scope:

The present document defines optional Optimal Media Routeing (OMR) procedures that can be applied by entities in the IP Multimedia Subsystem (IMS) that control media resources and are capable of manipulating the Session Description Protocol (SDP) as defined by IETF RFC 4566 [6].

The OMR procedures in the present specification relate to the handling of OMR-specific SDP attributes that are documented in TS 24.229 [4]. The OMR procedures use SDP offer/answer related procedures in IETF RFC 3264 [5] and in 3GPP TS 24.229 [4] in a backward-compatible manner.

The 3GPP network architecture, including the configuration and network entities of the IMS, is defined in 3GPP TS 23.002 [2]. The Stage 2 of the IMS is defined 3GPP TS 23.228 [3]. Annex Q of 3GPP TS 23.228 [3] documents the architecture and call flows for OMR.

The OMR procedures in this document are applicable to the following IMS entities that perform as an IMS-ALG or UA according to 3GPP TS 24.229 [4] and that control media resources:

- an IBCF acting as an IMS-ALG;

- a P-CSCF acting as IMS-ALG;

- an AS acting as B2BUA and adapting IMS-ALG procedures to control an MRF;

- an AS acting as B2BUA and adapting UA procedures to control an MRF; and

- an MGCF acting as UA.

NOTE 1: An AS acting as B2BUA to perform application functions such as conferencing or announcements will normally perform separate originating and terminating UA procedures, treating the media resource as an endpoint. An AS acting as B2BUA offering transcoding options will typically follow IMS-ALG procedures.

NOTE 2: The controlled media resource can be a TrGW, IMS-AGW, MRF, or a media function of the entity performing OMR.

The OMR procedures are not applicable for an UE.