The present document provides the protocol details for the messaging service within the IP Multimedia CN Subsystem (IMS) based on the Session Initiation Protocol (SIP), the Session Description Protocol (SDP) and, the Message Session Relay Protocol (MSRP) . The document covers immediate messaging, session based messaging and session-based messaging conferences, as described in TS 22.340. Where possible the present document specifies the requirements for this protocol by reference to specifications produced by the IETF within the scope of SIP, SDP and, MSRP, either directly, or as modified by TS 24.229. The present document is applicable to Application Servers (ASs) , Media Resource Function Controllers (MRFCs), Media Resource Function Processors (MRFPs) and to User Equipment (UE) providing messaging capabilities. This document does not cover the signalling between a MRFC and a MRFP.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

The basic services for the IP Multimedia core network Subsystem (IMS), as defined in TS 24.229, allow a user to initiate, modify and terminate media sessions using the Session Initiation Protocol, as defined in RFC 3261. Although these basic mechanisms already allow the exchange of instant messaging information using SIP, this functionality can be extended to provide a richer service within the IMS. The messaging service within the IM CN subsystem provides the means for a user to send or receive single messages immediately to / from another user and to create and participate in a messaging conference with one ore more other users. Participants to such message based communication may be internal or external to the home network. When to use an immediate message and when to use a session-based messaging session will depend on the application. For immediate messaging the procedures for page-mode messaging, as defined in RFC 3428 or for session-mode messaging, as defined in RFC 4975, RFC 6714 and RFC 6135 are utilized. When to use an page-mode messaging and when to use session-mode messaging session for the purpose of immediate messaging will depend on the application. For session-based messaging and session-based messaging conferences, the Message Session Relay Protocol (MSRP) is utilized to transport messages. The architecture for the 3GPP messaging is specified in TS 23.228 and TS 23.218. The 3GPP recommended media formats and codecs are specified in TS 26.141. The 3GPP2 recommended media formats and codecs are specified in 3GPP2 C.S0050-B [17]. The functional split for session-mode messaging between an AS, MRFC and MRFP is that same as that described in clause 4 of TS 24.147 for SIP based conferences. The functional split between the AS, MRFC and MRFP for page-model messaging is out of scope of the present document.