transmission of audio/video byte streams in ... (Motorola, Cisco, Nortel Networks) with the goal of ..... over the Internetâ, Proceedings of IEEE Conference on.
InvestigationoMPEG-4 f VideoStreamingover SCTP AlexBalk,Marc Sigler,Mario Gerla,M.Y.Sanadidi NetworkResearchLaboratory,ComputerScienceDepartment,U LosAngeles,CA 90024-1596 {abalk,shill,gerla,medy}@cs.ucla.edu
ABSTRACT TheStreamControlTransmissionProtocol(SCTP)isanew transportlayerprotocoldefinedinRFC2960.Itwasdeveloped byresearchersfromindustryanduniversitieswiththefocusof transportingtelecommunicationssignalinginformationoverIP basednetworks.Itisawindowbased,reliableprotocolwitha congestioncontrolmechanismsimilartoTCP,butiat lsoha severalnewfeaturesthatmakeitwellsuitedforstreami mediacontent. NewfeaturesprovidedbySCTPincludemultiplestreams withinanassociationbetweentwoendpoints,bundlingodata f andcontrolinformationtogetherinthesamepacket,and flexiblepacketdelivery (notstrictly inorder). Inthispape useNS-2simulationtoexplorethenewfeaturesoSf CTPand compare the performance with existing techniques for streaming media.WeimplementourownapplicationinNS-2thatis “SCTPaware”inordertousetheSCTPAPI totakeadvantag ofencodingpropertiesotfheMPEG-4formatandattemptto deliverbetterquality video.
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resultingfromitsmechanismsforreliabledatadelivery. SeveralimplementationsoS f CTParefreelyavailableon Internetincodeform[3]. SCTPwasintendedtorunoverIP, andtobeimplementedentirelyincommunicationendpoints, usuallyathe t operatingsystemlevel. SCTPassumesnoQ functionality inrouters,no activequeue management (AQM) or explicitcongestionnotification(ECN).Itcould,however, benefitfromsuchnetworklevelfacilities.Inthispaper,we focusonlyonthecaseorfealtimestreaminginastrict effort network, and dnoot address possible interactions between SCTPandAQMorECN. ThemainnewfeatureoSCTP f itshe concept of an whichisageneralizationoftheTCPconnection.An associationbetweentwoSCTPendpointsiaconnection s over whichbothapplication data and control data can bteransported, withflexiblereliability andorderof delivery.
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Weshowthatintypical a networktopology,severalfeature SCTPcanbeexploitedtodelivermoreoftheMPEG-4 bitstreamtothereceiver.WeconcludethatSCTPcan potentiallyprovideasignificant performancebenefitto streaming video applications. Directionsfor future resear also considered.
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Theassociationconcept inSCTPincorporatescommon functionalitythatwouldnormallyberequiredbymany streamingapplicationsandprovidesanAPIfortheapplication tousetoaccessthefunctionality. Thiscanmakethetasko implementingstreamingapplicationsothe f futuremucheasier. TheSCTPAPIcanbeseenasprovidingstreamingmedia transportmuchliketheTCPAPI providestransportforreliabl data.
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ch are 2.OVERVIEWOFSCTP
Keywords:MPEG-4,VideoStreaming,Real-time,Congestion Control.
FEATURES
PacketsinSCTParedividedintochunks,andeachchunkmay belongtoadifferentlogicalstreamwithintheassociatio Figure1showstheorganizationopafacket.
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1.INTRODUCTION AwidelyheldvisionoftheInternetotfhenearfutureias s mediumofdistributionforfilm,television,sportingevents, digitized lectures, and other content that requires the transmission of audio/video byte streams in However,thelargescaledeploymentofstreamingapplicati isproblematic: currently thereino stransport layer protoc providesaservicemodelwhichfitstherequirementsorfeal timestreams. Thefullyreliableend-to-endsemanticsof donotsuitthelosstolerantanddelaysensitivenatureof multimediastreaming. Inthispaperweassesstheapplicabili oftheStreamControlTransmissionProtocol(SCTP)to streaming pre-recordedMPEG-4encodedvideo. SCTPisanewtransportlayerprotocoldefinedinRFC2960 [1].Itwasmainlydevelopedbyresearchersinindustry (Motorola, Cisco, Nortel Networks) with the goal of overcomingsomeoftheperformancelimitationsofTCP
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real-time. ons ol that TCP ty
Thedatasegmentsotfhechunksmaybeodf ifferentsizes,so longasthetotalpacketsizeislessthanorequaltothel MTU. SeveralchunktypesarepredefinedtheSCTPRFC, includingchunksforinitialization,payloaddata,selective acknowledgements(sacks),etc.
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AnSCTPsenderwillperformchunkbundlingbymultiplexing severalchunksintoonepacketifithastheopportunity. Correspondingly,demultiplexing,or de-bundling,isperformed by the SCTP receiver. There are two key parameters that can be specifiedonper-chunk a basis: reliability andorderof delivery . ReliabilityinSCTPids efinedatshenumberofretransmissio n attemptsforlostpacketsasdeterminedbythesenderfrom examiningthesacks. Areliabilitylevelozf eroindicates no retransmissionsshouldbeattempted,areliabilitylevelof one indicatesamost t oneretransmissionattempt,andsoOr n. der ofdeliverycanbesettoavoidtheso-called head-of-line blockingwhichoccursinTCP. If theunordereddelivery flag is
set,SCTPwillpassdatattohe application itnhe order in w itisreceived,insteadowaiting f forretransmissionswhe is gap a insequencenumbers.
hich nthere
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theframesiGOV. na Errors in the transmission oI-fra f willcausedegradationoqf ualitynotonlyforthatI-frame, alsoinallP-framesandB-framesthatarepredictedfrom frame,aphenomenonknownaspropagationoferrors. Ifthe errorrateonI-framedataihsighenough tcoause the decoder “discard”theI-frame,theplayoutqualityotfhevideostrea willbeaffectedseverely.
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SCTP Common Header
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Figure 1: AnSCTPPacket TheSCTPcongestion control scheme ialmost s identical to NewReno. Acongestionwindowiskeptforeachassociation, andifollows t thewellknownAdditiveIncreaseMultiplicative Decrease(AIMD)algorithmwiththeattendantslowstart, congestionavoidance,fastretransmitandfastrecoveryphas Thisleadstogoodinter-protocolfriendlinesswithTCPand intra-protocolfairness,asstudiedinsomedetailin[4]. However,thecostofusingAIMDishighervarianceinthe packetreceptionrateathe t receiver,whichmustbecountered byappropriatebuffering. Thecharacteristicsotfhebuffering requiredtovercomethefluctuationsinbandwidthseenathe t receiverisoneothe f maintopicsothis f paper.
Figure 2: GroupoVideo f ObjectPlanes(GOV)inMPEG-4. TCP
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3.MPEG-4VIDEOSTREAMINGWITHSCTP Inthissection,weassesstheadvantagesofusingSCTPto deliverrealtime,pre-recorded,non-interactiveMPEG-4vide overunicastconnections. InordertouseSCTPtotransmit videointheNS-2simulationenvironment,weimplementedan “SCTP-aware”applicationthatusestheSCTPAPI to senddata accordingtoatraceoMPEG-4 f framesforthemovie“Jur Park”[11]. Itmaybeusefultobrieflyconsidersomeotfheproperties MPEG-4encodingtounderstandhowSCTPcouldbbeestused fortransmission. Figure depicts 2 taypical sequence oMPE f 4videoframes,knownasaGroupofVideoObjectPlanes (GOV)[13]. Therearethree different types of frames in the MPEG-4 for (1) Intra-coded frames (I-frames) that are encoded independently,andcanbeconsideredreferenceor“key” frames;(2)Predictedframes(P-frames)thatdependon preceding or IP-framesandcontainpredictedmotiondataand errorinformation;and(3)Bi-directionally predictedframes frames)thatdependonboth previousandnextframes. As evidentfrom thefigure,thereihigh as inter-dependency among
Undertheseconsiderations,SCTP’sfeaturesofvariable reliability,outoforderdelivery,andchunkbundlingcanbe usedtooptimizethequalityotfhetransmittedMPEG-4video bitstreamoverbesteffortnetworks.Inthesimulation,we configuretwoscenariosandcomparethemdirectly.Inthefir st case,ourSCTP-awareapplicationsendsI-framedataat reliability level1,whileP and B-frame data isent s at r eliability level0.Thatis,thereceiverrequestsretransmissions( via duplicatesacks)amost t onetimeonalldatachunksbelonging totheI-framestream. Inthesecondcase,noretransmiss ions areattempted;theentirebitstreamistransmittedunreliably . Additionally,unordereddeliveryisenabledforbothcases,so theSCTPreceiverpassesdatattoheapplicationaisarr t ives on theinterface. SCTPsender
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Figure 3: SimulationTopology Withchunkbundling,iitspossibletosendbothI-framedata and B-frame data (for example) in the same packet. This can be beneficialwhenthereislittlemotioninconsecutiveframes resultinginsmall(possibly100byte)frames.Bundlingis enabledinbothcases.TheSCTPsenderreactsaccordinglyfor eachsackreceived:ifduplicatesacksarriveforastream reliability enabled,itwillretransmitasnecessary. Inthesimulationtopology(Figure3),theSCTPsenderand receiversharethebottlenecklinkwithseveralothernodes.A
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mixoTCP f andUDPbackgroundtraffic iinjected s btyhe other nodesintothenetworkstarting a15 t secondsandlasting for15 seconds(i.e.,from 15t3o0seconds). The total simulation t isoneminute.
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establishabout300Kbytesofbuffereddatabeforeplayout. Figure4showsthebufferoccupancywithrespecttotimefor threedifferentvaluesoround f triptime(RTT). ItisclearthatretransmittingI-framedataisadvantage During the congestion episode (between 1a5nd 3s0econds), the bufferis “draining” since the throughput is less than the playout rate. In the30msRTTcase,theretransmissionspreventthe bufferfrombeingunderrun.Inthe50mcsase,althoughbuffer underrunisnotcompletelyprevented,theamountoftime duringwhichtheplayoutwouldbe“jagged”orunsteadyis reduced.
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Figure 5:PercentageoIf-Framedatareceivedwithrespectto round trip time. Solid line indicates I-Frame data retransmissionsareenabled.
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Itshouldbenotedthattherequirementtosettheplayoutstart timea6stecondsaftertheinitiationothe f associationi theneedtocompensatefortherapidlychangingthroughput deliveredbySCTP. TheAIMDalgorithmemployedbySCTP reactstocongestionbyrepeatedlyhalvingitscongestion window, causing the packet reception rate athe t receiver to considerably. Figure5illustratesthepercentageothe f I-f datareceivedunderbothexperimentalscenarios.
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Inthefigure,apacketwithI-framedataicsonsideredrecei ifiat rrivesbeforeitsscheduledplayouttime. Theratio receivedI-framepacketstosentI-framepacketsishownw respectto varying RTT.
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Figure 4:Bufferoccupancyfordifferentvaluesorfoundtrip time. Solid lines represent experimental case where retransmissionoI-frame f dataiesnabled. Dashedlinesdenot casewherenroetransmissionsareattempted.
Thebottleneckcapacityi5s 12Kbps,themeanplayoutrateof the video stream is 270 Kbps, and 400 byte packets are used. At thereceiver,theinitialplayoutstarttimeis6secondsa connectionestablishment,givingthereceiveranopportunity to
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UsingthevariablereliabilityfeaturetoretransmitI-fr amedata resultsihigher na portionothe f totalI-frame data itnhe stream beingreceivedbeforeitsscheduledplayout. Thisholdsforall casesoRTT, f withthegreatestadvantagebeing inthecase of a 50 mRTT. s This leads to the conclusion that the propagation of errorsfrommissingI-framedataisreduced,whichinturn providesmoreconsistentvideo quality. Aquantitivemeasureopf erceivedvideoqualitybyusing,say, theframerateotfheobservedvideoplayoutatthereceive
irs
codecspecificnotpossibleinsimulation. However,these resultsshowthatundergeneralconditions,someadvantagecan begainedusingthevariablereliabilityfeatureofSCTPto retransmitonlydatathatisusedtoconstructI-framesint decoder.
[7]N.Feamster,“AdaptiveDeliveryoReal-Time f Strea Video”,MastersThesis,MITLaboratoryforComputer Science,May 2001. he [8]R.Rejaie,M.Handley,andDEstrin, . “Quality Adaptatio forUnicastAudioandVideo”,ProceedingsofACM SIGCOMM,September1999.
4.CONCLUSIONS Inthispaperwe xaminedvideostreamingsupportofferedby SCTP, naew transport layer protocol. We demonstrated sev advantagesoSCTP. f Namely,thevariablereliability,o delivery,andchunkbundling featurescanbuesedbsytreaming applicationstodelivermoreconsistent,higherquality real streams. SCTPisalsobeneficialfromanapplicationdeveloper standpoint. ItpresentsanAPItotheapplicationlevel,which canbeusedbymanysoftwarestreamingsystems,not necessarilyonlythosefortheMPEG-4format. Bothcontro andapplicationdatacanbesentoverthesameSCTP association,reducingtheoperatingsystemoverheadcommonly associated with starting independent connections for control and data.
eral rderof
[10]RRejaie, . M.Handley,andDEstrin, . “RAP:AnEnd-toEndRate-BasedCongestionControlMechanismforRealtime Streams in the Internet”,Proceedings of INFOCOMM 99,March1999. l
5.REFERENCES [1]RFC2960,http://www.ietf.org/rfc/rfc2960.txt
[3]SCTPdownloadpage,http://www.sctp.de/download/ [4]A.Jungmaeir,MS. chopp,andM.Tuxen,“Performance Evaluationothe f StreamControlTransmissionProtocol”, Proceedingsothe f IEEEConferenceonHighPerformance Switching andRouting,June2000. [5]S.Floyd,M.Handley,J.Padhye,andJ.Widmer, “Equation-Based Congestion Control for Unicast Applications”,ProceedingsoA f CMSIGCOMM,August 2000. [6]N.Feamster,DB . ansal,andH.Balakrishnan,“Onthe InteractionsBetweenLayeredQualityAdaptationand Congestion Control for Streaming Video”, 11th InternationalPacketVideoWorkshop(PV2001),April 2001.
[9]R.Rejaie,M.Handley,andD.Estrin,“Architectural ConsiderationsforPlaybackofQualityAdaptiveVideo overtheInternet”,ProceedingsofIEEEConferenceon Networks,September2000.
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SCTPifsairwithotherSCTPconnectionsand friendly with the TCPprotocol[4]. However,friendlinesscomesasomewha t of t adisadvantage:thethroughputseenbytheSCTPreceiver changescontinuallyduetoAIMDcongestioncontrol. Sizeable buffersmustbeputinplacetoabsorbtheseratevariations. In futurework, we would like tionvestigate either using varia tions ofAIMDcongestioncontrol,e.g.bandwidthestimationadsone inTCPWestwood[2]orequationbasedcongestioncontrol[5]. AdoptingsomeofSCTP’sfavorablefeatures,especially variable reliability tother streaming protocols, possibl [y12], is also being considered.
[2]C.Casetti,M.Gerla,S.S.Lee,S.Mascolo,andM Sanadidi,“TCPwithFasterRecovery”,Proceedingsof MILCOM2000.October2000.
ming
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[11]FF. itzek,andM.Reisslein,“MPEG-4andH.263Video TracesforNetworkPerformanceEvaluation”,Technical UniversityofBerlinTechnicalReport,October2000. http://www-tkn.ee.tu-berlin.de/research/trace/trace.html [12]D.Chanady“LayeredVideoProtocolUsingBandwidth Estimation”, Masters Thesis, UCLA CSD, December 2001. [13]TheMPEGhomepage. http://mpeg.telecomitalialab.com/ [14]DT . anandA.Zahkor,“Real-timeInternetVideoUsing ErrorResilientScalableCompressionandTCP-friendly TransportProtocol”,IEEETransactionsonMultimedia, May 1999.
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