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clarify requirements of simulcast support #243

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clarify requirements of simulcast support #243

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clarify requirements of simulcast support
DanilChapovalov Aug 23, 2024
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Update av1-rtp-spec.md
DanilChapovalov Sep 18, 2024
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Update av1-rtp-spec.md
DanilChapovalov Sep 19, 2024
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6 changes: 3 additions & 3 deletions av1-rtp-spec.md
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Expand Up @@ -298,11 +298,11 @@ The general function of a MANE or SFM is to selectively forward packets to recei

### 6.1 Simulcast

The RTP payload defined in this specification supports two distinct modes for transport of simulcast encodings. In either mode, simulcast transport MUST only be used to convey multiple encodings from the same source. Also, in either mode, a sequence header OBU SHOULD be aggregated with each spatial layer. Both modes MUST be supported by implementations of this specification.
The RTP payload defined in this specification supports two distinct modes for transport of simulcast encodings. In either mode, simulcast transport MUST only be used to convey multiple encodings from the same source. Also, in either mode, a sequence header OBU SHOULD be aggregated with each spatial layer.

When simulcast encodings are transported each on a separate RTP stream, each simulcast encoding utilizes a distinct bitstream containing its own distinct Sequence Header and Scalability Metadata OBUs. This mode utilizes distinct SSRCs and Restriction Identifiers (RIDs) for each encoding as described in [RFC8852] and, as a result, RTCP feedback can be provided for each simulcast encoding. This mode of simulcast transport, which MUST be supported by SFMs, utilizes Session Description Protocol (SDP) signaling as described in [RFC8851] and [RFC8853].
When simulcast encodings are transported each on a separate RTP stream, each simulcast encoding utilizes a distinct bitstream containing its own distinct Sequence Header and Scalability Metadata OBUs. This mode utilizes distinct SSRCs and Restriction Identifiers (RIDs) for each encoding as described in [RFC8852] and, as a result, RTCP feedback can be provided for each simulcast encoding. This mode of simulcast transport utilizes Session Description Protocol (SDP) signaling as described in [RFC8851] and [RFC8853].

When simulcast encodings are transported on a single RTP stream, RIDs are not used and the Sequence Header and Scalability Metadata OBUs (utilizing an 'S' mode) convey information relating to all encodings. This simulcast transport mode is possible since AV1 enables multiple simulcast encodings to be provided within a single bitstream. However, in this mode, RTCP feedback cannot be provided for each simulcast encoding, but only for the aggregate, since only a single SSRC is used. This mode of simulcast transport MAY be supported by SFMs.
When simulcast encodings are transported on a single RTP stream, RIDs are not used and the Sequence Header and Scalability Metadata OBUs (utilizing an 'S' mode) convey information relating to all encodings. This simulcast transport mode is possible since AV1 enables multiple simulcast encodings to be provided within a single bitstream. However, in this mode, RTCP feedback cannot be provided for each simulcast encoding, but only for the aggregate, since only a single SSRC is used.


### 6.1.1 Example
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