rtpatransportprotocolforrealtimeapplications.docx
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rtpatransportprotocolforrealtimeapplications.docx
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rtpatransportprotocolforrealtimeapplications
RTPATransportProtocolforReal-TimeApplications
NetworkWorkingGroupAudio-VideoTransportWorkingGroup
RequestforComments:
1889H.Schulzrinne
Category:
StandardsTrackGMDFokus
S.Casner
PreceptSoftware,Inc.
R.Frederick
XeroxPaloAltoResearchCenter
V.Jacobson
LawrenceBerkeleyNationalLaboratory
January1996RTP:
ATransportProtocolforReal-TimeApplicationsStatusofthisMemoThisdocumentspecifiesanInternetstandardstrackprotocolforthe
Internetcommunity,andrequestsdiscussionandsuggestionsfor
improvements.Pleaserefertothecurrenteditionofthe"Internet
OfficialProtocolStandards"(STD1)forthestandardizationstate
andstatusofthisprotocol.Distributionofthismemoisunlimited.AbstractThismemorandumdescribesRTP,thereal-timetransportprotocol.RTP
providesend-to-endnetworktransportfunctionssuitablefor
applicationstransmittingreal-timedata,suchasaudio,videoor
simulationdata,overmulticastorunicastnetworkservices.RTPdoes
notaddressresourcereservationanddoesnotguaranteequality-of-
serviceforreal-timeservices.Thedatatransportisaugmentedbya
controlprotocol(RTCP)toallowmonitoringofthedatadeliveryina
mannerscalabletolargemulticastnetworks,andtoprovideminimal
controlandidentificationfunctionality.RTPandRTCParedesigned
tobeindependentoftheunderlyingtransportandnetworklayers.The
protocolsupportstheuseofRTP-leveltranslatorsandmixers.TableofContents1.Introduction........................................3
2.RTPUseScenarios...................................5
2.1SimpleMulticastAudioConference...................5
2.2AudioandVideoConference..........................6
2.3MixersandTranslators..............................6
3.Definitions.........................................7
4.ByteOrder,Alignment,andTimeFormat..............9
5.RTPDataTransferProtocol..........................10
5.1RTPFixedHeaderFields.............................10
5.2MultiplexingRTPSessions...........................13Schulzrinne,etalStandardsTrack[Page1]
RFC1889RTPJanuary19965.3Profile-SpecificModificationstotheRTPHeader.....14
5.3.1RTPHeaderExtension................................14
6.RTPControlProtocol--RTCP........................15
6.1RTCPPacketFormat..................................17
6.2RTCPTransmissionInterval..........................19
6.2.1Maintainingthenumberofsessionmembers...........21
6.2.2Allocationofsourcedescriptionbandwidth..........21
6.3SenderandReceiverReports.........................22
6.3.1SR:
SenderreportRTCPpacket.......................23
6.3.2RR:
ReceiverreportRTCPpacket.....................28
6.3.3Extendingthesenderandreceiverreports...........29
6.3.4Analyzingsenderandreceiverreports...............29
6.4SDES:
SourcedescriptionRTCPpacket................31
6.4.1CNAME:
Canonicalend-pointidentifierSDESitem.....32
6.4.2NAME:
UsernameSDESitem...........................34
6.4.3EMAIL:
ElectronicmailaddressSDESitem............34
6.4.4PHONE:
PhonenumberSDESitem.......................34
6.4.5LOC:
GeographicuserlocationSDESitem.............35
6.4.6TOOL:
ApplicationortoolnameSDESitem............35
6.4.7NOTE:
Notice/statusSDESitem.......................35
6.4.8PRIV:
PrivateextensionsSDESitem..................36
6.5BYE:
GoodbyeRTCPpacket............................37
6.6APP:
Application-definedRTCPpacket................38
7.RTPTranslatorsandMixers..........................39
7.1GeneralDescription.................................39
7.2RTCPProcessinginTranslators......................41
7.3RTCPProcessinginMixers...........................43
7.4CascadedMixers.....................................44
8.SSRCIdentifierAllocationandUse..................44
8.1ProbabilityofCollision............................44
8.2CollisionResolutionandLoopDetection.............45
9.Security............................................49
9.1Confidentiality.....................................49
9.2AuthenticationandMessageIntegrity................50
10.RTPoverNetworkandTransportProtocols............51
11.SummaryofProtocolConstants.......................51
11.1RTCPpackettypes...................................52
11.2SDEStypes..........................................52
12.RTPProfilesandPayloadFormatSpecifications......53
A.Algorithms..........................................56
A.1RTPDataHeaderValidityChecks.....................59
A.2RTCPHeaderValidityChecks.........................63
A.3DeterminingtheNumberofRTPPacketsExpectedand
Lost................................................63
A.4GeneratingSDESRTCPPackets........................64
A.5ParsingRTCPSDESPackets...........................65
A.6GeneratingaRandom32-bitIdentifier...............66
A.7ComputingtheRTCPTransmissionInterval............68Schulzrinne,etalStandardsTrack[Page2]
RFC1889RTPJanuary1996A.8EstimatingtheInterarrivalJitter..................71
B.SecurityConsiderations.............................72
C.AddressesofAuthors................................72
D.Bibliography........................................731.IntroductionThismemorandumspecifiesthereal-timetransportprotocol(RTP),
whichprovidesend-to-enddeliveryservicesfordatawithreal-time
characteristics,suchasinteractiveaudioandvideo.Thoseservices
includepayloadtypeidentification,sequencenumbering,timestamping
anddeliverymonitoring.ApplicationstypicallyrunRTPontopofUDP
tomakeuseofitsmultiplexingandchecksumservices;bothprotocols
contributepartsofthetransportprotocolfunctionality.However,
RTPmaybeusedwithothersuitableunderlyingnetworkortransport
protocols(seeSection10).RTPsupportsdatatransfertomultiple
destinationsusingmulticastdistributionifprovidedbythe
underlyingnetwork.NotethatRTPitselfdoesnotprovideanymechanismtoensuretimely
deliveryorprovideotherquality-of-serviceguarantees,butrelies
onlower-layerservicestodoso.Itdoesnotguaranteedeliveryor
preventout-of-orderdelivery,nordoesitassumethattheunderlying
networkisreliableanddeliverspacketsinsequence.Thesequence
numbersincludedinRTPallowthereceivertoreconstructthe
sender'spacketsequence,butsequencenumbersmightalsobeusedto
determinetheproperlocationofapacket,forexampleinvideo
decoding,withoutnecessarilydecodingpacketsinsequence.WhileRTPisprimarilydesignedtosatisfytheneedsofmulti-
participantmultimediaconferences,itisnotlimitedtothat
particularapplication.Storageofcontinuousdata,interactive
distributedsimulation,activebadge,andcontrolandmeasurement
applicationsmayalsofindRTPapplicable.ThisdocumentdefinesRTP,consistingoftwoclosely-linkedparts:
othereal-timetransportprotocol(RTP),tocarrydatathathas
real-timeproperties.otheRTPcontrolprotocol(RTCP),tomonitorthequalityof
serviceandtoconveyinformationabouttheparticipantsinan
on-goingsession.ThelatteraspectofRTCPmaybesufficient
for"looselycontrolled"sessions,i.e.,wherethereisno
explicitmembershipcontrolandset-up,butitisnot
necessarilyintendedtosupportallofanapplication'scontrol
communicationrequirements.Thisfunctionalitymaybefullyor
partiallysubsumedbyaseparatesessioncontrolprotocol,Schulzrinne,etalStandardsTrack[Page3]
RFC1889RTPJanuary1996whichisbeyondthescopeofthisdocument.RTPrepresentsanewstyleofprotocolfollowingtheprinciplesof
applicationlevelframingandintegratedlayerprocessingproposedby
ClarkandTennenhouse[1].Thatis,RTPisintendedtobemalleable
toprovidetheinformationrequiredbyaparticularapplicationand
willoftenbeintegratedintotheapplicationprocessingratherthan
beingimplementedasaseparatelayer.RTPisaprotocolframework
thatisdeliberatelynotcomplete.Thisdocumentspecifiesthose
functionsexpectedtobecommonacrossalltheapplicationsforwhich
RTPwouldbeappropriate.Unlikeconventionalprotocolsinwhich
additionalfunctionsmightbeaccommodatedbymakingtheprotocol
moregeneralorbyaddinganoptionmechanismthatwouldrequire
parsing,RTPisintendedtobetailoredthroughmodificationsand/or
additionstotheheadersasneeded.ExamplesaregiveninSections
5.3and6.3.3.Therefore,inadditiontothisdocument,acompletespecificationof
RTPforaparticularapplicationwillrequireoneormorecompanion
documents(seeSection12):
oaprofilespecificationdocument,whichdefinesasetof
payloadtypecodesandtheirmappingtopayloadformats(e.g.,
mediaencodings).Aprofilemayalsodefineextensionsor
modificationstoRTPthatarespecifictoaparticularclassof
applications.Typicallyanapplicationwilloperateunderonly
oneprofile.Aprofileforaudioandvideodatamaybefoundin
thecompanionRFCTBD.opayloadformatspecificationdocuments,whichdefinehowa
particularpayload,suchasanaudioorvideoencoding,isto
becarriedinRTP.Adiscussionofreal-timeservicesandalgorithmsfortheir
implementationaswellasbackgrounddiscussiononsomeoftheRTP
designdecisionscanbefoundin[2].SeveralRTPapplications,bothexperimentalandcommercial,have
alreadybeenimplementedfromdraftspecifications.These
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