Q.1 General

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

The purpose of optimal media routing (OMR) is to identify and remove unnecessary media functions from the media path for each media stream associated with a session.

The IP Multimedia Subsystem has the option to deploy media functions such as TrGWs on the media path associated with each media stream associated with a session. These media functions can perform only transport level functions such as firewall or NAT, or can also perform application level functions such as transcoding or conferencing. These media functions are typically allocated proactively during SDP offer/answer signalling within a session since it is unknown which of the functions are actually needed for a media stream until the SDP offer/answer signalling completes. For example, a transcoder can be allocated during session establishment but whether transcoding is needed is determined once the SDP offer reaches the far endpoint. In another example, the IBCFs at the boundary of a network allocate TrGWs to protect media functions within the network or to provide address translation to the private address space used within the network, however it might be determined later during session establishment that no media resources are needed within the network, thus making the TrGWs unnecessary.

Any SIP signalling entity within the IMS network that allocates, to a session flow, a media function that might later be determined to be unnecessary may implement the procedures in this Annex to assist in the removal of unnecessary media functions. In particular, any entity with an IMS-ALG may implement the OMR algorithm. This includes the IBCF (see Annex I) and P-CSCF (see Annex G). Any AS performing as a B2BUA controlling media resources may also implement the OMR procedures. Annex Q shows every controlling OMR entity as an IMS-ALG and every controlled media resources as a TrGW, but other options are possible. An MGCF or AS performing as UA may also implement OMR procedures to assist in the removal of unnecessary media functions in some cases. MGCF and AS procedures can be derived from the procedures in this annex by collocating an IMS-ALG with the MGCF or AS.

The OMR procedures identify and name the IP address realm used for each media path segment among UAs and TrGWs. The terms IP realm and realm are equivalent to the term IP address realm in this annex. An IP realm name is associated with each set of IP endpoints that are mutually reachable via IP routing and without address translation. Endpoints in different IP realms usually require allocation of a TrGW between those IP realms for connectivity and possibly for NAT.

When endpoints in different IP realms are mutually reachable without allocation of a TrGW, then OMR procedures may use provisioned information about such connected IP realms to determine possible optimal media paths through these connected realms.

NOTE: Connected IP realms are particularly useful when there are bilateral IP transport connections between operator networks, e.g., using IP tunnels via IP transport networks. In this case, each operator network can manage its own IP realm for inter-operator interconnection and provision the names of connected IP realms. Without the concept of connected IP realms, each bilateral connection (e.g. IP tunnel) in this example would need to be defined as its own IP realm.

IMS-ALGs implementing OMR shall include information in forwarded SDP regarding IP realm, codec and IP connectivity information for TrGWs on the media path to assist in bypassing unnecessary TrGWs. A TrGW can be bypassed when it is not required to transcode, when it is unnecessary to protect a network resource, and when a successive TrGW on the path is reachable by a previous TrGW on the path via a common IP realm.

The OMR procedures have the following additional characteristics:

– They build on the ALG NAT traversal model that is an alternative to the ICE NAT traversal model.

– They usually complete within a single end-to-end SDP offer/answer transaction. Some transcoding scenarios require additional signalling to complete optimisation.

– They apply independently to each media stream established by an SDP offer/answer transaction.

– They apply to media streams established between any types of endpoints (e.g., UEs, media servers, media gateways).

– They apply to media streams established using SIP 3pcc procedures. OMR applies to the endpoints of an SDP offer/answer transaction and not necessarily to the endpoints of a SIP dialog.

– They apply separately to each dialog when forking occurs. An IMS-ALG shall delay the release of a TrGW for OMR until it is clear that no forked dialog needs the TrGW.

– For early media negotiated with the same SDP as normal media, the OMR procedures have no direct impact on early media handling since path modifications are in place as soon as the SDP offer/answer transaction completes. An IMS-ALG can anchor in place any TrGW needed for blocking of unauthorized early media by removing OMR SDP extension attributes as necessary. For separate early-session disposition SDP the OMR algorithm shall not be applied.

– They do not require endpoints to support new procedures, although some additional optimisations are possible in some special cases.