The present document defines a call control protocol for use in the IP Multimedia (IM) Core Network (CN) subsystem based on the Session Initiation Protocol (SIP), and the associated Session Description Protocol (SDP).

The present document is applicable to:

- the interface between the User Equipment (UE) and the Call Session Control Function (CSCF);

- the interface between the CSCF and any other CSCF;

- the interface between the CSCF and an Application Server (AS);

- the interface between the CSCF and the Media Gateway Control Function (MGCF);

- the interface between the S-CSCF and the Media Resource Function Controller (MRFC)

- the interface between the CSCF and the Breakout Gateway Control Function (BGCF);

- the interface between the BGCF and the MGCF; - the interface between the BGCF and any other BGCF; and

- the interface between the CSCF and an external Multimedia IP network.

Where possible the present document specifies the requirements for this protocol by reference to specifications produced by the IETF within the scope of SIP and SDP. Where this is not possible, extensions to SIP and SDP are defined within the present document. The document has therefore been structured in order to allow both forms of specification.

NOTE 1: This document assumes the existence of the Go interface between the ICN Bearer Control Point and the P-CSCF. However, since the Go interface is not supported in the MMD Revision 0, the procedures that are dependent on the Go interface are not mandatory. This also pertains to the Service Based Local Policy (SBLP) which is also not supported in the MMD Revision 0.

NOTE 2: The present document covers only the usage of SIP and SDP to communicate with the entities of the IM CN subsystem. It is perfectly possible, and not precluded, to use only the capabilities of the IP Connectivity Network to allow a terminal containing a SIP UA to communicate with SIP servers outside the IM CN subsystem, and therefore utilise the services provided by those SIP servers. Such usage is outside the scope of the present document.