtualPaththroughanon-MPLSATMnetwor ,thentheVPIfieldisnotnecessarilyavailableforusebyMPLS.

Whenthisencodingtechniqueisused,theATM-LSRattheegress

oftheVPeffectivelydoesa"pop"operation.

3.SVPMultipointEncoding

UsetheVPIfieldtoencodethelabelwhichisatthetopofthelabelstac ,usepartoftheVCIfieldtoencodethesecondlabelonthestac

,ifoneispresent,andusetheremainderoftheVCIfieldtoidentifytheLSPingress.Ifthistechniqueisused,conventionalATMVP-switchingcapabilitiescanbeusedtoprovidemultipoint-to-pointVPs.Cellsfromdifferent

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pac etswillthencarrydifferentVCIvalues.Asweshallseeinsection3.26,thisenablesustodolabelmerging,withoutrunningintoanycellinterleavingproblems,onATMswitcheswhichcanprovidemultipoint-to-pointVPs,butwhichdonothavetheVCmergecapability.

Thistechniquedependsontheexistenceofacapabilityforassigning16-bitVCIvaluestoeachATMswitchsuchthatnosingleVCIvalueisassignedtotwodifferentswitches.(Ifan


adequatenumberofsuchvaluescouldbeassignedtoeachswitch,itwouldbepossibletoalsotreattheVCIvalueasthesecondlabelinthestac .)

Iftherearemorelabelsonthestac thancanbeencodedintheATMheader,theATMencodingsmustbecombinedwiththegenericencapsulation.

3.25.3.InteroperabilityamongEncodingTechniques

IfisasegmentofaLSP,itispossiblethatR1willuseoneencodingofthelabelstac whentransmittingpac etPtoR2,butR2willuseadifferentencodingwhentransmittingapac etPtoR3.Ingeneral,theMPLSarchitecturesupportsLSPswithdifferentlabelstac encodingsusedondifferenthops.Therefore,whenwediscusstheproceduresforprocessingalabeledpac et,wespea inabstracttermsofoperatingonthepac et'slabelstac .Whenalabeledpac etisreceived,theLSRmustdecodeittodeterminethecurrentvalueofthelabelstac ,thenmustoperateonthelabelstac todeterminethenewvalueofthestac ,andthenencodethe

newvalueappropriatelybeforetransmittingthelabeledpac

ettoitsnexthop.

Unfortunately,ATMswitcheshavenocapabilityfortranslatingfromoneencodingtechniquetoanother.TheMPLSarchitecturethereforerequiresthatwheneveritispossiblefortwoATMswitchestobesuccessiveLSRsalongalevelmLSPforsomepac et,thatthosetwoATMswitchesusethesameencodingtechnique.

NaturallytherewillbeMPLSnetwor swhichcontainacombinationofATMswitchesoperatingasLSRs,andotherLSRswhichoperateusinganMPLSshimheader.Insuchnetwor

stheremaybesomeLSRswhichhaveATMinterfacesaswellas"MPLSShim"interfaces.Thisisone

exampleofanLSRwithdifferentlabelstac

encodingsondifferenthops.SuchanLSRmayswapoffanATMencodedlabelstac onanincominginterfaceandreplaceitwithanMPLSshimheaderencodedlabelstac ontheoutgoinginterface.

3.26.LabelMerging

SupposethatanLSRhasboundmultipleincominglabelstoaparticularFEC.Whenforwardingpac etsinthatFEC,onewouldli etohaveasingleoutgoinglabelwhichisappliedtoallsuchpac ets.

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Thefactthattwodifferentpac etsintheFECarrivedwithdifferentincominglabelsisirrelevant;onewouldli

etoforwardthemwiththesameoutgoinglabel.Thecapabilitytodosois nownas"labelmerging".


LetussaythatanLSRiscapableoflabelmergingifitcanreceivetwopac etsfromdifferentincominginterfaces,and/orwithdifferentlabels,andsendbothpac

etsoutthesameoutgoinginterfacewiththesamelabel.Oncethepac etsaretransmitted,theinformationthattheyarrivedfromdifferentinterfacesand/orwithdifferentincominglabelsislost.

LetussaythatanLSRisnotcapableoflabelmergingif,foranytwopac etswhicharrivefromdifferentinterfaces,orwithdifferentlabels,thepac etsmusteitherbetransmittedoutdifferentinterfaces,ormusthavedifferentlabels.ATM-LSRsusingtheSVCor

SVPEncodingscannotperformlabelmerging.Thisisdiscussedinmoredetailinthenextsection.

IfaparticularLSRcannotperformlabelmerging,theniftwopac etsinthesameFECarrivewithdifferentincominglabels,theymustbeforwardedwithdifferentoutgoinglabels.Withlabelmerging,thenumberofoutgoinglabelsperFECneedonlybe1;withoutlabelmerging,thenumberofoutgoinglabelsperFECcouldbeaslargeasthenumberofnodesinthenetwor .

Withlabelmerging,thenumberofincominglabelsperFECthataparticularLSRneedsisneverbelargerthanthenumberoflabeldistributionadjacencies.Withoutlabelmerging,thenumberof

incominglabelsperFECthataparticularLSRneedsisaslargeasthenumberofupstreamnodeswhichforwardtrafficintheFECtotheLSRinquestion.Infact,itisdifficultforanLSRtoevendeterminehowmanysuchincominglabelsitmustsupportforaparticularFEC.

TheMPLSarchitectureaccommodatesbothmergingandnon-mergingLSRs,butallowsforthefactthattheremaybeLSRswhichdonotsupportlabelmerging.ThisleadstotheissueofensuringcorrectinteroperationbetweenmergingLSRsandnon-mergingLSRs.TheissueissomewhatdifferentinthecaseofdatagrammediaversusthecaseofATM.Thedifferentmediatypeswillthereforebediscussedseparately.

3.26.1.Non-mergingLSRs

TheMPLSforwardingproceduresisverysimilartotheforwardingproceduresusedbysuchtechnologiesasATMandFrameRelay.Thatis,aunitofdataarrives,alabel(VPI/VCIorDLCI)isloo edupina"cross-connecttable",onthebasisofthatloo

upanoutputportischosen,andthelabelvalueisrewritten.Infact,itispossibletousesuchtechnologiesforMPLSforwarding;alabeldistributionprotocolcanbeusedasthe"signallingprotocol"forsettingupthe

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cross-connecttables.


Unfortunately,thesetechnologiesdonotnecessarilysupportthelabelmergingcapability.InATM,ifoneattemptstoperformlabelmerging,theresultmaybetheinterleavingofcellsfromvariouspac ets.Ifcellsfromdifferentpac etsgetinterleaved,itisimpossibletoreassemblethepac

ets.SomeFrameRelayswitchesusecellswitchingontheirbac planes.Theseswitchesmayalsobeincapableofsupportinglabelmerging,forthesamereason--cellsofdifferentpac etsmaygetinterleaved,andthereisthennowaytoreassemblethepac ets.

Weproposetosupporttwosolutionstothisproblem.First,MPLSwillcontainprocedureswhichallowtheuseofnon-mergingLSRs.Second,MPLSwillsupportprocedureswhichallowcertainATMswitchestofunctionasmergingLSRs.

SinceMPLSsupportsbothmergingandnon-mergingLSRs,MPLSalsocontainsprocedurestoensurecorrectinteroperationbetweenthem.

3.26.2.LabelsforMergingandNon-MergingLSRs

AnupstreamLSRwhichsupportslabelmergingneedstobesentonlyonelabelperFEC.AnupstreamneighborwhichdoesnotsupportlabelmergingneedstobesentmultiplelabelsperFEC.However,thereisnowayof nowingapriorihowmanylabelsitneeds.ThiswilldependonhowmanyLSRsareupstreamofitwithrespecttotheFECinquestion.

IntheMPLSarchitecture,ifaparticularupstreamneighbordoesnot

supportlabelmerging,itisnotsentanylabelsforaparticularFECunlessitexplicitlyas sforalabelforthatFEC.Theupstreamneighbormayma emultiplesuchrequests,andisgivenanewlabeleachtime.Whenadownstreamneighborreceivessucharequestfromupstream,andthedownstreamneighbordoesnotitselfsupportlabelmerging,thenitmustinturnas itsdownstreamneighborforanotherlabelfortheFECinquestion.

Itispossiblethattheremaybesomenodeswhichsupportlabelmerging,butcanonlymergealimitednumberofincominglabelsintoasingleoutgoinglabel.Supposeforexamplethatduetosomehardwarelimitationanodeiscapableofmergingfourincominglabelsintoasingleoutgoinglabel.Supposehowever,thatthisparticular

nodehassixincominglabelsarrivingatitforaparticularFEC.Inthiscase,thisnodemaymergetheseintotwooutgoinglabels.

WhetherlabelmergingisapplicabletoexplicitlyroutedLSPsisforfurtherstudy.

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3.26.3.MergeoverATM

3.26.3.1.MethodsofEliminatingCellInterleave

ThereareseveralmethodsthatcanbeusedtoeliminatethecellinterleavingprobleminATM,therebyallowingATMswitchestosupportstreammerge:

1.VPmerge,usingtheSVPMultipointEncoding

WhenVPmergeisused,multiplevirtualpathsaremergedintoavirtualpath,butpac etsfromdifferentsourcesaredistinguishedbyusingdifferentVCIswithintheVP.

2.VCmerge

WhenVCmergeisused,switchesarerequiredtobuffercellsfromonepac etuntiltheentirepac etisreceived(thismaybedeterminedbyloo ingfortheAAL5endofframeindicator).

VPmergehastheadvantagethatitiscompatiblewithahigherpercentageofexistingATMswitchimplementations.Thisma esitmoreli elythatVPmergecanbeusedinexistingnetwor s.Unli eVCmerge,VPmergedoesnotincuranydelaysatthemergepointsandalsodoesnotimposeanybufferrequirements.However,ithasthedisadvantagethatitrequirescoordinationoftheVCIspacewithineachVP.Thereareanumberofwaysthatthiscanbeaccomplished.Selectionofoneormoremethodsisforfurtherstudy.

ThistradeoffbetweencompatibilitywithexistingequipmentversusprotocolcomplexityandscalabilityimpliesthatitisdesirablefortheMPLSprotocoltosupportbothVPmergeandVCmerge.Inorderto

dosoeachATMswitchparticipatinginMPLSneedsto

nowwhetheritsimmediateATMneighborsperformVPmerge,VCmerge,ornomerge.

3.26.3.2.Interoperation:VCMerge,VPMerge,andNon-Merge

TheinteroperationofthevariousformsofmergingoverATMismosteasilydescribedbyfirstdescribingtheinteroperationofVCmergewithnon-merge.

InthecasewhereVCmergeandnon-mergenodesareinterconnectedtheforwardingofcellsisbasedinallcasesonaVC(i.e.,theconcatenationoftheVPIandVCI).Foreachnode,ifanupstreamneighborisdoingVCmergethenthatupstreamneighborrequiresonly

asingleVPI/VCIforaparticularstream(thisisanalogoustotherequirementforasinglelabelinthecaseofoperationoverframemedia).Iftheupstreamneighborisnotdoingmerge,thenthe



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neighborwillrequireasingleVPI/VCIperstreamforitself,plusenoughVPI/VCIstopasstoitsupstreamneighbors.ThenumberrequiredwillbedeterminedbyallowingtheupstreamnodestorequestadditionalVPI/VCIsfromtheirdownstreamneighbors(thisisagainanalogoustothemethodusedwithframemerge).

AsimilarmethodispossibletosupportnodeswhichperformVPmerge.InthiscasetheVPmergenode,ratherthanrequestingasingleVPI/VCIoranumberofVPI/VCIsfromitsdownstreamneighbor,insteadmayrequestasingleVP(identifiedbyaVPI)butseveralVCIswithintheVP.Furthermore,supposethatanon-mergenodeisdownstreamfromtwodifferentVPmergenodes.ThisnodemayneedtorequestoneVPI/VCI(fortrafficoriginatingfromitself)plustwoVPs(oneforeachupstreamnode),eachassociatedwithaspecifiedsetofVCIs(asrequestedfromtheupstreamnode).

InordertosupportallofVPmerge,VCmerge,andnon-merge,itisthereforenecessarytoallowupstreamnodestorequestacombinationofzeroormoreVCidentifiers(consistingofaVPI/VCI),pluszeroormoreVPs(identifiedbyVPIs)eachcontainingaspecifiednumberofVCs(identifiedbyasetofVCIswhicharesignificantwithinaVP).VPmergenodeswouldthereforerequestoneVP,withacontainedVCIfortrafficthatitoriginates(ifappropriate)plusaVCIfor

eachVCrequestedfromabove(regardlessofwhetherornottheVCispartofacontainingVP).VCmergenodewouldrequestonlyasingleVPI/VCI(sincetheycanmergeallupstreamtrafficintoasingleVC).Non-mergenodeswouldpassonanyrequeststhattheygetfromabove,plusrequestaVPI/VCIfortrafficthattheyoriginate(ifappropriate).

3.27.TunnelsandHierarchy

SometimesarouterRuta esexplicitactiontocauseaparticularpac ettobedeliveredtoanotherrouterRd,eventhoughRuandRdarenotconsecutiveroutersontheHop-by-hoppathforthatpac et,andRdisnotthepac et'sultimatedestination.Forexample,this

maybedonebyencapsulatingthepac

etinsideanetwor

layerpac

etwhosedestinationaddressistheaddressofRditself.Thiscreatesa"tunnel"fromRutoRd.Werefertoanypac etsohandledasa"TunneledPac

et".

3.27.1.Hop-by-HopRoutedTunnel

IfaTunneledPac

etfollowstheHop-by-hoppathfromRutoRd,wesaythatitisinan"Hop-by-HopRoutedTunnel"whose"transmitendpoint"isRuandwhose"receiveendpoint"isRd.



3.27.2.ExplicitlyRoutedTunnel

IfaTunneledPac ettravelsfromRutoRdoverapathotherthanthe

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Hop-by-hoppath,wesaythatitisinan"ExplicitlyRoutedTunnel"whose"transmitendpoint"isRuandwhose"receiveendpoint"isRd.Forexample,wemightsendapac etthroughanExplicitlyRoutedTunnelbyencapsulatingitinapac etwhichissourcerouted.

3.27.3.LSPTunnels

ItispossibletoimplementatunnelasaLSP,anduselabelswitchingratherthannetwor layerencapsulationtocausethepac ettotravelthroughthetunnel.ThetunnelwouldbeaLSP,whereR1isthetransmitendpointofthetunnel,andRnisthereceiveendpointofthetunnel.Thisiscalleda"LSPTunnel".

Thesetofpac

etswhicharetobesentthoughtheLSPtunnelconstitutesaFEC,andeachLSRinthetunnelmustassignalabeltothatFEC(i.e.,mustassignalabeltothetunnel).Thecriteriaforassigningaparticularpac ettoanLSPtunnelisalocalmatteratthetunnel'stransmitendpoint.Toputapac etintoanLSPtunnel,thetransmitendpointpushesalabelforthetunnelontothelabelstac andsendsthelabeledpac ettothenexthopinthetunnel.

Ifitisnotnecessaryforthetunnel'sreceiveendpointtobeabletodeterminewhichpac etsitreceivesthroughthetunnel,asdiscussedearlier,thelabelstac maybepoppedatthepenultimate

LSRinthetunnel.

A"Hop-by-HopRoutedLSPTunnel"isaTunnelthatisimplementedasanhop-by-hoproutedLSPbetweenthetransmitendpointandthereceiveendpoint.

An"ExplicitlyRoutedLSPTunnel"isaLSPTunnelthatisalsoanExplicitlyRoutedLSP.

3.27.4.Hierarchy:LSPTunnelswithinLSPs

ConsideraLSP.LetussupposethatR1receivesunlabeledpac etP,andpushesonitslabelstac thelabeltocause

ittofollowthispath,andthatthisisinfacttheHop-by-hoppath.However,letusfurthersupposethatR2andR3arenotdirectlyconnected,butare"neighbors"byvirtueofbeingtheendpointsofanLSPtunnel.SotheactualsequenceofLSRstraversedbyPis.


[Page33]


WhenPtravelsfromR1toR2,itwillhavealabelstac ofdepth1.R2,switchingonthelabel,determinesthatPmustenterthetunnel.R2firstreplacestheIncominglabelwithalabelthatismeaningfultoR3.Thenitpushesonanewlabel.Thislevel2labelhasavaluewhichismeaningfultoR21.Switchingisdoneonthelevel2labelbyR21,R22,R23.R23,whichisthepenultimatehopintheR2-R3tunnel,popsthelabelstac beforeforwardingthepac etto

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R3.WhenR3seespac etP,Phasonlyalevel1label,havingnowexitedthetunnel.SinceR3isthepenultimatehopinP'slevel1LSP,itpopsthelabelstac ,andR4receivesPunlabeled.

Thelabelstac mechanismallowsLSPtunnelingtonesttoanydepth.

3.27.5.LabelDistributionPeeringandHierarchy

Supposethatpac etPtravelsalongaLevel1LSP,andwhengoingfromR2toR3travelsalongaLevel2LSP.FromtheperspectiveoftheLevel2LSP,R2'slabeldistributionpeerisR21.FromtheperspectiveoftheLevel1LSP,R2'slabeldistributionpeersareR1andR3.Onecanhavelabeldistributionpeersateachlayerofhierarchy.Wewillseeinsections4.6and4.7somewaystoma euseofthishierarchy.Notethatinthisexample,R2andR21mustbeIGPneighbors,butR2andR3neednotbe.

WhentwoLSRsareIGPneighbors,wewillrefertothemas"locallabeldistributionpeers".WhentwoLSRsmaybelabeldistributionpeers,butarenotIGPneighbors,wewillrefertothemas"remotelabeldistributionpeers".Intheaboveexample,R2andR21arelocallabeldistributionpeers,butR2andR3areremotelabeldistributionpeers.

TheMPLSarchitecturesupportstwowaystodistributelabelsatdifferentlayersofthehierarchy:ExplicitPeeringandImplicitPeering.

Oneperformslabeldistributionwithone'slocallabeldistributionpeerbysendinglabeldistributionprotocolmessageswhichareaddressedtothepeer.Onecanperformlabeldistributionwithone'sremotelabeldistributionpeersinoneoftwoways:

1.ExplicitPeering

Inexplicitpeering,onedistributeslabelstoapeerby

sendinglabeldistributionprotocolmessageswhichareaddressedtothepeer,exactlyasonewoulddoforlocallabeldistributionpeers.Thistechniqueismostusefulwhenthenumberofremotelabeldistributionpeersissmall,orthe



numberofhigherlevellabelbindingsislarge,ortheremote

labeldistributionpeersareindistinctroutingareasordomains.Ofcourse,oneneedsto nowwhichlabelstodistributetowhichpeers;thisisaddressedinsection4.1.2.

Examplesoftheuseofexplicitpeeringisfoundinsections4.2.1and4.6.

2.ImplicitPeering

InImplicitPeering,onedoesnotsendlabeldistribution

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protocolmessageswhichareaddressedtoone'speer.Rather,todistributehigherlevellabelstoonesremotelabeldistributionpeers,oneencodesahigherlevellabelasanattributeofalowerlevellabel,andthendistributesthelowerlevellabel,alongwiththisattribute,toone'slocallabeldistributionpeers.Thelocallabeldistributionpeersthenpropagatetheinformationtotheirlocallabeldistributionpeers.Thisprocesscontinuestilltheinformationreachestheremotepeer.

Thistechniqueismostusefulwhenthenumberofremotelabeldistributionpeersislarge.Implicitpeeringdoesnotrequireann-squarepeeringmeshtodistributelabelstotheremotelabeldistributionpeersbecausetheinformationispiggybac

edthroughthelocallabeldistributionpeering.However,implicitpeeringrequirestheintermediatenodestostoreinformationthattheymightnotbedirectlyinterestedin.

Anexampleoftheuseofimplicitpeeringisfoundinsection4.3.

3.28.LabelDistributionProtocolTransport

AlabeldistributionprotocolisusedbetweennodesinanMPLS

networ

toestablishandmaintainthelabelbindings.InorderforMPLStooperatecorrectly,labeldistributioninformationneedstobetransmittedreliably,andthelabeldistributionprotocolmessagespertainingtoaparticularFECneedtobetransmittedinsequence.Flowcontrolisalsodesirable,asisthecapabilitytocarrymultiplelabelmessagesinasingledatagram.

OnewaytomeetthesegoalsistouseTCPastheunderlyingtransport,asisdonein[MPLS-LDP]and[MPLS-BGP].



3.29.WhyMorethanoneLabelDistributionProtocol?

Thisarchitecturedoesnotestablishhardandfastrulesforchoosingwhichlabeldistributionprotocoltouseinwhichcircumstances.However,itispossibletopointoutsomeoftheconsiderations.

3.29.1.BGPandLDP

Inmanyscenarios,itisdesirabletobindlabelstoFECswhichcanbeidentifiedwithroutestoaddressprefixes(seesection4.1).Ifthereisastandard,widelydeployedroutingalgorithmwhichdistributesthoseroutes,itcanbearguedthatlabeldistributionisbestachievedbypiggybac ingthelabeldistributiononthedistributionoftheroutesthemselves.

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Forexample,BGPdistributessuchroutes,andifaBGPspea erneedstoalsodistributelabelstoitsBGPpeers,usingBGPtodothelabeldistribution(see[MPLS-BGP])hasanumberofadvantages.Inparticular,itpermitsBGProutereflectorstodistributelabels,thusprovidingasignificantscalabilityadvantageoverusingLDPtodistributelabelsbetweenBGPpeers.

3.29.2.LabelsforRSVPFlowspecs

WhenRSVPisusedtosetupresourcereservationsforparticularflows,itcanbedesirabletolabelthepac etsinthoseflows,sothattheRSVPfilterspecdoesnotneedtobeappliedateachhop.ItcanbearguedthathavingRSVPdistributethelabelsaspartofitspath/reservationsetupprocessisthemostefficientmethodofdistributinglabelsforthispurpose.

3.29.3.LabelsforExplicitlyRoutedLSPs

InsomeapplicationsofMPLS,particularlythoserelatedtotrafficengineering,itisdesirabletosetupanexplicitlyroutedpath,fromingresstoegress.Itisalsodesirabletoapplyresourcereservationsalongthatpath.

Onecanimaginetwoapproachestothis:

-Startwithanexistingprotocolthatisusedforsettingupresourcereservations,andextendittosupportexplicitroutingandlabeldistribution.

-Startwithanexistingprotocolthatisusedforlabeldistribution,andextendittosupportexplicitroutingandresourcereservations.



Thefirstapproachhasgivenrisetotheprotocolspecifiedin[MPLS-RSVP-TUNNELS],thesecondtotheapproachspecifiedin[MPLS-CR-LDP].

3.30.Multicast

Thissectionisforfurtherstudy

4.SomeApplicationsofMPLS

4.1.MPLSandHopbyHopRoutedTraffic

AnumberofusesofMPLSrequirethatpac etswithacertainlabelbeforwardedalongthesamehop-by-hoproutedpaththatwouldbeusedforforwardingapac etwithaspecifiedaddressinitsnetwor layerdestinationaddressfield.

4.1.1.LabelsforAddressPrefixes

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Ingeneral,routerRdeterminesthenexthopforpac etPbyfindingtheaddressprefixXinitsroutingtablewhichisthelongestmatchforP'sdestinationaddress.Thatis,thepac etsinagivenFECarejustthosepac etswhichmatchagivenaddressprefixinR'sroutingtable.Inthiscase,aFECcanbeidentifiedwithanaddressprefix.

Notethatapac etPmaybeassignedtoFECF,andFECFmaybeidentifiedwithaddressprefixX,evenifP'sdestinationaddressdoesnotmatchX.

4.1.2.DistributingLabelsforAddressPrefixes

4.1.2.1.LabelDistributionPeersforanAddressPrefix

LSRsR1andR2areconsideredtobelabeldistributionpeersforaddressprefixXifandonlyifoneofthefollowingconditionsholds:

1.R1'sroutetoXisaroutewhichitlearnedaboutviaaparticularinstanceofaparticularIGP,andR2isaneighborofR1inthatinstanceofthatIGP

2.R1'sroutetoXisaroutewhichitlearnedaboutbysomeinstanceofroutingalgorithmA1,andthatrouteis

redistributedintoaninstanceofroutingalgorithmA2,andR2isaneighborofR1inthatinstanceofA2


3.R1isthereceiveendpointofanLSPTunnelthatiswithinanotherLSP,andR2isatransmitendpointofthattunnel,andR1andR2areparticipantsinacommoninstanceofanIGP,andareinthesameIGParea(iftheIGPinquestionhasareas),andR1'sroutetoXwaslearnedviathatIGPinstance,orisredistributedbyR1intothatIGPinstance

4.R1'sroutetoXisaroutewhichitlearnedaboutviaBGP,andR2isaBGPpeerofR1

Ingeneral,theserulesensurethatiftheroutetoaparticularaddressprefixisdistributedviaanIGP,thelabeldistributionpeersforthataddressprefixaretheIGPneighbors.Iftherouteto

aparticularaddressprefixisdistributedviaBGP,thelabeldistributionpeersforthataddressprefixaretheBGPpeers.InothercasesofLSPtunneling,thetunnelendpointsarelabeldistributionpeers.

4.1.2.2.DistributingLabels

InordertouseMPLSfortheforwardingofpac etsaccordingtothehop-by-hoproutecorrespondingtoanyaddressprefix,eachLSRMUST:

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1.bindoneormorelabelstoeachaddressprefixthatappearsinitsroutingtable;

2.foreachsuchaddressprefixX,usealabeldistributionprotocoltodistributethebindingofalabeltoXtoeachofitslabeldistributionpeersforX.

ThereisalsoonecircumstanceinwhichanLSRmustdistributealabelbindingforanaddressprefix,evenifitisnottheLSRwhichboundthatlabeltothataddressprefix:

3.IfR1usesBGPtodistributearoutetoX,namingsomeotherLSRR2astheBGPNextHoptoX,andifR1 nowsthatR2hasassignedlabelLtoX,thenR1mustdistributethebindingbetweenLandXtoanyBGPpeertowhichitdistributesthatroute.

TheserulesensurethatlabelscorrespondingtoaddressprefixeswhichcorrespondtoBGProutesaredistributedtoIGPneighborsifandonlyiftheBGProutesaredistributedintotheIGP.Otherwise,thelabelsboundtoBGProutesaredistributedonlytotheotherBGPspea ers.

Theserulesareintendedonlytoindicatewhichlabelbindingsmust

bedistributedbyagivenLSRtowhichotherLSRs.


4.1.3.UsingtheHopbyHoppathastheLSP

Ifthehop-by-hoppaththatpac etPneedstofollowis,thencanbeanLSPaslongas:

1.thereisasingleaddressprefixX,suchthat,foralli,1

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AnLSRRisconsideredtobean"LSPEgress"LSRforaddressprefixXifandonlyifoneofthefollowingconditionsholds:

1.RhasanaddressY,suchthatXistheaddressprefixinR'sroutingtablewhichisthelongestmatchforY,or

2.RcontainsinitsroutingtablesoneormoreaddressprefixesYsuchthatXisaproperinitialsubstringofY,butR's"LSPprevioushops"forXdonotcontainanysuchaddressprefixesY;thatis,Risa"deaggregationpoint"foraddressprefixX.

AnLSRR1isconsideredtobean"LSPProxyEgress"LSRforaddressprefixXifandonlyif:

1.R1'snexthopforXisR2,andR1andR2arenotlabeldistributionpeerswithrespecttoX(perhapsbecauseR2doesnotsupportMPLS),or

2.R1hasbeenconfiguredtoactasanLSPProxyEgressforX



ThedefinitionofLSPallowsfortheLSPEgresstobeanodewhichdoesnotsupportMPLS;inthiscasethepenultimatenodeintheLSPistheProxyEgress.

4.1.5.TheImplicitNULLLabel

TheImplicitNULLlabelisalabelwithspecialsemanticswhichanLSRcanbindtoanaddressprefix.IfLSRRu,byconsultingitsILM,

seesthatlabeledpac

etPmustbeforwardednexttoRd,butthatRdhasdistributedabindingofImplicitNULLtothecorrespondingaddressprefix,theninsteadofreplacingthevalueofthelabelontopofthelabelstac

,Rupopsthelabelstac

,andthenforwardstheresultingpac ettoRd.

LSRRddistributesabindingbetweenImplicitNULLandanaddressprefixXtoLSRRuifandonlyif:

1.therulesofSection4.1.2indicatethatRddistributestoRualabelbindingforX,and

2.Rd nowsthatRucansupporttheImplicitNULLlabel(i.e.,

thatitcanpopthelabelstac

),and

3.RdisanLSPEgress(notproxyegress)forX.

ThiscausesthepenultimateLSRonaLSPtopopthelabelstac .Thisisquiteappropriate;iftheLSPEgressisanMPLSEgressforX,thenifthepenultimateLSRdoesnotpopthelabelstac

,theLSPEgresswillneedtoloo upthelabel,popthelabelstac ,andthenloo upthenextlabel(orloo uptheL3address,ifnomorelabelsarepresent).ByhavingthepenultimateLSRpopthelabelstac ,the

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LSPEgressissavedthewor ofhavingtoloo uptwolabelsinordertoma

eitsforwardingdecision.

However,ifthepenultimateLSRisanATMswitch,itmaynothavethecapabilitytopopthelabelstac .HenceabindingofImplicitNULLmaybedistributedonlytoLSRswhichcansupportthatfunction.

IfthepenultimateLSRinanLSPforaddressprefixXisanLSPProxyEgress,itactsjustasiftheLSPEgresshaddistributedabindingofImplicitNULLforX.

4.1.6.Option:Egress-TargetedLabelAssignment

TherearesituationsinwhichanLSPIngress,Ri,

nowsthatpac

etsofseveraldifferentFECsmustallfollowthesameLSP,terminatingat,say,LSPEgressRe.Inthiscase,properroutingcanbeachieved


byusingasinglelabelforallsuchFECs;itisnotnecessarytohaveadistinctlabelforeachFEC.If(andonlyif)thefollowingconditionshold:

1.theaddressofLSRReisitselfintheroutingtableasa"hostroute",and

2.thereissomewayforRitodeterminethatReistheLSPegressforallpac etsinaparticularsetofFECs

ThenRimaybindasinglelabeltoallFECSintheset.Thisis nownas"Egress-TargetedLabelAssignment."

HowcanLSRRideterminethatanLSRReistheLSPEgressforallpac etsinaparticularFEC?Thereareanumberofpossibleways:

-Ifthenetwor isrunningalin stateroutingalgorithm,andallnodesintheareasupportMPLS,thentheroutingalgorithmprovidesRiwithenoughinformationtodeterminetheroutersthroughwhichpac

etsinthatFECmustleavetheroutingdomainorarea.

-Ifthenetwor isrunningBGP,Rimaybeabletodeterminethatthepac

etsinaparticularFECmustleavethenetwor

viasomeparticularrouterwhichisthe"BGPNextHop"forthatFEC.

-Itispossibletousethelabeldistributionprotocoltopassinformationaboutwhichaddressprefixesare"attached"towhichegressLSRs.Thismethodhastheadvantageofnotdependingonthepresenceoflin staterouting.

Ifegress-targetedlabelassignmentisused,thenumberoflabelsthatneedtobesupportedthroughoutthenetwor maybegreatlyreduced.ThismaybesignificantifoneisusinglegacyswitchinghardwaretodoMPLS,andtheswitchinghardwarecansupportonlya

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limitednumberoflabels.

Onepossibleapproachwouldbetoconfigurethenetwor touseegress-targetedlabelassignmentbydefault,buttoconfigureparticularLSRstoNOTuseegress-targetedlabelassignmentforoneormoreoftheaddressprefixesforwhichitisanLSPegress.Weimposethefollowingrule:

-IfaparticularLSRisNOTanLSPEgressforsomesetofaddressprefixes,thenitshouldassignlabelstotheaddressprefixesinthesamewayasisdonebyitsLSPnexthopforthoseaddressprefixes.Thatis,supposeRdisRu'sLSPnext


hopforaddressprefixesX1andX2.IfRdassignsthesamelabeltoX1andX2,Rushouldaswell.IfRdassignsdifferentlabelstoX1andX2,thenRushouldaswell.

Forexample,supposeonewantstoma eegress-targetedlabelassignmentthedefault,buttoassigndistinctlabelstothoseaddressprefixesforwhichtherearemultiplepossibleLSPegresses(i.e.,forthoseaddressprefixeswhicharemulti-homed.)OnecanconfigureallLSRstouseegress-targetedlabelassignment,andthenconfigureahandfulofLSRstoassigndistinctlabelstothoseaddressprefixeswhicharemulti-homed.Foraparticularmulti-homedaddressprefixX,onewouldonlyneedtoconfigurethisinLSRswhichareeitherLSPEgressesorLSPProxyEgressesforX.

ItisimportanttonotethatifRuandRdareadjacentLSRsinanLSPforX1andX2,forwardingwillstillbedonecorrectlyifRuassigns

distinctlabelstoX1andX2whileRdassignsjustonelabeltothebothofthem.ThisjustmeansthatR1willmapdifferentincominglabelstothesameoutgoinglabel,anordinaryoccurrence.

Similarly,ifRdassignsdistinctlabelstoX1andX2,butRuassignstothemboththelabelcorrespondingtotheaddressoftheirLSPEgressorProxyEgress,forwardingwillstillbedonecorrectly.RuwilljustmaptheincominglabeltothelabelwhichRdhasassignedtotheaddressofthatLSPEgress.

4.2.MPLSandExplicitlyRoutedLSPs

Thereareanumberofreasonswhyitmaybedesirabletouseexplicit

routinginsteadofhopbyhoprouting.Forexample,thisallowsroutestobebasedonadministrativepolicies,andallowstheroutesthatLSPsta

etobecarefullydesignedtoallowtrafficengineering[MPLS-TRFENG].

4.2.1.ExplicitlyRoutedLSPTunnels

Insomesituations,thenetwor administratorsmaydesiretoforwardcertainclassesoftrafficalongcertainpre-specifiedpaths,wherethesepathsdifferfromtheHop-by-hoppaththatthetrafficwould

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ordinarilyfollow.Thiscanbedoneinsupportofpolicyrouting,orinsupportoftrafficengineering.Theexplicitroutemaybeaconfiguredone,oritmaybedetermineddynamicallybysomemeans,e.g.,byconstraint-basedrouting.

MPLSallowsthistobeeasilydonebymeansofExplicitlyRoutedLSPTunnels.Allthatisneededis:


1.Ameansofselectingthepac etsthataretobesentintotheExplicitlyRoutedLSPTunnel;

2.AmeansofsettinguptheExplicitlyRoutedLSPTunnel;

3.Ameansofensuringthatpac etssentintotheTunnelwillnotloopfromthereceiveendpointbac tothetransmitendpoint.

Ifthetransmitendpointofthetunnelwishestoputalabeledpac etintothetunnel,itmustfirstreplacethelabelvalueatthetopofthestac withalabelvaluethatwasdistributedtoitbythetunnel'sreceiveendpoint.Thenitmustpushonthelabelwhichcorrespondstothetunnelitself,asdistributedtoitbythenexthopalongthetunnel.Toallowthis,thetunnelendpointsshouldbeexplicitlabeldistributionpeers.ThelabelbindingstheyneedtoexchangeareofnointeresttotheLSRsalongthetunnel.

4.3.LabelStac sandImplicitPeering

SupposeaparticularLSRReisanLSPproxyegressfor10address

prefixes,anditreacheseachaddressprefixthroughadistinctinterface.

Onecouldassignasinglelabeltoall10addressprefixes.ThenReisanLSPegressforall10addressprefixes.Thisensuresthatpac etsforall10addressprefixesgetdeliveredtoRe.However,Rewouldthenhavetoloo upthenetwor layeraddressofeachsuchpac

etinordertochoosetheproperinterfacetosendthepac

eton.

Alternatively,onecouldassignadistinctlabeltoeachinterface.ThenReisanLSPproxyegressforthe10addressprefixes.ThiseliminatestheneedforRetoloo

upthenetwor

layeraddressesinordertoforwardthepac ets.However,itcanresultintheuseofa

largenumberoflabels.

Analternativewouldbetobindall10addressprefixestothesamelevel1label(whichisalsoboundtotheaddressoftheLSRitself),andthentobindeachaddressprefixtoadistinctlevel2label.Thelevel2labelwouldbetreatedasanattributeofthelevel1labelbinding,whichwecallthe"Stac

Attribute".Weimposethefollowingrules:

-WhenLSRRuinitiallylabelsahithertounlabeledpac et,if

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thelongestmatchforthepac et'sdestinationaddressisX,andRu'sLSPnexthopforXisRd,andRdhasdistributedtoRuabindingoflabelL1toX,alongwithastac attributeofL2,then


1.RumustpushL2andthenL1ontothepac

et'slabelstac

,andthenforwardthepac ettoRd;

2.WhenRudistributeslabelbindingsforXtoitslabeldistributionpeers,itmustincludeL2asthestacattribute.

3.Wheneverthestac attributechanges(possiblyasaresultofachangeinRu'sLSPnexthopforX),Rumustdistributethenewstac attribute.

NotethatalthoughthelabelvalueboundtoXmaybedifferentateachhopalongtheLSP,thestac attributevalueispassedunchanged,andissetbytheLSPproxyegress.

ThustheLSPproxyegressforXbecomesan"implicitpeer"witheachotherLSRintheroutingareaordomain.Inthiscase,explicitpeeringwouldbetoounwieldy,becausethenumberofpeerswouldbecometoolarge.

4.4.MPLSandMulti-PathRouting

IfanLSRsupportsmultipleroutesforaparticularstream,thenitmayassignmultiplelabelstothestream,oneforeachroute.Thus

thereceptionofasecondlabelbindingfromaparticularneighborforaparticularaddressprefixshouldbeta enasmeaningthateitherlabelcanbeusedtorepresentthataddressprefix.

Ifmultiplelabelbindingsforaparticularaddressprefixarespecified,theymayhavedistinctattributes.

4.5.LSPTreesasMultipoint-to-PointEntities

Considerthecaseofpac etsP1andP2,eachofwhichhasadestinationaddresswhoselongestmatch,throughoutaparticularroutingdomain,isaddressprefixX.SupposethattheHop-by-hoppathforP1is,andtheHop-by-hoppathforP2is.Let'ssupposethatR3bindslabelL3toX,anddistributesthisbindingtoR2.R2bindslabelL2toX,anddistributesthisbindingtobothR1andR4.WhenR2receivespac

etP1,itsincominglabelwillbeL2.R2willoverwriteL2withL3,andsendP1toR3.WhenR2receivespac etP2,itsincominglabelwillalsobeL2.R2againoverwritesL2withL3,andsendP2ontoR3.

NotethenthatwhenP1andP2aretravelingfromR2toR3,theycarrythesamelabel,andasfarasMPLSisconcerned,theycannotbedistinguished.Thusinsteadoftal ingabouttwodistinctLSPs,

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R2,R3>and,wemighttal

ofasingle"Multipoint-to-PointLSPTree",whichwemightdenoteas.

ThiscreatesadifficultywhenweattempttouseconventionalATMswitchesasLSRs.SinceconventionalATMswitchesdonotsupportmultipoint-to-pointconnections,theremustbeprocedurestoensurethateachLSPisrealizedasapoint-to-pointVC.However,ifATMswitcheswhichdosupportmultipoint-to-pointVCsareinuse,thentheLSPscanbemostefficientlyrealizedasmultipoint-to-pointVCs.Alternatively,iftheSVPMultipointEncoding(section3.25.2)canbeused,theLSPscanberealizedasmultipoint-to-pointSVPs.

4.6.LSPTunnelingbetweenBGPBorderRouters

ConsiderthecaseofanAutonomousSystem,A,whichcarriestransit

trafficbetweenotherAutonomousSystems.AutonomousSystemAwillhaveanumberofBGPBorderRouters,andameshofBGPconnectionsamongthem,overwhichBGProutesaredistributed.Inmanysuchcases,itisdesirabletoavoiddistributingtheBGProutestorouterswhicharenotBGPBorderRouters.Ifthiscanbeavoided,the"routedistributionload"onthoseroutersissignificantlyreduced.However,theremustbesomemeansofensuringthatthetransittrafficwillbedeliveredfromBorderRoutertoBorderRouterbytheinteriorrouters.

ThiscaneasilybedonebymeansofLSPTunnels.SupposethatBGProutesaredistributedonlytoBGPBorderRouters,andnottotheinteriorroutersthatliealongtheHop-by-hoppathfromBorder

RoutertoBorderRouter.LSPTunnelscanthenbeusedasfollows:

1.EachBGPBorderRouterdistributes,toeveryotherBGPBorderRouterinthesameAutonomousSystem,alabelforeachaddressprefixthatitdistributestothatrouterviaBGP.

2.TheIGPfortheAutonomousSystemmaintainsahostrouteforeachBGPBorderRouter.EachinteriorrouterdistributesitslabelsforthesehostroutestoeachofitsIGPneighbors.

3.Supposethat:

a)BGPBorderRouterB1receivesanunlabeledpac etP,

b)addressprefixXinB1'sroutingtableisthelongestmatchforthedestinationaddressofP,

c)theroutetoXisaBGProute,

d)theBGPNextHopforXisB2,

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e)B2hasboundlabelL1toX,andhasdistributedthisbindingtoB1,

f)theIGPnexthopfortheaddressofB2isI1,

g)theaddressofB2isinB1'sandI1'sIGProutingtablesasahostroute,and

h)I1hasboundlabelL2totheaddressofB2,anddistributedthisbindingtoB1.

Thenbeforesendingpac etPtoI1,B1mustcreatealabelstac forP,thenpushonlabelL1,andthenpushonlabelL2.

4.SupposethatBGPBorderRouterB1receivesalabeledPac etP,wherethelabelonthetopofthelabelstac correspondstoanaddressprefix,X,towhichtherouteisaBGProute,andthatconditions3b,3c,3d,and3eallhold.Thenbeforesendingpac etPtoI1,B1mustreplacethelabelatthetopofthe

labelstac

withL1,andthenpushonlabelL2.

Withtheseprocedures,agivenpac etPfollowsalevel1LSPallofwhosemembersareBGPBorderRouters,andbetweeneachpairofBGPBorderRoutersinthelevel1LSP,itfollowsalevel2LSP.

TheseprocedureseffectivelycreateaHop-by-HopRoutedLSPTunnelbetweentheBGPBorderRouters.

SincetheBGPborderroutersareexchanginglabelbindingsforaddressprefixesthatarenoteven nowntotheIGProuting,theBGProutersshouldbecomeexplicitlabeldistributionpeerswitheachother.

ItissometimespossibletocreateHop-by-HopRoutedLSPTunnelsbetweentwoBGPBorderRouters,eveniftheyarenotinthesameAutonomousSystem.Suppose,forexample,thatB1andB2areinAS1.SupposethatB3isanEBGPneighborofB2,andisinAS2.Finally,supposethatB2andB3areonsomenetwor whichiscommontobothAutonomousSystems(a"DemilitarizedZone").Inthiscase,anLSPtunnelcanbesetupdirectlybetweenB1andB3asfollows:

-B3distributesroutestoB2(usingEBGP),optionallyassigninglabelstoaddressprefixes;

-B2redistributesthoseroutestoB1(usingIBGP),indicating

thattheBGPnexthopforeachsuchrouteisB3.IfB3hasassignedlabelstoaddressprefixes,B2passestheselabelsalong,unchanged,toB1.



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-TheIGPofAS1hasahostrouteforB3.

4.7.OtherUsesofHop-by-HopRoutedLSPTunnels

TheuseofHop-by-HopRoutedLSPTunnelsisnotrestrictedtotunnelsbetweenBGPNextHops.AnysituationinwhichonemightotherwisehaveusedanencapsulationtunnelisoneinwhichitisappropriatetouseaHop-by-HopRoutedLSPTunnel

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