RFC2327SDPSession Description Protocol文档格式.docx
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Internetcommunity,andrequestsdiscussionandsuggestionsfor
improvements.Pleaserefertothecurrenteditionofthe"
Internet
OfficialProtocolStandards"
(STD1)forthestandardizationstate
andstatusofthisprotocol.Distributionofthismemoisunlimited.
CopyrightNotice
Copyright(C)TheInternetSociety(1998).AllRightsReserved.
Abstract
ThisdocumentdefinestheSessionDescriptionProtocol,SDP.SDPis
intendedfordescribingmultimediasessionsforthepurposesof
sessionannouncement,sessioninvitation,andotherformsof
multimediasessioninitiation.
ThisdocumentisaproductoftheMultipartyMultimediaSession
Control(MMUSIC)workinggroupoftheInternetEngineeringTask
Force.Commentsaresolicitedandshouldbeaddressedtotheworking
group'
smailinglistatconfctrl@isi.eduand/ortheauthors.
1.Introduction
OntheInternetmulticastbackbone(Mbone),asessiondirectorytool
isusedtoadvertisemultimediaconferencesandcommunicatethe
conferenceaddressesandconferencetool-specificinformation
necessaryforparticipation.Thisdocumentdefinesasession
descriptionprotocolforthispurpose,andforgeneralreal-time
multimediasessiondescriptionpurposes.Thismemodoesnotdescribe
multicastaddressallocationorthedistributionofSDPmessagesin
detail.Thesearedescribedinaccompanyingmemos.SDPisnot
intendedfornegotiationofmediaencodings.
2.Background
TheMboneisthepartoftheinternetthatsupportsIPmulticast,and
thuspermitsefficientmany-to-manycommunication.Itisused
extensivelyformultimediaconferencing.Suchconferencesusually
havethepropertythattightcoordinationofconferencemembershipis
notnecessary;
toreceiveaconference,auseratanMbonesiteonly
hastoknowtheconference'
smulticastgroupaddressandtheUDP
portsfortheconferencedatastreams.
Sessiondirectoriesassisttheadvertisementofconferencesessions
andcommunicatetherelevantconferencesetupinformationto
prospectiveparticipants.SDPisdesignedtoconveysuchinformation
torecipients.SDPispurelyaformatforsessiondescription-it
doesnotincorporateatransportprotocol,andisintendedtouse
differenttransportprotocolsasappropriateincludingtheSession
AnnouncementProtocol[4],SessionInitiationProtocol[11],Real-
TimeStreamingProtocol[12],electronicmailusingtheMIME
extensions,andtheHypertextTransportProtocol.
SDPisintendedtobegeneralpurposesothatitcanbeusedfora
widerrangeofnetworkenvironmentsandapplicationsthanjust
multicastsessiondirectories.However,itisnotintendedto
supportnegotiationofsessioncontentormediaencodings-thisis
viewedasoutsidethescopeofsessiondescription.
3.GlossaryofTerms
Thefollowingtermsareusedinthisdocument,andhavespecific
meaningwithinthecontextofthisdocument.
Conference
Amultimediaconferenceisasetoftwoormorecommunicatingusers
alongwiththesoftwaretheyareusingtocommunicate.
Session
Amultimediasessionisasetofmultimediasendersandreceivers
andthedatastreamsflowingfromsenderstoreceivers.A
multimediaconferenceisanexampleofamultimediasession.
SessionAdvertisement
Seesessionannouncement.
SessionAnnouncement
Asessionannouncementisamechanismbywhichasession
descriptionisconveyedtousersinaproactivefashion,i.e.,the
sessiondescriptionwasnotexplicitlyrequestedbytheuser.
SessionDescription
Awelldefinedformatforconveyingsufficientinformationto
discoverandparticipateinamultimediasession.
3.1.Terminology
Thekeywords"
MUST"
"
MUSTNOT"
REQUIRED"
SHALL"
SHALLNOT"
"
SHOULD"
SHOULDNOT"
RECOMMENDED"
MAY"
and"
OPTIONAL"
inthis
documentaretobeinterpretedasdescribedinRFC2119.
4.SDPUsage
4.1.MulticastAnnouncements
SDPisasessiondescriptionprotocolformultimediasessions.A
commonmodeofusageisforaclienttoannounceaconferencesession
byperiodicallymulticastinganannouncementpackettoawellknown
multicastaddressandportusingtheSessionAnnouncementProtocol
(SAP).
SAPpacketsareUDPpacketswiththefollowingformat:
|--------------------|
|SAPheader|
|textpayload|
|//////////
TheheaderistheSessionAnnouncementProtocolheader.SAPis
describedinmoredetailinacompanionmemo[4]
ThetextpayloadisanSDPsessiondescription,asdescribedinthis
memo.Thetextpayloadshouldbenogreaterthan1Kbyteinlength.
IfannouncedbySAP,onlyonesessionannouncementispermittedina
singlepacket.
4.2.E