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 Multimedia Resource Function Controller (MRFC);
• the interface between the Application Server (AS) and the Multimedia 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 CSCF and an IBCF;
• the interface between the E-CSCF and the Location Retrieval Function (LRF);
• the interface between the BGCF and any other BGCF;
• the interface between the CSCF and an external Multimedia IP network; and
• the interface between the E-CSCF and the EATF.

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.

As the IM CN subsystem is designed to interwork with different IP-Connectivity Access Networks (IP-CANs), the IPCAN independent aspects of the IM CN subsystem are described in the main body and annex A of this specification. Aspects for connecting a UE to the IM CN subsystem through specific types of IP-CANs are documented separately in the annexes or in separate documents.

The document also specifies media-related requirements for the NAT traversal mechanisms defined in this specification.

NOTE: The present document covers only the usage of SIP and SDP to communicate with the enitities of the IM CN subsystem. It is possible, and not precluded, to use the capabilities of IP-CAN to allow a terminal containing a SIP UA to communicate with SIP servers or SIP UAs outside the IM CN subsystem, and therefore utilise the services provided by those SIP servers. The usage of SIP and SDP for communicating with SIP servers or SIP UAs outside the IM CN subsystem is outside the scope of the present document.