Download

688 SIGNALING FOR VoIPFigure 21.3-2. Tag value assignment and acquisition.format as SIP URIs, except for “sips” instead of “sip.” For examples:sip:bob@biloxi.comsips:1212@gateway.comSIP also allows the use of Internet URIs in lieu of SIP URIs.Tag. This is a parameter in the “To” and “From” headers, consisting of a randomlygenerated but unique character string. Two tag values, local and remote, are used byeach UA, and together with the Call-ID they identify a dialog (Dialog ID). A UAassigns its own local tag value and acquires the remote tag value from the remote UA.In a request message, the “To” header carries the remote tag; prior to its acquisition,the field is omitted. The “From” header carries the local tag. In a responsemessage, the roles are reversed: “To” carries the local tag and “From” carries theremote tag.The process for assigning the local tag and for acquiring the remote tag is shownin Fig. 21.3-2. The calling UA inserts its local value (tag¼A) in the “From” headerof the INVITE message, starting the dialog. Upon receipt of the INVITE, the calledUA acquires A as its remote tag value and assigns its own local value (tag¼B).Tag ¼ B is inserted in the “To” header of the first response message (e.g.,Ringing) sent to the calling UA, which acquires it as its remote tag. At this pointthe dialog is established (the Call-ID was assigned by UA and sent in theINVITE). In Fig. 21.3-2 the INVITE and Ringing messages may be sent directlyor relayed by proxy servers (not shown).21.3.5 SIP Call SequencesA SIP session is conceptually very simple, as shown in the example of Fig. 21.3-3,taken from RFC 3261 [8]. The example is based on a point-to-point session