The INVITE is a SIP method that specifies the action that the requester (Calling Party) wants the server (Called Party) to take. The INVITE request contains ...
SIPINVITEMethod
TheINVITEisaSIPmethodthatspecifiestheactionthattherequester(CallingParty)wantstheserver(CalledParty)totake.TheINVITErequestcontainsanumberofheaderfields.Headerfieldsarenamedattributesthatprovideadditionalinformationaboutamessage.TheonespresentinanINVITEincludeauniqueidentifierforthecall,thedestinationaddress,CallingPartyAddress,andinformationaboutthetypeofsessionthattherequesterwishestoestablishwiththeserver.
Example:
INVITEsip:[email protected]/2.0
Via:SIP/2.0/UDPpc33.atlanta.com;branch=z9hG4bK776asdhds
Max-Forwards:70
To:Bob
From:Alice;tag=1928301774
Call-ID:[email protected]
CSeq:314159INVITE
Contact:
Content-Type:application/sdp
Content-Length:142
Thefirstlineofthetext-encodedmessagecontainsthemethodname(INVITE).Thelinesthatfollowarealistofheaderfields.Thisexamplecontainsaminimumrequiredset.Theheaderfieldsarebrieflydescribedbelow:
Via-Containstheaddress(pc33.atlanta.com)atwhichAliceisexpectingtoreceiveresponsestothisrequest. Italsocontainsabranchparameterthatidentifiesthistransaction.
To-Containsadisplayname(Bob)andaSIPorSIPSURI(sip:[email protected])towardswhichtherequestwasoriginallydirected.
From-Alsocontainsadisplayname(Alice)andaSIPorSIPSURI(sip:[email protected])thatindicatetheoriginatoroftherequest.Thisheaderfieldalsohasatagparametercontainingarandomstring(1928301774)thatwasaddedtotheURIbythesoftphone. Itisusedforidentificationpurposes.
Call-ID-Containsagloballyuniqueidentifierforthiscall,generatedbythecombinationofarandomstringandthesoftphone'shostnameorIPaddress. ThecombinationoftheTotag,Fromtag,andCall-IDcompletelydefinesapeer-to-peerSIPrelationshipbetweenAliceandBobandisreferredtoasadialog.
CSeq-orCommandSequence,containsanintegerandamethodname. TheCSeqnumberisincrementedforeachnewrequestwithinadialogandisatraditionalsequencenumber.
Contact-ContainsaSIPorSIPSURIthatrepresentsadirectroutetocontactAlice,usuallycomposedofausernameatafullyqualifieddomainname(FQDN).WhileanFQDNispreferred,manyendsystemsdonothaveregistereddomainnames,soIPaddressesarepermitted.WhiletheViaheaderfieldtellsotherelementswheretosendtheresponse,theContactheaderfieldtellsotherelementswheretosendfuturerequests.
Max-Forwards-Servestolimitthenumberofhopsarequestcanmakeonthewaytoitsdestination. Itconsistsofanintegerthatis decrementedbyoneateachhop.
Content-Type-Containsadescriptionofthemessagebody(notshown).
Content-Length-Containsanoctet(byte)countofthemessagebody.
Re-INVITE
TheINVITEmethodisusedtoestablishmediasessionsbetweenUserAgents.TheRE-INVITEmessagepermitsthe2020IMGtochangeparametersofanexistingorpendingcall.Thefollowingissupported
RE-INVITEmessagesthatchangetheporttowhichmediashouldbesent.
RE-INVITEmessagesthatchangetheconnectionaddressormediatype.
Hold/ReleaseandSUSPEND/RESUMErtpstreams(connectionaddressiszero).SIPCallHold
InitialINVITEmessagesonhold.
RE-INVITEmessagesforFAX(T.38andBypass).
Copyright©2016DialogicCorporation.
AllRightsReserved.
