ACK sip:travis@135.18.10.10 SIP/2.0
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: Deby Russell
One other note regarding the BRANCH value: the first seven digits are always
z9hG4bK (indicating this came from SIP 2.0 in accordance with RFC 3261).
There is of course much more to the entire process that is outside the scope of this
book. You can refer to the RFC 3261 for detailed specifications for SIP processing and
session establishment, or refer to my other book, ???Session Initiation Protocol (SIP):
Controlling Convergent Networks (McGraw-Hill, 2008).
Emergency Session Establishment
The P-CSCF is responsible for emergency session establishment. The P-CSCF will store
and maintain an operator configurable list of local (local to the P-CSCF and within its
Establishing a Session in the IMS 135
domain) emergency numbers and URIs. The P-CSCF can also maintain a list of emergency
numbers and URIs for any roaming and interconnect partners.
When a session request (i.e., INVITE) is received by the P-CSCF containing the TEL
or SIP URI of an emergency number (such as 911), the P-CSCF will route this request
directly to the appropriate destination.
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