SIP (Session Initiation Protocol) [172] is the main signaling protocol of the IMS and thus used by the Multimedia Telephony communication service. SIP is a text-based rendezvous protocol that provides user mobility and session establishment.

SIP provides user mobility through registrations. A user is always reachable under the same identifier regardless of the user's current location. Every time a user becomes available at a user agent, the user agent registers its location with the user's registrar, which is located in the user's home domain. This way, the home domain is informed about how to route incoming session requests to the user agent where the user is currently available.

In SIP, users are identified by URIs. SIP URIs have a similar format to email addresses (e.g. <sip:[email protected]>).

SIP provides session establishment through a two-way session description exchange called the offer/answer model [170]. A user agent generates a session description that contains

IMS Multimedia Telephony over Cellular Systems S. Chakraborty, T. Frankkila, J. Peisa and P. Synnergren © 2007 John Wiley & Sons, Ltd the information needed to establish the session (e.g. IP addresses to be used to transfer the media) and sends it to the remote user agent. This session description is referred to as the offer.

On receiving the offer, the remote user agent generates its own session description, which is referred to as the answer.

Both the offer and the answer are written in a session description format which needs to be understood by both user agents. The default session description format is SDP (Session Description Protocol) [84].

Once the offer/answer exchange completes, the user agents can start exchanging media between them. At that point, the session is considered to be established.

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