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Chapter 6.  Providing IPv6 Multicast Services

Chapter 6.  Providing IPv6 Multicast Services

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ignoredinIPv6,asshowninTable6-1.Ontheotherhand,IPv6

leveragesitsaddresssizeandstructurethroughnew,specific

features.Theimplementationisalsocleanerorsimplerattimes

becausemulticastwaspartoftheIPv6developmentfromday

oneandavoidedsomeconstraintsintroducedbyunicast

featuresorconcepts.InTable6-1,IPv4multicastfeaturesthat

areexplicitlynotconsideredinIPv6aremarkedNC.

Table6-1.TaxonomyofMulticastProtocolsandTheir

AvailabilityinIPv6

ProtocolFunction MulticastProtocol



IPv4 IPv6



Groupmembership

management



X



InternetGroupManagement

Protocol(IGMP)v1,v2,v3

MulticastListenerDiscovery(MLD)

v1,v2



Multicastsupportat Snooping(IGMPorMLD)

layer2



X



X



CiscoGroupManagementProtocol X

(CGMP)



Multicastrouting



X

NC



RouterGroupManagement

Protocol(RGMP)



X



GARP(GeneralizedAttribute

RegistrationProtocol)Multicast

RegistrationProtocol



X



ProtocolIndependentMulticastDenseMode(PIM-DM)



X



NC



ProtocolIndependentMulticastSparseMode(PIM-SM)



X



X



ProtocolIndependentMulticastSourceSpecificMulticast(PIMSSM)



X



X



ProtocolIndependentMulticast-



X



X



Bidirectional(PIM-Bidir)

PragmaticGeneralMulticast(PGM) X

Rendezvouspoint

management



Static-RP



X



Auto-RP



X



Bootstraprouter(BSR)



X



EmbeddedRP

MulticastSourceDiscovery

Protocol



X



X

X



X



NC



Table6-1isnotmeanttobecompletefromahistorical

perspective.Itintendsonlytosummarizethemostrelevant

multicastprotocolsofbothIPv4andIPv6.TheIPv6-specific

onesarediscussedintheremainderofthischapter.Itis

assumedthatthereaderisfamiliarwiththeIPv4multicast

concepts.Someoftheseconceptsarebrieflydescribedhere,

butforacompleteanddetailedpresentation,refertotheCisco

PressbookDevelopingIPMulticastNetworks,Volume1.

Thischaptercoversthefollowingtopics:

AreviewofIPmulticastconceptsandtheirimplementation

inIPv6

IPv6multicastdeploymentconsiderations

ExamplesofconfiguringIPv6multicast

Bytheendofthechapter,youwillmostlikelyconcludethat,for



themostpart,IPv6multicastisnotdramaticallydifferentfrom

itspredecessor,butitenablescleanerservicedeployments.







IPv6Multicast

Animportantaspectofthemulticastserviceisaddressing.

Whereasaunicastaddressidentifiesanode,amulticast

addressidentifiesagroupofnodesinterestedinthesamedata.

Therearenoconstraintsonthelocationofagroup'smembers.

Apacketwithamulticastdestinationaddress(DA)isdelivered

toallmembersofthegroupidentifiedbythataddress.

IPv6multicast,likeunicast,benefitsfromthenewaddressing

architecturedefinedinIPv6forthefollowingreasons:

Largeraddressingspaceimpliestheavailabilityofplenty

ofaddressesformulticastgroups(unicast-basedmulticast

addressdescribedinChapter2,"AnIPv6Refresher,"isa

goodexample).IPv6addressingliftsmajordeployment

constraintsthatplaguedIPv4;itfacilitatesandsimplifies

multicastservicedeployments.



Note

IANAassignedtheIPv4ClassDaddresses,from

224.0.0.0to239.255.255.255,todesignate

IPv4multicastgroups.Formoreinformation,

visittheIANAwebsiteathttp://www.iana.org.



Addressscopingoffersacleanerwaytocontainthe

multicasttrafficwithintheintendeddomain.



Note



InIPv4only,thefixedmulticastscoping(see

Chapter2)isdefined.



Chapter2presentstheIPv6multicastaddressingarchitecture

forbothlayer2andlayer3oftheOSImodel.Thischapter

buildsonthatknowledgetoprovideareviewofkeyconcepts

suchasmulticastgroup,multicastdomain,multicasttrees,and

service-supportingprotocols.



GroupMembershipManagement

Multicastaddressesdefine"forums"forapplicationstodeliver

theircontentto.Theseforums(calledgroups)haveadynamic

audiencethatuserscanjoinorleave.Onalink,thegroupis

managedthroughseveralprotocolsatbothlayer2andatlayer

3oftheOSImodel.



MulticastListenerDiscovery

TheInternetGroupManagementProtocol(IGMP)isusedin

IPv4toenablehostsonalinktojoinamulticastgroup,leaveit,

orsimplycommunicatetoaroutertheirgroupmembership.In

short,itmanagesthemulticast-relatedinteractionbetween

listenersandrouters.IGMPwentthroughseveraldevelopmental

iterations:IGMPv1,IGMPv2,andIGMPv3.Thelaterversionsare

backwardcompatible,buteachaddsfeaturesthatenhancethe

operationofitspredecessor.



ProtocolDescription



TheMulticastListenerDiscovery(MLD)protocolisperforming

IGMP'sfunctioninIPv6.Therearetwoversionsoftheprotocol,

MLDv1andMLDv2.Theymapidenticallythelasttwoversions

ofIGMP,asshowninTable6-2.SimilartoIGMP,MLDisbuilton

topofICMP.

Table6-2.MLDMessageTypes

MLD



IGMP



MLDMessageTypes



ICMPv6

Type



IGMPv1(RFC

1112)

MLDv1(RFC2710) IGMPv2(RFC

2236)



Multicastlistener

query



130



Multicastlistener

report



131



Multicastlistenerdone 132

MLDv2(MLDv2)



IGMPv3(RFC

3376)



Multicastlistener

query



130



Multicastlistener

report



143



Foreachofitslinks,arouterhastokeeptrackofallmulticast

groupsthathavelisteners.Itneedstomaintainthisstateto

decidewhetheritshouldaccepttrafficforamulticastgroupand

whetheritshouldforwardthattrafficoutitsinterfaces.

Ontheotherhand,bydefault,routersdonothavetotrack

everylisteneronaninterface.Theyjusthavetoknowwhether

thereisatleastoneactivelistener.Toperformthisfunction,on

eachlinkasinglerouteriselectedtoqueryforlisteners.Inthis



sense,onalinkaroutercanbeinaquerier(sendingperiodic

generalqueries)oranonquerierstate.Allroutersstartas

queriers,butonlytheoneusingthelowestsourceaddress(SA)

(link-localaddressisusedastheSA)onitsqueriesforagiven

linkremainsactive.Thequeriersendsgeneralqueries("Any

listeneroutthere?")orspecificqueries("Anylistenersforgroup

G?").Whenarouterisinformedofalistener'sdeparture,the

latterquerytyperepresentsanoptimalwaytoverifywhether

anyotherlistenersremaininthegroupthatwasjustleft.



Note

Ifarouterqueriesaspecificgroup,thepacketis

senttothemulticastaddressofthatgroup.The

responsetothequeryissentwiththesameDA,and

therouterignoresitiftherouterhasnotsubscribed

tothatgroup,too.Forthis,reasonahop-by-hop

extensionheaderwithaRouterAlertoption(seethe

"Hop-by-HopOptionsHeader"sectioninChapter2)

isusedwiththeMLDICMPv6packets.Itforces

routerstoexaminemessagesdestinedtomulticast

groupsthattherouterisnotsubscribedto.



Nodesrespondtoquerieswithreportmessagesthatindicate

thegroupsandsourcestheirinterfacesarelisteningto.Report

(MLDv2)orDone(MLDv1)messagesarealsosentbynodesto

indicateachangeinthelisteningstateofoneoftheir

interfaces.

TheMLDv1messagesareusedtoperformthefollowing

functions:

MulticastlistenerqueryUsedtoidentifywhetheragiven



grouphaslistenersonalink.Therearetwotypesofquery.

Thegeneralquerysenttothelink-local,all-nodesmulticast

address(FF02::1),andwiththeMLDMulticastAddressfield

setto::isusedtolearnwhichmulticastgrouphas

listeners.Themulticastaddress-specificqueryisusedto

identifythelistenersforagivengroupthatislistedinthe

MLDMulticastAddressfieldofthemessageandissentto

thequeriedmulticastaddress.

MulticastlistenerreportThemessageusedinresponse

toaquery.TheIPv6DAforareportisthemulticastaddress

beingreported.

MulticastlistenerdoneSentbyanodetoindicatethatit

stoppedlisteningtoamulticastaddress.TheIPv6DAfor

thedonemessageisthelink-local,all-routersmulticast

address(FF02::2).



Note

AllMLDpacketsaresentwithalink-localaddressas

theIPv6SA.Thehoplimitissetto1.Thisisdoneto

makesurethepacketisnotroutedoutsideofthe

link.



MLDv2enhancesMLDv1byenablinganodetoexpressor

reportinterestinaparticularsourceforamulticastgroup.This

capabilityoptimizesthemulticastoperationthroughamore

discretecontrolofgroupmembership.Italsoprovidesthe

supportfortheSourceSpecificMulticast(SSM)deployment

modelthatisdiscussedlaterinthischapter.

TheMLDv2querymessageperformsthesamefunctionsasits



MLDv1counterpart.Inaddition,itsupportsmulticast-source

specificqueries.Thecapabilitiesofthereportmessagewere

alsoenhancedforMLDv2.Itconcatenatesasetofrecords,each

recordcontaininginformationthatrelatestoagivenmulticast

address.ThisstructureoffersenoughflexibilitytotheMLDv2

reportmessagetoperformthefunctionoftheMLDv1done

message,too.MLDv2doesnotusedonemessages.



Note

MLDv2isbackwardcompatiblewithMLDv1,andfor

thisreasonitmustsupporttheMLDv1messages,

includingthe131and132types(seeTable6-2).



Figure6-1summarizessomeaspectsoftheMLDv2-governed

listener-routerinteractiononalink.



Figure6-1.ConceptualRepresentationof

Listener-RouterInteractiononaLink



[Viewfullsizeimage]



OnCiscorouters,MLDv2isenabledbydefaultonallinterfaces

assoonasmulticastroutingisgloballyenabledontherouter

(usingtheglobalconfigurationcommandipv6multicastrouting).Example6-1showsanexampleoftheMLD

operationalstatusonaninterface.



Example6-1.MLDOperationalStatusonaRouter

Interface



Router#showipv6mldinterfaceGE-WAN1/1.1

GE-WAN1/1.1isup,lineprotocolisup

Internetaddressis::/10

MLDisenabledoninterface

CurrentMLDversionis2

MLDqueryintervalis125seconds

MLDqueriertimeoutis255seconds

MLDmaxqueryresponsetimeis10seconds

Lastmemberqueryresponseintervalis1seconds

MLDactivity:4joins,0leaves



MLDqueryingrouterisFE80::20D:29FF:FEE1:4DC0(thissystem)



TheMLD-specificparameterssuchasqueryinterval,timeout,or

maxresponsetimethatarehighlightedintheexampleabove

canbeconfiguredundereachinterface.Theoutputalsolists

thequerierforthelink;inthiscase,itis

FE80::20D:29FF:FEE1:4DC0,therouterintheexample.



SourceSpecificMulticastMappingforMLDv1

TheSSMservicemodeldiscussedlaterinthischapterrequiresa

hosttospecifyboththemulticastgroupitintendstojoinand

thespecificsourceitintendstolistento.OnlyMLDv2supports

thisfunctionalityonthehosts.AlthoughSSMisapopular

deploymentmodel,MLDv2isnotcommonlyimplementedon

IPv6stacksatthetimeofthiswriting,soasolutionis

necessarytomakeSSMworkwithMLDv1.Thissolutioniscalled

SSMmappingforMLDv1,anditoperatesintwomodes:

StaticallyconfiguredmappingAsource(S)isstatically

mappedtoagivengroup(G)ontherouter.Therouter

mapsany(*,G)MLDv1reporttoan(S,G)basedonthe

configuredmapping.Thismappingfeatureisoffbydefault

onarouter.Itisenabledwiththeglobalcommandipv6

mldssm-mapenable.Thestaticmappingisconfigured

withtheglobalcommandipv6mldssm-mapstatic

source,wheretheaccesscontrollist

(ACL)identifiesthegroupsmappedtoagivensource.

DynamicallyconfiguredmappingAnAAAArecord(see

Chapter3,"DeliveringIPv6UnicastServices")isconfigured

forGinaDNSserver.WhentherouterreceivesanMLDv1

reportfor(*,G),itdoesareverseDNSlookupqueryingfor



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