The Request-Line-URI includes the destination of the call. It contains the same information as the To field, omitting the display name. Via:.
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ThemainSIPINVITEHeaderFieldsexplained
PostedonMarch8th,2010byNickGalea,CTO&Founder,3CX
ThisarticleexplainsthemainfieldsincludedinaSIPINVITE,whichissenttoset-upaVoIPcall.ASIPINVITEmessagecontainstypicallybetween4and6headerentrieswithcontactinformationinsidethem.Differentdevicesorprovidersusetheseheadersindifferentwaysandtherefore,anunderstandingofthe“normal”scopeoftheseheaders,togetherwithacloseanalysisoflogmessages,willhelptounderstandthecauseofcertainissues,andalsotoconfiguredevicesorprovidersinthe3CXManagementConsolesoastobeidentifiedcorrectly. TheimagebelowshowsatypicalSIPINVITE.
Request-Line-URI:
TheRequest-Line-URIincludesthedestinationofthecall.ItcontainsthesameinformationastheTofield,omittingthedisplayname.
Via:
Everyproxyintherequestpathaddstotopofthe“Via”theaddressandportonwhichitreceivedthemessage,thanforwardsitonwards. Whenprocessingresponses,eachproxyinthereturnpathprocessesthecontentsofthe“Via”fieldinreverseorder,removingitsaddressfromthetop.
From:
The“From”headerfieldindicatestheidentityoftheinitiatoroftherequestfromthepointofviewofthePBXServer–similarinconstructiontoemailaddresses([email protected]–where“user”is,forexample,theextensionnumber,and“domain”istheserverdomainorIPaddress).Likethe“To”headerfield,itcontainsaURIandoptionallyadisplayname.Itisusedtodeterminewhichprocessingrulestoapplytoarequest.
FromtheSIPRFC3261:The“From”headerfieldallowsforadisplayname.AUAC(i.eaphone)SHOULDusethedisplayname"Anonymous",alongwithasyntacticallycorrect,butotherwisemeaninglessURI(e.g.From:“Anonymous”),iftheidentityoftheclientistoremainhidden.
Typically,thehostIPAddresswillbetheinternalIPaddressofthePBXServer.
To:
The“To”headerfieldfirstandforemostspecifiesthedesired"logical"recipientoftherequest,ortheaddress-of-recordoftheuserorresourcethatisthetargetofthisrequest.Thismayormaynotbetheultimaterecipientoftherequest.The“To”headerfieldMAYcontainaSIPURI,butitmayalsomakeuseofotherURIschemes(thetelURL(RFC2806[9]),forexample)whenappropriate.AllSIPimplementationsMUSTsupporttheSIPURIscheme.
TheToheaderfieldallowsforadisplayname(e.g.To:“TargetUser”).
Typically,the“To”fieldcontainsaSIPURIthatpointstothefirst(next)hop(proxy)thatwillprocesstherequest,butnotnecessarilytheSIPURIoftheeventualrecipient.
Contact:
The“Contact”headerfieldprovidesasingleSIPURIthatcanbeusedtocontactthesenderoftheINVITEforsubsequentrequests.TheContactheaderfieldMUSTbepresentandcontainexactlyoneSIPURIinanyrequestthatcanresultintheestablishmentofadialog–inthiscase,specificallyaSIPINVITE.Fortheserequests,thescopeoftheContactisglobal.Thatis,theContactheaderfieldvaluecontainstheURIatwhichthesenderisexpectingtoreceiverequests,andthisURIMUSTbevalidevenifusedinsubsequentrequestsoutsideofanydialogs(inourcase,meaningestablishedcalls).
Allow:
Thisfieldlists,incomma-separatedformat,theSIPMethodsthatthecallercansupportanduse.SIPdefinesthefollowingmethods:ACK,BYE,CANCEL,INFO,INVITE,NOTIFY,OPTIONS,PRACK,REFER,REGISTER,SUBSCRIBE,UPDATE
Definitions
SIPURI-ASIPURIisauser’sSIPphonenumber.TheSIPURIresemblesane-mailaddressandiswritteninthefollowingformat:SIPURI=sip:[email protected]:Port
FurtherinformationaboutSIP,SDP
SIPInviteHeaderFieldsDemystified
SIPMessages(Request,Status)Explained
SessionDescriptionProtocol-Part 1-BirdsEyeView
SessionDescriptionProtocol-Part2-Adeepdive
DTMF,SIP,RFC2833-howtheydancetogether
ByNickGalea|March8th,2010|CommentsOffonThemainSIPINVITEHeaderFieldsexplained
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