RTP Control Protocol (RTCP) Messages for Green Metadata

Format of RTCP Feedback Messages This document extends the RTCP feedback messages defined in the RTP/AVPF and by defining a Green Metadata feedback message. The message can be used by the receiver to inform the sender of the desirable coding spatial resolution and temporal resolution (frame rate) of the bitstream delivered, and by the sender to indicate the coding spatial and temporal resolution it will use henceforth. RTCP Green Metadata feedback message follows a similar message format as RTCP Temporal-Spatial Trade-off Request and Notification . The message may be sent in a regular full compound RTCP packet or in an early RTCP packet, as per the RTP/AVPF rules. This document specifies two additional payload-specific feedback messages: Temporal-Spatial Resolution Request (TSRR) and Temporal-Spatial Resolution Notification (TSRN)

Temporal-Spatial Resolution Request The TSRR feedback message is identified by RTCP packet type value PT=PSFB and FMT=11. The FCI field MUST contain one or more TSRR FCI entries.

Message format The content of the FCI entry for the Temporal-Spatial Resolution Request is depicted in .

SSRC (32 bits): The Synchronization Source (SSRC) of the media sender that is requested to apply the frame rate and picture resolution. Seq nr. (8 bits): Request sequence number. The sequence number space is unique for pairing of the SSRC of request source and the SSRC of the request target. The sequence number SHALL be increased by 1 modulo 256 for each new command. A repetition SHALL NOT increase the sequence number. The initial value is arbitrary. Reserved (14 bits): All bits SHALL be set to 0 by the sender and SHALL be ignored on reception. Frame Rate (10 bits): frames_per_second. This field specifies the frame rate as defined in clause 5.3 of . An integer value between 1 and 1023 that indicates the coding frame rate that is requested. The value of Frame Rate equal to 0 is illegal. Picture Width (14 bits): pic_width_in_luma_samples. This field specifies the picture width as defined in clause 5.3 of . An integer value between 1 and 16383 that indicates the coding picture width in the units of luma samples that is requested. The value of Picture Width equal to 0 is illegal. Picture Height (14 bits): pic_height_in_luma_samples. This specifies the picture height as defined in clause 5.3 of . An integer value between 1 and 16383 that indicates the coding picture height in the units of luma samples that is requested. The value of Picture Height equal to 0 is illegal.

Semantics A decoder can suggest a temporal-spatial resolution by sending a TSRR message to an encoder. If the encoder is capable of adjusting its temporal-spatial resolution, it SHOULD take into account the received TSRR message for future coding of pictures. The reaction to the reception of more than one TSRR message by a media sender from different media receivers is left open to the implementation. The selected Frame Rate, Picture Width and Picture Height SHALL be communicated to the media receivers by means of the TSRN message (see section ). Within the common packet header for feedback messages (as defined in section 6.1 of ), the "SSRC of packet sender" field indicates the source of the request, and the "SSRC of media source" is not used and SHALL be set to 0. The SSRCs of the media senders to which the TSRR applies are in the corresponding FCI entries. A TSRR message MAY contain requests to multiple media senders, using one FCI entry per target media sender.

Timing Rules The timing follows the rules outlined in section 3 of . This request message is not time critical and SHOULD be sent using regular RTCP timing. Only if it is known that the user interface requires quick feedback, the message MAY be sent with early or immediate feedback timing.

Handling of Message in Mixers and Translators A mixer or media translator that encodes content sent to the session participant issuing the TSRR SHALL consider the request to determine if it can fulfill it by changing its own encoding parameters. A media translator unable to fulfill the request MAY forward the request unaltered towards the media sender. A mixer encoding for multiple session participants will need to consider the joint needs of these participants before generating a TSRR on its own behalf towards the media sender.

Temporal-Spatial Resolution Notification (TSRN) The TSRN message is identified by RTCP packet type value PT=PSFB and FMT=12. The FCI field SHALL contain one or more TSRN FCI entries.

Message format The content of the FCI entry for the Temporal-Spatial Resolution Notification is depicted in .

SSRC (32 bits): The Synchronization Source (SSRC) of the source of the TSRR that resulted in this notification. Seq nr. (8 bits): The sequence number value from the TSRR that is being acknowledged. Reserved (14 bits): All bits SHALL be set to 0 by the sender and SHALL be ignored on reception. Frame Rate (10 bits): The frame rate the media sender is using henceforth. Picture Width (14 bits): The coding picture width the media sender is using henceforth. Picture Height (14 bits): The coding picture height the media sender is using henceforth. It is to note that the returned value (Frame Rate, Picture Width, Picture Height) may differ from the requested one, for example, in cases where a media encoder cannot change its frame rate or picture resolution, or when pre-recorded content is used.

Semantics This feedback message is used to acknowledge the reception of a TSRR. For each TSRR received targeted at the session participant, a TSRN FCI entry SHALL be sent in a TSRN feedback message. A single TSRN message MAY acknowledge multiple requests using multiple FCI entries. The Frame Rate, Picture Width and Picture Height value included SHALL be the same in all FCI entries of the TSRN message. Including an FCI for each requestor allows each requesting entity to determine that the media sender received the request. The notification SHALL also be sent in response to TSRR repetitions received. If the request receiver has received TSRR with several different sequence numbers from a single requestor, it SHALL only respond to the request with the highest (modulo 256) sequence number. Note that the highest sequence number may be a smaller integer value due to the wrapping of the field. Appendix A.1 of has an algorithm for keeping track of the highest received sequence number for RTP packets; it could be adapted for this usage. The TSRN SHALL include the Temporal-Spatial Resolution Frame Rate, Picture Width and Picture Height that will be used as a result of the request. This is not necessarily the same Frame Rate, Picture Width and Picture Height as requested, as the media sender may need to aggregate requests from several requesting session participants. It may also have some other policies or rules that limit the selection. Within the common packet header for feedback messages (as defined in section 6.1 of ), the "SSRC of packet sender" field indicates the source of the Notification, and the "SSRC of media source" is not used and SHALL be set to 0. The SSRCs of the requesting entities to which the Notification applies are in the corresponding FCI entries.

Timing Rules The timing follows the rules outlined in section 3 of . This acknowledgement message is not extremely time critical and SHOULD be sent using regular RTCP timing.

Handling of TSRN in Mixers and Translators A mixer or translator that acts upon a TSRR SHALL also send the corresponding TSRN. In cases where it needs to forward a TSRR itself, the notification message MAY need to be delayed until the TSRR has been responded to.