18.27.0>
CONTENT-TYPE: application/SDP
CONTENT-LENGTH: 154
This INVITE contains the basic headers needed for session establishment. In an IMS
environment there are numerous other headers, but they have been eliminated in this
example for simplicity.
Notice that the TAG value has been calculated in the FROM header, but not yet the
TO header. This is because the destination must calculate its own unique TAG value
and append it to the response. Here is an example of a 200 OK response:
SIP/2.0 200 OK
VIA: SIP/2.0/UDP pchome101@aol.com:5060; branch=z9hG4bK74gh5
FROM: Deby Russell
; tag=9hz34567sl
TO: Travis Russell ; tag=1df789jkf
MAX FORWARDS: 70
CALL-ID: 82167534@126.18.27.0
CSeq: 1 INVITE
CONTACT: Travis Russell
CONTENT-TYPE: application/SDP
CONTENT-LENGTH: 154
In this example, the TAG value has now been entered into the TO header, and the
request originator can now calculate the dialog ID. The parameters shown in bold are
used for calculating the dialog ID.
The request originator will now generate the ACK in response to the 200 OK and the
session is now established. Notice in the ACK that the BRANCH value in the VIA header
is the same as in the INVITE. This is how stateful proxies within a SIP environment
correlate requests and responses (for example, any CSCF acting as a stateful proxy).
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