T.2 Application of network policy for the support of transcoding

24.2293GPPIP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP)Release 18Stage 3TS

When providing transcoding functions at the P-CSCF, at the IBCF and at an AS, the set of codecs to be forwarded as the SDP offer to the remote user is subject to network policy. In order to give support to the codecs and media quality originally requested by the offerer, the network policy shall meet the following requirements.

NOTE 1: RFC 3264 [27B] recommends to list codecs in priority order, so by adding network inserted codecs to the end of the codec list will give higher priority to previous codecs that might have been inserted by the originating UE.

A) An intermediate entity should attempt to support the original request for codecs from the UE.

B) An intermediate entity should only remove a codec from the codec list to meet policy requirements of the local access of the user.

C) A modification (i.e. any combination of reordering, removal or addition) to the codec list should only be made, such that the resultant SDP offer / answer exchange results in media of equal or better end-to-end quality than if the modification had not been made, subject to policy restrictions of the access of the local user.

NOTE 2: Transcoding between codecs of higher quality can provide better end-to-end quality than using a common codec of lower quality.

D) A modification (i.e. any combination of reordering, removal or addition) to the codec list should only be made, such that the resultant SDP offer / answer exchange prefers solutions that do not use a transcoder rather than ones that do use transcoder, subject to meeting the policy restrictions in B) and meeting the best end-to-end media quality in C) above.

E) Additions to the codec list that are provided by the network entity shall be supported by transcoding between at least one of the offered codecs contained in the SDP offer and the added codecs.

F) An intermediate entity shall not insert a codec to the codec list if end-to-end media security mechanism is required for the related media.

G) If an intermediate entity performs transcoding during an ongoing session and receives a SIP message containing a subsequent SDP offer including the codec that is currently in use on the incoming call leg, the entity should include the codec that is currently in use on the outgoing call leg when forwarding the SIP message.

H) If an intermediate entity performs transcoding during an ongoing session and receives a SIP message containing a subsequent SDP offer not including the codec that is currently in use on the incoming call leg, the entity should include the codec that is currently in use on the outgoing call leg when forwarding the SIP message, subject to meeting the policy restrictions in E).

I) If an intermediate entity does not perform transcoding during an ongoing session and receives a SIP message containing a subsequent SDP offer including the codec that is currently in use, the entity should not add any codecs in the subsequent SDP offer.

Annex U (normative):
IP-Connectivity Access Network specific concepts when using 5GS to access IM CN subsystem