英语翻译

TheSessionInitiationProtocol(SIP)isanapplicationprotocol,definedinRFC2543[4],thatisbeingdesignedbythe

IETFMMUSIC(MultipartyMultimediaSessionControl)workinggrouptoenableuserstoparticipateinmultimedia

sessions,thatis,toestablish,modifyandterminatemultimediasessionscalls.MMUSICworkinggroup[5]focuson

looselycoupledconferencesastheyexiststodayontheMBONE.Oneofthemainissuesinthisareaisrelatedwith

howtoinformusersaboutforthcomingsessions,mediarequirements,addresses,etc.Therearetwobasicwaysto

locateandtoparticipateinamultimediasession:

·Advertisement.Sessionsareadvertisedinvariouswayslikeemail,webpages,newsgroupsoramulticast

advertisementsviaSessionsAnnouncementProtocol(SAP)likeintheMBONE.

·Invitation.UsersareinvitedbyotherstoparticipatebyusingtheSessionInitiationProtocol(SIP).

SIPhasbeenproposedasagenericunicastandmulticastinitiationprotocolandalsoasanIPTelephonycallset-up

protocol.Itisbasedonaclient-serverprotocol.SIPclientssendaRequestMessageforaservice,andaserver

handlestherequest,answeringwithaResponseMessage.SIPterminalscanbothgenerateandreceiverequestas

theyarecomposedbyaUserAgentClient(UAC)andaUserAgentServer(UAS).

SIPterminalscanestablishvoicecallsdirectlywithoutrequiringanyotherelement.Figure6showsanexamplein

whichuser1callsuser2bysendinganINVITERequestprimitivecontaininguser1supportedcapabilitiesforreceiving

audioandaUDPport(port12345andmlawcodec).Whenuser2receivesINIVITERequest,hecanestablishavoice

channelto12345UDPportofuser1whileheacceptstherequestbysendinganOKResponsemessage.Inaddition,

user2responseincludesitsownmediacapabilities,whichareusedbyuser1toestablishavoicechannel(GSMcodec

at54321portintheexample)andsendanACKmessagetoacknowledgeuser2’sresponse.Inordertoterminatethe

connection,anyofthepartiescansendaBYEMessage,whichmustbeacknowledgedbyanOKresponse.