10.2.18 Coordination of Video Orientation

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

10.2.18.1 General

The IBCF and the TrGW may support the Coordination of Video Orientation (CVO) as defined in 3GPP TS 26.114 [36].

If the IBCF receives an SDP body containing the RTP header extension attribute(s) "a=extmap", as defined in IETF RFC 5285 [45], and if "a=extmap" attribute indicates a CVO URN(s) (i.e. the CVO URN for a 2 bit granularity of rotation and/or the CVO URN for a higher granularity of rotation) as defined in 3GPP TS 26.114 [36], then:

a) if the IBCF and the TrGW support the CVO feature:

– the IBCF shall include an "extended RTP header for CVO" information element when seizing resources in the TrGW to indicate to the TrGW that it shall allow the RTP header extension for CVO to pass; and

– the IBCF shall forward within SIP signalling the SDP body received from the preceding node to the succeeding node with unmodified "a=extmap" attribute(s); or

b) if the TrGW does not support the CVO feature the IBCF shall forward within SIP signalling, the SDP body received from the preceding node to the succeeding node without any "a=extmap" attributes.

If the TrGW supports the CVO feature and has been instructed as described above to pass on the extended RTP header for CVO for both incoming and outgoing terminations then:

– if the TrGW does not apply video transcoding, it shall pass any received RTP CVO header extension to succeeding RTP streams; or

– if the TrGW applies video transcoding, it shall keep the video orientation unchanged during the transcoding and copy the received RTP CVO header extension to the succeeding outgoing RTP stream(s) after transcoding the associated group of packets.

NOTE 1: IETF RFC 5285 [45] provides a framework for header extensions and can also be used for non-CVO related purposes. It is an implementation decision of the TrGW if it only passes CVO related RTP header extensions, or if it passes any RTP header extension when being instructed with the "extended RTP header for CVO" information element.

NOTE 2: The behaviour of the TrGW when being instructed with the "extended RTP header for CVO" information element only at one termination is an implementation decision.

10.2.18.2 Message sequence chart

Figure 10.2.18.2.1 shows the message sequence chart example for indicating extended RTP header for Coordination of Video Orientation.

Figure 10.2.18.2.1: Procedure to indicate RTP extension header for CVO

The IBCF shall send the "extended RTP header for CVO" information element only with supported CVO URN(s) to the TrGW. If the TrGW does not support the CVO feature, the IBCF shall not send the "extended RTP header for CVO" information element to the TrGW.

NOTE: If the IBCF receives an SDP answer which includes the "a=extmap" attribute with a CVO URN with a granularity that the TrGW has not included in its response, or if the SDP answer does not include any "a=extmap" CVO related attribute, it is not necessary to modify the TrGW for this reason alone. Doing that would only add unnecessary signalling without requiring any action or changes in the TrGW. However if the IBCF needs to modify the media attributes for other purposes, in particular due to transcoding then the TrGW is updated in accordance with the received SDP answer, that is, either with the "extended RTP header for CVO" information element containing the received CVO URN if the CVO related "a=extmap" attribute has been included in the received SDP answer or without the "extended RTP header for CVO" information element if the CVO related "a=extmap" attribute has not been included in the received SDP answer (thus removing the requirement for sending the received RTP header extension with CVO bits for the transcoded stream.