10.2.25 WebRTC Media Plane Optimization

29.1623GPPInterworking between the IM CN subsystem and IP networksRelease 17TS

10.2.25.1 General

The IBCF and the TrGW may support the WebRTC media plane optimization as defined in 3GPP TS 23.228 [8], annex U.2.4, and 3GPP TS 24.229 [1], subclause 6.7.1.5.

The purpose of WebRTC media plane optimization procedures is to convey media between WebRTC clients without bearer level protocol conversion. When both ends are WebRTC IMS clients (WIC), the TrGW remain allocated but media plane interworking is disabled.

The SDP attributes associated with WebRTC media plane optimization procedures "tra-contact", "tra-m-line", "tra-att", "tra-SCTP-association", "tra-media-line-number" and "tra-bw" are defined in 3GPP TS 24.229 [1], subclause 7.5.4.

10.2.25.2 SDP offer handling

If the IBCF receives an SDP offer that contains any "tra-contact" SDP attribute, and the IBCF decides to include a TrGW in the media path, the IBCF shall:

1) reserve resources at the TrGW that are suitable for the media described in the SDP offer outside the "tra-m-line", "tra-att" and "tra-bw" SDP attributes.

2) include the address information as received from the TrGW in that contact line and also encapsulate the address information into each received "tra-contact" attribute, replacing previous information; and

3) transparently pass (in the forwarded SDP offer) all received "tra-m-line", "tra-att", "tra-SCTP-association", "tra-media-line-number" and "tra-bw" SDP attributes.

10.2.25.3 SDP answer handling

If the IBCF receives an SDP answer and the SDP answer includes "tra-m-line" media level SDP attributes, the IBCF shall:

1) configure the TrGW to transparently pass the media described in the received "tra-m-line", "tra-att", "tra-SCTP-association", and "tra-bw" SDP attributes; and

2) transparently pass all received "tra-m-line", "tra-att", "tra-SCTP-association" and "tra-bw" SDP attributes.

NOTE: If interconnected H.248 Stream endpoints or terminations at the TrGW are configured with transport "UDP", they will pass the payload within UDP without modifications, known as UDP transparent forwarding. For WebRTC media plane optimization, the UDP payload will be either DTLS encapsulating SCTP and data channel(s), DTLS-SRTP encapsulating key management information, or SRTP encapsulating audio or video media.