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Multicast

Mul)cast Waysofaddressing:

  unicast(traditional):transmissiontoasingledestinationIPaddress(uniqueontheInternet/localnetwork)

  broadcast:addressing"allreceivers"inasubnetwork(e.g.lookingforarouterorserver,urgentmessage);doesn'tdeliverpacketsoutsidethenetwork

  Howtotransmitonlytoaselectedgroupofaddresses,evenoutsidethelocalnetwork?  multicastaddressingallowsdeliverytogroupsofreceiversregardlessofthebordersofthesubnetworks

  IGMP(InternetGroupManagementProtocol)isusedformanagingthosegroups

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Mul)cast

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Mul)cast‐example

Wewanttotransmitto4of6computersinanetwork.How?

1.   unicast:weneed6copiesofthesamepacket;multipletransmissionscanoverloadthemedium.

2.  broadcast:addressallcomputers;filteringtherightreceiversisleftuptohigherlayerprotocols.

3.  multicast:wetransmittoa"special"addressrepresentingaGROUPofreceiversthatlistentothepacketstargetedatthataddress

  similartobroadcast:everyonereceivesthepacket  but:filteringoccursatthenetworklevel‐IP

(sometimesevenatthedata‐linklayer)

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Mul)cast:packetrou)ng   broadcastpacketsarenotforwardedbyrouters(everyonewouldreceivethem!),

meaningtheystayinsidethelocalnetwork

  multicastroutingispractical:asinglepacketisreplicatedbytherouterandforwardedonlythroughthoseinterfaceswheretherearelistenerstothatpacket.Groupnamesare32bitnumbers(almost).

  Challengesfortheprotocol:  findingoutwherethepacketreceiversare,  multicastrequiresadditionalwork:routingprotocols,forwardinginformationabout

thelisteners,  multicastaddressesdon'tforma(sub)network‐>themaskhas32bits.Thereforethey

requirespecialinputinroutingtables  challenge:theycanalsohavemorespecialinputs.Why?

  security:aneavesdropper(illicitlistener)cansubscribetolisteningthepacketsandthusbecomealegitimatereceiver

  whattodowhenonlyonereceiversignalizesitdidn'treceivethepacket

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Mul)cast

subscribingtomulticasttraffic(IGMP)

multicastrouting(PIM)

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Mul)castapplica)ons  sendinglargefilesoveranetwork(centralofficeto

affiliates)‐reliabletransmission  softwareupdatesinalargenetwork  datastreaming(e.g.sendinginformationaboutshares

toallfinancialcompanies  audio/videostreaming  videoondemand(watchingaTVprogram)  conferenceholding(consideration:betteruseofa

conferencecenterthatdecideswhocanspeakandwhosepacketsaretobeforwardedtoothers)  challenge:thinkabouthowaconferenceisheldusing

amulticastapproach  real‐timeapplicationswithRTP,whichisusedfor

ensuringcontinuousandqualitydeliverieswithinenvironmentsthatusemulticast

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IPv4addressing   multicastgroupnamesareactuallyspeciallyreservedIPv4addresses:

224.0.0.0 - 239.255.255.255 (classD)  Specialaddressesinsidethatrange:

Addressrange Description

224.0.0.0 - 224.0.0.255 Reservedforwell‐knownmulticastaddresses

224.0.0.1 Allsystems(interfacesandrouters)

224.0.0.2 Allrouters

224.0.1.0 - 238.255.255.255 Globally‐scopedmulticastaddresses(Internet)

239.0.0.0 - 239.255.255.255 Locally‐scopedmulticastaddresses(localnetwork)

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IPv6addressing

1.)multicastgroupnamesare128bitnumbers‐IPv6address,startingwithFF

2.)FF02::1(linklocal:allINTERFACES)3.)FF02::2(linklocal:allROUTERS)4.)IPv6addressstructure:

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Addressmapping  EthernetandFDDIframeworksuse48bitaddresses.Mul)castgroupaddressesrange

from01‐00‐5e‐00‐00‐00to01‐00‐5e‐ff‐ff‐ff.  The01‐00‐5eprefixrepresentsthemul)castframe,thenextbitis0,andtherest23

bitscons)tutethenameofthemul)castgroup.

  sincemul)castIPaddressesarecomprisedof28variablebits,themappingisn'tunique!Onlythe23low‐orderbitsaremappedtotheframe.Thatmeansthat32(25)addressesaremappedtothesameaddressonthesecondlayer.  challenge:whatdoestherouterhavetodothen?

  Thenetworklayerdecideswhetherthedatagramsareimportantforreceivingornot.

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IGMPprotocol  networklayerprotocolIPv4,protocolnumber2  RFC2236,InternetGroupManagementProtocol,Version2,RFC3376,

InternetGroupManagementProtocol,Version3  required:finditontheInternetandreadit‐furtherreading!  challenge:findtheotherRFCdocumentsrelatedtoIGMP

  IGMPtakescareofmanagingwhothemulticastreceiversare.Itallows:  establishinggroupmemberships  leavingagroup  detectingotherinterfacesinthegroup

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IGMPprotocol  theIGMPcommunicationoccursbetweenahostandanimmediately‐

neighboringmulticastrouter

  routersgetthetaskofconnectingtothemulticasttreestructurebasedontheIGMPprotocol

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IGMPversionsThereare3versions:IGMPv1,v2andv3.

  IGMPv1:Interfacescanconnecttogroups.Therearenomessagesforleavingagroup.Routersusetimeoutstodetectgroupsofnoconcernfortheinterface.

  IGMPv2:Messagesforleavingagroupareadded.Thatallowsforfasternotificationaboutunnecessarytraffictermination.

  IGMPv3:Biggerchangesintheprotocol.InterfacescandetermineaLISTofotherinterfacesfromwhichtheywishtoreceivetraffic.Thenetworkblocksalltrafficfromotherinterfaces.

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IGMPprotocol  IGMPmessagesare8byteslong

  type–typeofthemessage:  17 (0x11): Group Membership Query   18 (0x12): Group Membership Report IGMP v1   22 (0x16): Group Membership Report IGMP v2   34 (0x22): Group Membership Report IGMP v3

  23 (0x17): Leave Group Report IGMP v2

  responsetime‐maximumtimeallowedforanIGMPGroupMembershipQueryrecipienttorespond

  checksum‐(doesn’tcovertheIPheader)  multicastgroupaddress‐IPv4addressofthemulticastgroup

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IGMPprotocol

Ac#on IGMPmessage IPDes#na#onAddressIGMPMul#castGroupAddress

joinagroup GroupMembershipReport groupaddress groupaddress

listofgroupmembers GroupMembershipQuery groupaddress groupaddress

listofexis#nggroups GroupMembershipQuery allinterfaces(224.0.0.1) 0.0.0.0

acknowledgebeingamemberofthequeriedgroup GroupMembershipReport groupaddress groupaddress

leavethegroup GroupLeaveReport allrouters(224.0.0.2)groupaddress

  HowtoaccomplishgroupmanagementwithIGMP

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IGMPProtocol  Specialmessage:IGMPv3GroupMembershipreport

  Type=0x22  theresponsesfromallinterfacesinthegroupareinthesamepacket  theinterfacewaitsfortheresponsesoftheotherrecipientsinthegroupbefore

itanswers  thespecialformatofthepackageensurestheavoidanceofmultipliedmulticasttraffic

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IGMPprotocol:subscrip)ontoasource  forjoiningagroup,aGMRmessageissentwithvalueTTL=1(deliveredonlyto

thenearestrouter)  therouterrecognizesthatitmustforwardthegrouppacketstothenew

subscriber(how?mappedmulticastaddress/datagramcopiestotheIPaddress)

  therouterinformstheneighboringroutersthatithasanewsubscriber.Ifeveryrouterweretopassthesamemessageonwards,therewouldbeaproblem‐thepacketswouldbecross‐forwardedacrossallconnectionsinthenetwork.Solutions:  useoftheRPLalgorithm(ReversePathLookup):werejectallmulticast

packetscomingfromroutersthatdon'tconnecttothesourceofthepacketthroughtheshortestpath

  routershavespecialroutingprotocolsformulticasttraffic:e.g.thePIM‐SMprotocol(ProtocolIndependentMulticast‐SparseMode)

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Reversepathlookup:example

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3 4

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XY

ZC

B

A

rejects

rejects20

MLDprotocol  MulticastListenerDiscovery,RFC2710,MulticastListenerDiscovery

(MLD)forIPv6  required:finditontheInternetandreadit‐furtherreading!  challenge:lookforthedifferencesbetweenMLDandIGMP  challenge:whataboutthecoexistenceofIGMP(IPv4)andMLD(IPv6)?

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MLDprotocol  Basicallyit'samulticastprotocolforIPv6andhasthesamefunctionality

asIGMP

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IGMPandMLDprotocol  MLD:

  IGMP:

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0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Type | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Maximum Response Delay | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| | + +

| | + Multicast Address +

| | + +

| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Type | Max Resp Time | Checksum |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Group Address |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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Trafficmul)cast   packetsmoveintheformofmulticasttrees  atreecanoptimizedifferentcriteria:

  figure1:totalpathlength(numberofhops)ofalldatagrams  figure2:shortestpathforeverydatagramseparately(minimumspanningtree)

8hopstotal,thedatagrammakes6hops

9hopstotal,thedatagrammakes5hops

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Mul)castrou)ng  Dutyoftherouting:findatreeofconnectionsthatconnectsallrouters

inthesamemulticastgroup  Forcommunicationbetweenroutersweneedmulticastrouting

algorithms(workingatthenetworklayer),like:PIM,DVMRP,MOSFPandBGP.

howtoconnecttheredroutersintoacommontree?

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Twosolu)onsforfindingamul)casttree  usingasingletreeforallroutersforroutingmulticasttraffic,wefindasingle

tree(group‐sharedtree)‐leftfigure

  determiningaseparatetreeforeverymemberinthegroup(source‐basedtree);forNmembersofthegroupwehaveNtrees(foreverymulticastgroup)‐rightfigure

separatetreeforA(blue)andtreeforB(pink)

commontree

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Determiningacommontree(group‐shared)1.  findingatreewiththeminimaltotalcost(Steiner'salgorithmisusedfor

spanningtrees;theproblemisNP‐hard),leftfigureor

2.  determiningthecentralnode("rendez‐vouspoint")(theunicastroutingrulesdefinehowtoroutetoit);therouterjoinsthetreewhen,onitswaytothecentralnode,itencountersthefirstnodealreadyinthetree,rightfigure

minimaltotalcost Eisthecentralnode

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Determiningtreesforseparatesenders(source‐based)

1.  Findingtheshortestpathtreeinagraph(usingDijkstra'salgorithmwhichconstructsatreeoftheshortestconnections(edges)relativetoagivenstartingnode),leftfigure

-  theroutershavetoknowthestatesofallconnections(edges)(link‐state)

or2.  UsingRPL(ReversePathLookup):doesn'tacceptmessagesfromrouters

thataren'tontheshortestpathtothesourceofthemessage,figureright

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Rou)ngprotocols  theyhandlethecommunicationbetweenroutersinanetwork

  dividedby2criteria(2x2=4groups)1.  sparse‐mode/dense‐mode

  sparse‐mode:certainnodesrequireinclusiontothetree(pullprinciple)  dense‐mode:wemulticastthemulticastpacketsaroundtheentirenetwork,

androutersarecutoffifthey'reunneeded(pushprinciple).Twoways:  broadcastandprune(usespruneandgraftmessages):thestructureisperiodically

reinitialized  domain‐widereporting(routersregisterclientsonthetrafficwithbroadcasting)

2.  intra(withinadomain)/inter‐domain(betweendomains)

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Rou)ngprotocols

Protocol Mode Treetype Type

PIM‐SM sparse common intraandinter‐domain

PIM‐DM dense separate intra‐domain

CBT sparse common intraandinter‐domain

MOSPF dense separate intra‐domain

BGMP dense separate intra‐domain

DVMRP dense separate intraandinter‐domain

  thereisaconnectionbetweentheoperationmodeandthetypeoftreebuiltbytheprotocol

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PIM‐SM(ProtocolIndependentMul8cast‐SparseMode)  PIM‐DM:dense‐mode,separatetree  PIM‐SM:sparse‐mode,commontree,occasionallyseparate

  challenge:readRFC4601andstudyit

  protocolsPIM‐SMandPIM‐DMaresuitableforroutersthatarealreadyrunningunicastrouting.Theyareindependentfromtheunicastprotocol

 messagesusetheIPnetworkprotocolwithprotocolnumber103

 messagesbetweenroutersareunicastormulticasttotheaddress224.0.0.13(allPIMrouters)

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PIM‐SMopera)on

architectureestablishment

• candidatebootstraprouters(c‐BSR)announcetheirpresence(messagetypeBOOTSTRAP)anddesignatethemainbootstraprouterBSR

• candidatecentral(rendezvous)routers(c‐RP)announcetheirpresencetotheBSRrouter(messagetypeCANDIDATE‐RP‐ADVERTISEMENT)

• TheBSRdesignatesthefinalrendezvousrouter(RP)foreverygroupandannouncesthemwithmessagesoftypeBOOTSTRAP

datatransmission

• PIM‐SMroutersdetecteachotherandmaintaincommunicationwithHELLOmessages• theinterfacethatsendsinformationtothegroupaddressmulticastsadatagramtothelocalsegmentofthenetwork• thedesignatedrouteronthenetworkencapsulatesthedatagraminaREGISTERtypemessageandsendsittotheRP• theRPdecapsulatesthedatagramandmulticastsitacrossthemulticasttree

subscriptionmaintenance

• whentheRPdetectstherearenomorerecipientsinthegroup,itsendsaREGISTER‐STOPmessagetoalldesignatedrouters• whenanewuserwantstojoinagroup,itsendsaJOIN/PRUNEmessagewithalistofalldesiredgroupsandallowedrecipientsprejemnikov

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Packetformat‐headercontent

  Theheaderis32bitslong  version=2  type:

value meaning

0 hello

1 register

2 registerstop

3 join/prune

4 bootstrap

5 assert

6candidate‐rp‐adver)sement

HEADER

packettypeHELLO

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PIM‐SMpacketformat‐HELLOpacket

  intendedforconnectionmaintenancebetweenrouters  iftherouterdesignatedfortransmittingmulticasttrafficdoesn't

respond,anotheroneisdesignated  thepacketcontainsasetofTLVvalues,suchase.g.timeout

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PIM‐SMpacketformat‐REGISTERandREGISTER‐STOP

  theREGISTERmessagecarriesthecontentsofthemulticastmessagetothecentralroute(unicast)  B(borderrouter)‐themessage

reachedtherouterdirectlyconnectedtotheinterface,

  N(null)‐thepacketisempty,forestablishingalink

  theREGISTERSTOPmessageissentbytherendezvousroutertothedesignatedroutertosignalnottosendanymessages(norecipients/alreadyreceivingmessagesfromelsewhere)

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PIM‐SMpacketformat‐JOIN/PRUNE

  allowsthehostto(un)subscribetoreceivingmul)casttraffic

  PIM‐SM'sNumberofPrunedSourcesis0(becauseitusesacommontree)

  (Un)subscrip)onfields:  EncodedJoinSourceAddress  EncodedPrunedSourceAddress

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Otherrou)ngprotocols

 MOSPF  MulticastOSPF  added:aspecialpacketformatthatsharesinformationaboutmulticasttraffic  challenge:findtheRFCdocumentsthatdescribeMOSPFandread

them!

  DVMRP  DistanceVectorMulticastRoutingProtocol  transmittedthroughIGMPpackets(type13)

  challenge:readRFC1075andstudyhowtheprotocolworks

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MBONE  connectionsbetweennetworkscapableofmulticasttraffic

  atfirstinsidetheInternet,usedbyworkstationswithvirtualconnections

  challenge:readRFC2715

  1995:MBONEcontains901routers(DVMRPisused)andit'spresentin20countries

  1999:4178routers,increaseduseofRTP,serviceprovidersbecomeoverloaded

  IETFsetsuptheMBONEtaskforcewiththetaskofestablishingmulticastroutingacrosstheentireInternet(developmentoftheMSDPprotocol:MulticastSourceDiscoveryProtocol)

  challenge:readRFC1112,whatisAnySourceMulticastarchitecture(ASM)?

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Wecon)nuenext)me!

  authentication,authorizationandaccounting‐AAA!

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