Understanding SIP INVITE method and messages - Wildix Blog

SIP INVITE method · Calls are started by means of the methods INVITE together with SDP (Session Description Protocol) which carry the information ... Skiptocontent CallsarestartedbymeansofthemethodsINVITEtogetherwithSDP(SessionDescriptionProtocol)whichcarrytheinformationnecessarytoallowtheendpointsofthecallstoexchangeaudioinformofRTP(RealTimeProtocol)packets. Let’sseeatypicalcalldialog: TheINVITEmethodcontainingSDPissenttothecalledpartywhichreplieswithaprovisionalmessageRinging(whichindicatesthattheremoteendpointisringing).BeforeRinging,aTryingisusuallysentbytheSIPProxytopreventthecallerfromretransmittingthemessage. ThecalleecantheneitheracceptthecallwithOKorrefuseitwithanerror.ThecalleecaninterruptthesessionatanytimebysendingaCANCEL. Let’sseeindetailwhichdataareexchangedandwhichheadersareimportanttousduringdebug: INVITEsip:[email protected]:5060;user=phoneSIP/2.0 Via:SIP/2.0/UDP10.135.0.12:5060;branch=z9hG4bKhye0bem20x.nx8hnt Max-Forwards:70 From:"CallingUser"<151>;tag=m3l2hbp To:<001234567890> Call-ID:[email protected] CSeq:10691INVITE Contact:<151>;+sip.instance="" Allow:INVITE,CANCEL,BYE,ACK,REGISTER,OPTIONS,REFER,SUBSCRIBE,NOTIFY,MESSAGE,INFO,PRACK,UPDATE Content-Disposition:session Supported:replaces,100rel User-Agent:WildixW-AIR03.55.00.249c751434072202:76:f3:d6:64 Content-Type:application/sdp Content-Length:254 v=0 o=15196559655INIP410.135.0.12 s=- c=INIP410.135.0.12 t=00 m=audio50024RTP/AVP80218 a=rtpmap:8PCMA/8000 a=rtpmap:0PCMU/8000 a=rtpmap:2G726-32/8000/1 a=rtpmap:18G729/8000 a=ptime:20 a=maxptime:80 a=sendrecv a=rtcp:50025 ThemethodisINVITEandtheSIPURIrequestedisthenumberdialed:001234567890(aleading0isaddedbythephonetousetheoutgoingline;thisleading0mightnotbepresent,or–incountriesliketheUnitedStates–itmightbesubstitutedwith9) Cseq:incasemethodsdifferentfromREGISTERareused,thenumberdoesnothavetostartwith1sotheUserAgentpicked10691 Contact:inthiscaseContactincludesalso+sip.instanceorauniqueidentifierwhichallowstheSIPRegistrar/Proxytoeasilydetectduplicatedregistrations Supported:indicatesthattheUserAgentsupportsReplaces(neededduringtransferstoreplaceanexistingsession)–100rel(indicatessupportforthereliabletransmissionofprovisionalresponses1XX) Content-Type:indicatesthattheSIPmessageincludesalsoanSDPbodywhichprovidestheinformationaboutthemediasupportedbythecallingparty SIP/2.0407ProxyAuthenticationRequired Via:SIP/2.0/UDP10.135.0.12:5060;branch=z9hG4bKhye0bem20x.nx8hnt;rport=5060 From:"CallingUser"<151>;tag=m3l2hbp To:<001234567890>;tag=b27e1a1d33761e85846fc98f5f3a7e58.d5d4 Call-ID:[email protected] CSeq:10691INVITE Proxy-Authenticate:Digestrealm="10.135.0.1",nonce="WWdAiVlnPjECocBoyYtBDshUs+RxMS+3" Server:WildixGW-4.2.5.35963 Content-Length:0 AsinthecaseoftheREGISTERmessage,theserverwillrequestauthenticationbeforeacceptingtherequest,astheinitialINVITEdoesnotcontainanyinformationneededforauthentication. Inthiscasethemessageisslightlydifferent–407ProxyAuthenticationRequired–toindicatethatitisaProxyrequestingtheauthentication. ACKsip:[email protected]:5060;user=phoneSIP/2.0 Via:SIP/2.0/UDP10.135.0.12:5060;branch=z9hG4bKhye0bem20x.nx8hnt Max-Forwards:70 From:"CallingUser"<151>;tag=m3l2hbp To:<001234567890>;tag=b27e1a1d33761e85846fc98f5f3a7e58.d5d4 Call-ID:[email protected] CSeq:10691ACK User-Agent:WildixW-AIR03.55.00.249c7514340722 Content-Length:0 Theclientacknowledgeshavingreceivedtheauthenticationchallenge. INVITEsip:[email protected]:5060;user=phoneSIP/2.0 Via:SIP/2.0/UDP10.135.0.12:5060;branch=z9hG4bKw5ij3wn4knq9hn8kkrmoa64.ahrre7d Max-Forwards:70 From:"CallingUser"<151>;tag=m3l2hbp To:<001234567890> Call-ID:[email protected] CSeq:10692INVITE Contact:<151>;+sip.instance="" Allow:INVITE,CANCEL,BYE,ACK,REGISTER,OPTIONS,REFER,SUBSCRIBE,NOTIFY,MESSAGE,INFO,PRACK,UPDATE Content-Disposition:session Proxy-Authorization:Digestusername="151",realm="10.135.0.1",nonce="WWdAiVlnPjECocBoyYtBDshUs+RxMS+3",uri="sip:[email protected]:5060;user=phone",response="b6194033119482068b09dc2688172baf" Supported:replaces,100rel User-Agent:WildixW-AIR03.55.00.249c751434072202:76:f3:d6:64 Content-Type:application/sdp Content-Length:254 v=0 o=15196559655INIP410.135.0.12 s=- c=INIP410.135.0.12 t=00 m=audio50024RTP/AVP80218 a=rtpmap:8PCMA/8000 a=rtpmap:0PCMU/8000 a=rtpmap:2G726-32/8000/1a=rtpmap:18G729/8000 a=ptime:20 a=maxptime:80 a=sendrecv a=rtcp:50025 ThesecondINVITElooksprettymuchthesame,buttheCSeqvaluehasincreasedandtheauthenticationinformationisnowpresent. SIP/2.0100trying--yourcallisimportanttous Via:SIP/2.0/UDP10.135.0.12:5060;branch=z9hG4bKw5ij3wn4knq9hn8kkrmoa64.ahrre7d;rport=5060 From:"CallingUser"<151>;tag=m3l2hbp To:<001234567890> Call-ID:[email protected] CSeq:10692INVITE Server:WildixGW-4.2.5.35963 Content-Length:0 ServerrepliesimmediatelywithTrying,toindicatethatthemessagehasbeenreceivedandavoidre-transmissionsfromtheclientwhichwouldhappenwithin500ms(thestandardT1Timer1forSIP)–1000ms(twicetheprevioustimeout)–2000ms(1000msx2)–4000ms(2000msx2)upto16000milliseconds.ThusintotalanSIPClientwillbydefaulttry32secondsbeforecancellingarequestwhichiscalledTimerB(64timesT1).T1canbemodifiedandshouldbeequaltotheestimatedmaximumroundtriptimebetweenclientandserver. SIP/2.0100Ringing Via:SIP/2.0/UDP10.135.0.12:5060;branch=z9hG4bKw5ij3wn4knq9hn8kkrmoa64.ahrre7d;rport=5060 From:"CallingUser"<151>;tag=m3l2hbp To:<001234567890> Call-ID:[email protected] CSeq:10692INVITE Server:WildixGW-4.2.5.35963 Content-Length:0 TheremotesideindicatesAlertingstate–theremotedeviceisringing. SIP/2.0183SessioninProgress Via:SIP/2.0/UDP10.135.0.12:5060;branch=z9hG4bKw5ij3wn4knq9hn8kkrmoa64.ahrre7d;rport= 5060 From:"CallingUser"<151>;tag=m3l2hbp To:<001234567890> Call-ID:[email protected] CSeq:10692INVITE Server:WildixGW-4.2.5.35963 Content-Type:application/sdp Content-Length:170 v=0 o=root16731402321673140232INIP410.135.0.1 s=Wildix35935 c=INIP410.135.0.1 t=00 m=audio13504RTP/AVP8 a=rtpmap:8PCMA/8000/1 a=ptime:20 a=sendrecv BesidesRingingtheremovesidemightprovideinbandinformationsuchasearlymediabeforeconnectionasintheexampleabove.Thisisusedforexamplebyoperatorstoprovidefreemessagesbeforetransferringacalltovoicemail. SIP/2.0200OK Via:SIP/2.0/UDP10.135.0.12:5060;rport=5060;branch=z9hG4bKw5ij3wn4knq9hn8kkrmoa64.ahrre7d From:"CallingUser"<151>;tag=m3l2hbp To:<001234567890>;tag=as27b249ee Call-ID:[email protected] CSeq:10692INVITE Server:WildixGW35935 Allow:INVITE,ACK,CANCEL,OPTIONS,BYE,REFER,SUBSCRIBE,NOTIFY,INFO,PUBLISH Supported:replaces,timer Session-Expires:600;refresher=uas Contact:<001234567890> P-Asserted-Identity:"+331XXXXXXXX" Content-Type:application/sdp Content-Length:170 v=0 o=root16731402321673140232INIP410.135.0.1 s=Wildix35935 c=INIP410.135.0.1 t=00 m=audio13504RTP/AVP8 a=rtpmap:8PCMA/8000/1 a=ptime:20 a=sendrecv PleasenotethattheSDPcontainsonlyonecodec.Itisalsoimportanttonoticethattheheader“P-Asserted-Identity”containstheinformationwhichwillbethendisplayedtothecalleronthescreen. 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