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Multicast transport protocols for high speed networks

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Multicast transport protocols for high speed networks
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  MulticastTransportProtocolsforHighSpeedNetworks  SanjoyPaulKrishanK.SabnaniDavidM.KristolAT&TBellLaboratoriesAT&TBellLaboratoriesAT&TBellLaboratoriesHolmdel,NJ07733Holmdel,NJ07733MurrayHill,NJ07974  Abstract Thispaperpresentsthedesignandanalysisofthreereliablemulticasttransportprotocolsforhighspeed networks.Thenoveltyoftheseprotocolsliesinthetechniqueusedincombiningtheacknowledgmentsof individualdestinationsalongtheunderlyingmulticasttreetopreventacknowledgementimplosionandinthetechniqueusedinpreventingunnecessaryretransmis-sionbyperforminglocalmulticasts.Theseproto-colsusetheperiodicexchangeofcompletestateinfor-mationNRS90]betweenthesourceandthedestina-tionsandablock-basedSelectiveRepeatretransmission schemetoimprovetheoverallperformanceinahigh speednetworkingenvironment.Performanceofeach protocolisanalyzedintermsofthroughput,end-to-end delay,buerrequirement,acknowledgmenttracand retransmissiontrac.Basedonthisanalysisandthecomplexityofimplementation,oneofthethreeproto-colsisrecommendedforreliablemulticastinginhigh speednetworks. 1Introduction  Multicastingisanimportantissuewhichneedsto beaddressedinthehighspeednetworks.Severalpapershaveaddressedtheissueofdesigningmul-ticastpacket-switchingnetworksTL90],E88]anda lotofworkhasalsobeendoneinmulticastrout-ingA84]D88],butthedesignofareliablemulti-casttransportprotocolinbroadbandhighspeednet-workshasnotreceivedasmuchattentionuntilrecently AFM92].APR93]discussesthedesignofareliablemulticastprotocolinadistributedenvironment.They donotfocusonthetransportofpacketsonwidearea networksandhencethequestionsofroundtripdelay,acknowledgmenttrac,remulticasttrac,orremul-ticasttimeoutdonotariseintheirdesign,whilethey doinours.ChangandMaxemchukCM84]designed areliablebroadcastprotocolinunreliablebroadcastnetworks.ReliablemulticastprotocolsforsatellitebroadcastchannelsSS85]havealsobeendesigned. Multicasting  isaverybroadtermanddierentmul-ticastingapplicationshave,ingeneral,dierentre-quirements.Forexample,anationwidevideoconfer-encing,whichisareal-timemultipoint-to-multipointmultimediamulticastingapplication,hasverydier-entrequirementsfromanelectronicdistributionofbooksbyapublishertobookstoresacrossthenation,whichisapoint-to-multipointreliabledatatransfermulticastingapplication.Inthispaper,weaddressmulticastingapplicationsofthesecondtypewhichre-quirereliabledatatransportwithhighthroughputand lowend-to-enddelay.However,manyoftheideaspre-sentedinthispapercanbeadaptedforreal-time,mul-timedia,multipoint-to-multipointapplicationsaswell.Inordertoachievehighthroughputandlowend-to-enddelay,weavoid  unnecessaryretransmission  bythesourceusingatechniquecalled  localmulticast .Inad-ditiontothat,weadoptanoveltechniqueof merging acknowledgments fromindividualreceiversalongthemulticasttreetoavoidthe acknowledgmentimplosion  probleminherentinanymulticastingscheme.Weusetheprincipleof periodicandfrequentexchangeofcom-pletestate betweenthetransmitterandthereceiverstoavoidcomplexerrorrecoveryproceduresNRS90].Finallyweuseablock-basedselectiverepeatretrans-missionschemetoachievehighthroughputNRS90].Inthispaper,wepresentthedesignandanalysisofthreemulticasttransportprotocolsforhighspeed networksforpoint-to-multipointmulticastingappli-cationswhichrequirehighthroughput,lowend-to-enddelayandreliabletransportofnon-real-timedata.Theprotocolsareverygeneralinthesensethatthey canbebuiltontopofeithervirtualcircuitnetworksordatagramnetworks.Theonlyserviceexpectedbytheprotocolsfromtheunderlyingnetworkistheestablish-mentofamulticasttreefromthesourcetothedes-tinations.Itdoesnotreallycareifthemulticasttreeissetup(includingresourcereservation)usingST-2PP92],RSVPZDESZ93]oranyconnectionoriented   protocol.Aslongasthetreeexists,theprotocolswillwork.Thefunctionoftheprotocolsistodeliverpack-etsfromthesourcetothedestinationsinsequencealongthemulticasttree,independentofhowthetreeiscreatedandresourcesareallocated.Thismakesourprotocoldeployableinvariousnetworksandthustheycantakeadvantageofanycleverresourceallo-cationtechniqueindatagramnetworkoranyecienttechniqueforsettingupvirtualcircuitsinconnection orientednetworks.InthecontextofbroadbandATM networks,theseprotocolscanbeusedintheAdapta-tionLayerformulticastingapplications.ThethreeprotocolsformulticastingaretheDesig-natedStatusProtocol(DSP),theConsolidatedSta-tusProtocol(CSP)andtheCombinedProtocol(CP).CSPintroducestheconceptofcombiningacknowledg-mentsfromthedestinationsalongthemulticasttree,DSPintroducestheconceptoflocalmulticast(multi-castingtothedestinationsinalocalregionratherthan toalldestinationsinthenetwork)andCPcombinestheideasfrombothprotocols.CPperformsthebestamongallthethreeprotocols.However,thecostpaid intermsofcommunicationandcomputationoverhead doesnotjustifyitsuse.DSP,whichisaclosesecond intermsofperformance,isourrecommendation.Thispaperisorganizedasfollows.Section2dis-cussestheassumptionswemakeinthedesignoftheprotocols.DesignatedStatusProtocol(DSP)isde-scribedinSection3,followedbythedescriptionsofConsolidatedStatusProtocol(CSP)andtheCom-binedProtocol(CP)inSections4and5respectively.Theperformanceoftheprotocolisanalyzedinsection 6,followedbythesimulationresultsinSection7.ThethreeprotocolsarecomparedinSection8,followedby theconclusion. 2Assumptions  Inthissection,welisttheassumptionscommonto allthreeprotocolsdescribedinthepaper.Laterwestatetheassumptionsspecictoeachprotocol.Thecommonassumptionsareasfollows:1.Theendpoints/usersareconnectedtoalocalex-change(LE)eitherdirectlyorthroughaccessnodes.Thelocalexchangesareinterconnected usingabackbonenetwork(Fig.1).2.AhierarchicaladdressingschemelikeE.164 (whichisverysimilartothecurrenttelephonenumberingsystem)isassumed.Thatis,given theaddressofanendpoint,itispossibletoinfer BACKBONENETWORKEi,1Ei,2EEi,4LLsSAccess NodesL - Local Exchange of SourceL - ith. Local Exchangesiii,3 Figure1:ModeloftheNetwork theareaitbelongsto.Forexample,iftheaddressofanendpointis908-317-9128,itcanbeinferred thattheendpointisservedbyareacode908and thatitisanaddressinNewJersey.Similarly,iftheendpointhasanaddress415-394-1127,itispossibletoconcludethatitisservedbyareacode415andthatitisanaddressinCalifornia.Infact,noneoftheaboveassumptionsarenec-essaryingeneral.However,theideaspresented inthispaperareeasytocomprehendwiththeseassumptionsinmind.Allthatisnecessaryhereissomesortof localization  informationwhichneedstobeusedbytheprotocols.Conceptuallythedestinationsina  local regionneedtobetreated asagroupforthepurposeofretransmissionofpackets.Thiseectoflocalizingrequestsforre-transmissioncanbeachieved,forexample,inIP byusingtheTTLeldofanIPpacket.Thede-tailsofhowtheTTLeldcanbeusedforthispurposeisthesubjectofadierentstudy.3.Weassumethatamulticasttreeissetupwiththeallocationofresourcesatthenetworklayer(ATM layerinthecontextofATMnetworks),rooted atthesourceSandspanningallthedestinations(orendpoints).Thisisreferredtoasthe globalmulticasttree inmanyoccasionstodistinguishitfromthe localmulticasttree whichservesasubsetofthedestinationsandisasmallportionoftheglobalmulticasttree.TheglobalmulticasttreeidentiesamulticastvirtualcircuitwhichwecallanMVC.TheMVCisshownbyboldlinesin Fig.2.Theendpointsinthelocalexchange L  i aredenotedby  E  i;j . L  i is not anendpoint.  S 2,2 3,1 3,23,4 4,1 4,24,34,4  3 2,1E 2L E2,3E2,4E E L4L EEE E EE3,3E 1,1E 1L Figure2:GlobalMulticastTreerootedat S  andLocalMulticastTreesrootedat E  i; 1 'sFinally,eachprotocolinthispaperisdescribed intermsofapoint-to-multipointmulticast,al-thoughmultipoint-to-multipointmulticastisalso possibleaslongasthemulticasttreesaresetup foreachsource. 3DesignatedStatusProtocol(DSP) DSPisaprotocolwhichchoosesaparticulardesti-nationineachlocalregionasarepresentativeoftheregionandthatdesignateddestinationisresponsibleforsendingthestatusmessagesonbehalfoftheregion tothesourceandisalsoresponsibleforlocallymul-ticastingapacketifeverydestinationinthatregion doesnotreceivethepacket.Moreformally,thesourcepicksanendpoint E  i; 1 ,whichcanbethoughtofasthe representative ofthegroupof E  i;j 's,ineachlocalexchange L  i (Fig.2). E  i; 1 istheendpointwithin  L  i whichis closest toSwith respecttotheglobalmulticasttreeamongthe E  i;j 'sbelongingto  L  i .This E  i; 1 isresponsibleforsending its own  statustothesource.Thus,ifthereare m  LE'swithnaldestinations(orendpoints),therewillbe m  designatedendpointswhicharesupposedtosendtheirstatustothesource.Infactthestatussentby  E  i; 1 willshowwhichblocks 1 havebeenreceivedby  E  i; 1 .How-ever,itwillnotsaywhichblockshavebeenreceived  1 Ablockisacollectionofpacketsanditistheunitchosenforselectiverepeatretransmissionasdescribedlaterinthepaper.InthecontextofATMnetwork,therewillbethreelevelsofhierarchy:block,packetandcell. bythe E  i;j 'sfor j 6 =1.Each  E  i; 1 willbereferredto asa  localsource (incontrasttotheglobalsource S  )becauseitwillmulticastpacketstothedestinationsin itslocalregion(asopposedtoallthedestinations).Weassumethatsomeestimateofroundtripdelay isavailablebetweenSandeach  E  i; 1 afterthemulti-casttreeissetup.Wewillrefertotheroundtrip delaycorrespondingto  E  i; 1 by  RTD  i .Peakband-width,packetsize,blocksizeandre-orderingbuersizerequiredateachdestinationarealsonegotiated attheconnection-establishmenttime.Wealsoassumethatseveral localmulticasttrees areformedbasedontheglobalmulticasttree.Alo-calmulticasttreerootedat E  i; 1 istheportionoftheglobalmulticasttreespanningthe E  i;j 'sinthe L  i .Lo-calmulticasttreesareshownbydashedlinesinFig.2.Suchalocalmulticasttreewillbeidentiedbyalocalmulticastvirtualcircuitidentier,whichwecallan LMVCI. 3.1Descriptionoftheprotocol Beforegoingtothedetailsofthepacketformatsand thedetaileddescriptionoftheprotocol,wedescribethemainideaoftheprotocolinthissub-section.Theprotocolworksasfollows:1. S  multicastsdatapacketsto  all thedestinations( E  i;j 's 8  i;j )usingtheglobalmulticasttree.Thismulticastwillbecalleda  globalmulticast .2.Each  E  i; 1 sendsits own  statusto  S  intheform ofcontrol(status)packetsatregularintervals.Thestatuspacketcontainstheinformationaboutwhichblockshavebeenreceivedby  E  i; 1 . S  glob-allymulticastsablockagainifthereexistsan  E  i; 1 whichhasnotreceivedtheblockwithintheex-pectedtime.Thistimedurationis,ingeneral,a multipleoftheround-trip-delaybetween  S  and the E  i; 1 inquestion.3.Each  E  i;j ( j 6 =1)sendsitsstatustothecorre-sponding  E  i; 1 atregularintervals. E  i; 1 locally multicastsablockifthereexistsan  E  i;j ( j 6 =1)whichhasnotreceivedtheblockduringglobalmulticast.Notethatan  E  i;j ( j 6 =1)dependson thecorresponding  E  i; 1 fortheretransmissionofa blockifitdidnotreceivetheblockeitherduring theglobalmulticastorduringlocalmulticasts.4. S  willmulticasta  new  blockprovidedeach  E  i; 1 hasavailablebuerspacefornewblocks.Notethatan  E  i; 1 hastwobuers:(1)a  reassembly  buerwhichisusedtoassemblethepacketsit  receivesfrom  S  and(2)a  retransmission  buerwhichisusedtoretainthepackets(blocks)thathavenotbeenreceivedbyallthe E  i;j 'sinits jurisdiction  2 .Ablockistransferredfromthere-assemblybuertotheretransmissionbuerwhen thereisspaceintheretransmissionbuer.Thistransfercreatesspaceinthereassemblybuerof E  i; 1 .If,however,bothbuersarefull,thereisno spacefornewblocks.Thestatusofspaceavail-ableinthereassemblybuerissentto  S  asa partofthestatusmessagefrom  E  i; 1 to  S  sothat S  knowsifitcanmulticastanewblock.Aslightlymodiedversionoftheprotocolinwhich thesourcemulticaststoalltherepresentatives E  i; 1 's(insteadoftoallthe E  i;j 'sforall i , j )andthe E  i; 1 'smulticasttothe E  i;j 's( j 6 =1),therebydividingtheconnectionalongeachsourcedestinationpairintotwo segments:onefromthesourceto  E  i; 1 andtheotherfromthe E  i; 1 to  E  i;j ( j 6 =1)canalsoconsidered.How-ever,wedonotconsiderithere.Theprotocolinvolvesseveraltypesofcontrolpack-etsanddatapacketsinadditiontothedatastruc-tureswhicharemodiedonthereceptionofthecon-trol/datapackets.Inthispaper,theformatsofcon-trolanddatapacketsaregiven.Thedetailsofdata structurescanbefoundinPSL94]. 3.2Formatofcontrolanddatapackets Inthissection,wegivethedetaileddescriptionofthepacketsinvolvedintheprotocol.Thefollowing packetsareusedintheprotocol:1.Acontrol(status)packetfroman  E  i; 1 tothesource.2.Acontrol(status)packetfroman  E  i;j ( j 6 =1)to  E  i; 1 .3.Amulticastdatapacketfromsource S  to  E  i;j 's 8  i;j .4.Amulticastdatapacketfroman  E  i; 1 to  E  i;j 8  j ( j 6 =1).ThenotationgiveninTable1isusedtorepresenttheeldsofthecontrolanddatapackets.NotethatMVCI(MulticastVirtualCircuitIdentier)isremi-niscentofVCI(VirtualCircuitIdentier)usedincon-nectionorientednetworks.However,MVCI,asused inthispaper,ismoregeneralanditsimplyidentiesaspecicmulticastconnectionwhichmayuseastatic 2 E  i;j 'sservedbyan  L  i aresaidtobeinthesamejurisdiction. allocationofresourcesattheconnectionestablishmenttimeoradynamicallocationofresourcesasdoneby RSVPZDESZ93].ThepacketformatsareshowninFig.3. MVCI Type = 0 Seq# k LW LOB Buffer_available Error Check EPILMVCI Type = 2 Seq# DATA Error CheckMVCI Type = 3 Seq# DATA Error CheckStatus Packet from E to SourceStatus Packet from E (j=/=1) to EData Packet from E to E (j=/=1)Data Packet from Source to E for all i, ji,1i,ji,ji,ji,1i,1rLMVCI Type = 1 Seq# k LW LOB Buffer_available Error Check LEPIr Figure3:PacketFormats 4ConsolidatedStatusProtocol(CSP) InDSP,an  E  i; 1 ischosenateach  L  i andthatdes-ignatedendpointsendsits own  statusmessagetothesource S  .InCSP,astatusmessageissenttothesourcebyeachlocalexchange L  i andthisstatusmes-sageisa  consolidated  statusmessageofalltheend-points/destinationsinitsjurisdiction.Thus,CSPusesthelocalexchangewhileDSPusesarepresentativedestinationinalocalregiontosendstatusmessagestothesourceS.Alsothedesignateddestinationin DSPsendsitsownstatuswhilethelocalexchangein CSPsendsthestatusofallthedestinationsinthelocalregioninacompact(consolidated)form.Theprotocolworksasfollows:1. S  multicastsablocktoallthedestinations E  i;j 's 8  i;j usingthemulticasttreewhichissetupattheconnectionestablishmentphase.2.The E  i;j 'ssendtheirstatusatregularintervalstothecorrespondinglocalexchanges L  i 's.Theformatofthestatusmessageisthesameasthatofthestatusmessagesentby  E  i;j 's(for j 6 =1)to  E  i; 1 inDSP.3. L  i consolidatesthestatusmessagesfromthe E  i;j 'sinitsjurisdiction.Periodically, L  i sends  NotationExplanation  MVCIamulticastvirtualcircuitidentier.LMVCIalocalmulticastvirtualcircuitidentier.Typetypeofapacket.type=0indicatesastatuspacketfrom  E  i; 1 tothesource.type=1indicatesastatuspacketfrom  E  i;j ( j 6 =1)to  E  i; 1 .type=2indicatesadatapacketfrom  E  i; 1 to  E  i;j ( j 6 =1).type=3indicatesadatapacketfrom  S  to  E  i;j 8  i;j .Seq#sequencenumberofapacket.Itisincrementedbysendereachtimeanew packetistransmitted. k  intervalbetweentwosuccessivestatuspacketsinunitsof T  IN  NRS90]. T  IN  =  max  ( RTD=kou;ipt ). kou  isaconstantbetween2and32. ipt istheintervalbetweentwosuccessivepackettransmissions. LW  r lowerendofwindowinreceiver,thatis,themaximumsequencenumberoftheblockbelowwhicheverypacketin everyblockhasbeencorrectly receivedasknownatthereceiver.Inthecontextoftheglobalmulticasttree, E  i; 1 'saretreatedasreceivers.Forlocalmulticasttrees, E  i;j 's( j 6 =1)aretreatedasreceivers.Lthelargestallowednumberofoutstandingblockschosenattheconnectionestablishmenttime.An outstandingblockisablockthathasbeenmulticastatleastoncebutnotyetbeenacknowledgedbyallthe E  i; 1 's.LOBlistofoutstandingblocks.Abitmap representingtheoutstandingblocksbetween  LW  r and( LW  r +  L  ?  1).Bueranintegerrepresentingthespaceavailableavailableinthe reassembly  buerofan  E  i; 1 intermsofthenumberofblocks.EPIanendpointidentierwhichisusedto identifyan  E  i; 1 .LEPIalocalendpointidentierwhichisusedtoidentifyan  E  i;j ( j 6 =1).Table1:ExplanationofNotationusedinPacketFor-matsitsconsolidatedstatusto  S  .Wewilldescribenexthow  L  i preparesaconsol-idated  LW  r ,aconsolidated  LOB  andaconsol-idated  bufferavailable fromthecorresponding eldsofindividualstatusmessagesof E  i;j 's.Firstwedescribehowtheconsolidated  LW  r isformed. LW  r ( consolidated ) =min( LW  ( j ) r )where LW  ( j ) r isthe LW  r eldofthestatusmessageof E  i;j to  L  i .Thatis, L  i choosesthelowestvalueof LW  r amongthe LW  r 'softhe E  i;j 'sstatusmessagesasthe LW  r ( consolidated ) .The LOB  consolidated isa bitwiseANDofthe properlyaligned  LOB  'softhe E  i;j 's.The bufferavailable eldintheconsolidated statusof L  i istheminimumofthe bufferavailable valuesatthe E  i;j 'sinitsjuris-diction. L  i sendstheconsolidatedstatusofthedestina-tionsinitsjurisdictionto  S  .Theformatofthisstatusmessageisexactlythesameasthatofthestatusmessagesentto  S  bythe E  i; 1 'sinDSP.4. S  re-multicastsablocktoallthedestinationsifthereexistsan  E  i;j thathasnotreceivedtheblock withintheexpectedtime.However, S  couldre-transmittheblockonlytothespecic E  i;j ,who hasnotreceivedtheblock,providedpoint-to-pointconnectionswereavailableinadditionto thepoint-to-multipointconnections.5. S  multicastsa  new  blockprovidedbuerspaceisavailableineach  L  i forreceivinganewblock.Here,ifeveryendpoint E  i;j inthelocalexchange L  i hasbueravailableforanewblock,wesay thatthe L  i hasbuerspaceavailableforanew block.Notethatwearetryingtoavoidusingthe L  i 'sas store and  forward  nodesbecausetheassembly ofpacketsatthe L  i 'swillintroduceunnecessary delayandunnecessarycomplexityattheswitches. 5CombinedProtocol(CP) CPcombinestheconceptof localmulticasting  from DSPwiththenotionofsendinga  consolidatedstatusmessage fromCSP.ThusCPusesthelocalexchange(likeCSP)tocombinethestatusmessagesofthedesti-nationsinthelocalregionandalsousesadestination ineachlocalregion(likeDSP)tolocallymulticast
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