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Substitute spatialScalabilty for referenceScaling #62

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Feb 2, 2022
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Substitute spatialScalabilty for referenceScaling #62

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69 changes: 39 additions & 30 deletions index.html
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Original file line number Diff line number Diff line change
Expand Up @@ -89,9 +89,9 @@ <h2>Terminology</h2>
(SFM) is defined in Section 3.7 of [[RFC7667]].
</p>
<p>
<dfn data-noexport data-idl data-dfn-for="VideoConfiguration">referenceScaling</dfn>
<dfn data-noexport data-idl data-dfn-for="VideoConfiguration">spatialScalability</dfn>
is a proposed addition to {{VideoConfiguration}} in
<a href="https:/w3c/media-capabilities/issues/182">discussion in the
<a href="https:/w3c/media-capabilities/pull/189">in the
Media Capabilities API</a>.
</p>
</section>
Expand Down Expand Up @@ -182,9 +182,10 @@ <h2>Negotiation</h2>
a codec supporting temporal scalability is negotiated.
</p>
<p>
There are situations where an <a title="Selective Forwarding Middlebox">SFM</a> may only support reception of a subset
of codecs and scalability modes. For example, an <a title="Selective Forwarding Middlebox">SFM</a> that parses codec
payloads may only support the H.264/AVC codec without scalability and
There are situations where an <a title="Selective Forwarding Middlebox">SFM</a>
may only support reception of a subset of codecs and scalability modes.
For example, an <a title="Selective Forwarding Middlebox">SFM</a> that parses
codec payloads may only support the H.264/AVC codec without scalability and
the VP8 codec with temporal scalability. On the other hand, the browser
may be able to encode VP8 with temporal scalability, VP9 with temporal
and spatial scalability and or H.264/AVC with temporal scalability.
Expand All @@ -194,33 +195,39 @@ <h2>Negotiation</h2>
To determine what codecs and scalability modes an application can send,
the {{RTCRtpSender}}'s <code>getCapabilities</code> method can be
used to determine the codecs and scalability modes supported by the
{{RTCRtpSender}}. The <a title="Selective Forwarding Middlebox">SFM</a> can provide information on the codecs and
scalability modes it can decode by providing its receiver capabilities.
After exchanging capabilities, the application can compute the intersection
of codecs and {{RTCRtpEncodingParameters/scalabilityMode}} values supported
by both the browser's {{RTCRtpSender}} and the <a title="Selective Forwarding Middlebox">SFM</a>'s receiver. This can be
used to determine the arguments passed to the browser's
{{RTCPeerConnection/addTransceiver()}} and {{RTCRtpSender/setParameters()}} methods.
{{RTCRtpSender}}. The <a title="Selective Forwarding Middlebox">SFM</a>
can provide information on the codecs and scalability modes it can
decode by providing its receiver capabilities. After exchanging
capabilities, the application can compute the intersection of codecs
and {{RTCRtpEncodingParameters/scalabilityMode}} values supported
by both the browser's {{RTCRtpSender}} and the
<a title="Selective Forwarding Middlebox">SFM</a>'s receiver. This
can be used to determine the arguments passed to the browser's
{{RTCPeerConnection/addTransceiver()}} and {{RTCRtpSender/setParameters()}}
methods.
</p>
<p>
Since sending simulcast encodings on a single stream is not negotiated within
Offer/Answer, an application using SDP signaling needs to determine whether
single stream simulcast transport is supported prior to the Offer/Answer negotiation.
This can be handled by having the <a title="Selective Forwarding Middlebox">SFM</a> send it's receiver capabilities to the
application prior to Offer/Answer. This allows the application to determine
whether single stream simulcast is supported, and if so, what scalability
modes the <a title="Selective Forwarding Middlebox">SFM</a> can handle. For example, an <a title="Selective Forwarding Middlebox">SFM</a> that can only support reception
of a maximum of 2 simulcast encodings on a single SSRC with the AV1 codec would
only indicate support for the "S2T1" and "S2T1h" scalability modes in its
receiver capabilities.
This can be handled by having the <a title="Selective Forwarding Middlebox">SFM</a>
send it's receiver capabilities to the application prior to Offer/Answer.
This allows the application to determine whether single stream simulcast is
supported, and if so, what scalability modes the
<a title="Selective Forwarding Middlebox">SFM</a> can handle. For example,
an <a title="Selective Forwarding Middlebox">SFM</a> that can only support
reception of a maximum of 2 simulcast encodings on a single SSRC with the
AV1 codec would only indicate support for the "S2T1" and "S2T1h" scalability
modes in its receiver capabilities.
</p>
<p>
To determine what codecs and scalability modes an <a title="Selective Forwarding Middlebox">SFM</a> can send to the
To determine what codecs and scalability modes an
<a title="Selective Forwarding Middlebox">SFM</a> can send to the
application, the application can utilize the [[?Media-Capabilities]] API.
If {{VideoConfiguration/referenceScaling}} is set to `false` or is absent, the decoder cannot decode
spatial scalability modes, but can decode all other
If {{VideoConfiguration/spatialScalability}} is set to `false` or is absent,
the decoder cannot decode spatial scalability modes, but can decode all other
{{RTCRtpEncodingParameters/scalabilityMode}} values supported by the
encoder. If {{VideoConfiguration/referenceScaling}} is set to `true` the
encoder. If {{VideoConfiguration/spatialScalability}} is set to `true` the
decoder can decode any {{RTCRtpEncodingParameters/scalabilityMode}}
value supported by the encoder.
</p>
Expand Down Expand Up @@ -285,13 +292,15 @@ <h2>Dictionary {{RTCRtpCodecCapability}} Members</h2>
</p>
<p class="note">
The {{scalabilityModes}} sequence represents the scalability modes supported
by a peer. For an <a title="Selective Forwarding Middlebox">SFM</a> the supported {{scalabilityModes}} may depend on the
negotiated RTP header extensions. For example, if the <a title="Selective Forwarding Middlebox">SFM</a> cannot parse codec
payloads (either because it is not designed to do so, or because the payloads
are encrypted), then negotiation of an RTP header extension (such as the
AV1 Dependency Descriptor defined in Appendix A of [[AV1-RTP]]) could be
a prerequisite for the <a title="Selective Forwarding Middlebox">SFM</a> to forward {{scalabilityModes}}. As a result, the
{{scalabilityModes}} supported by an <a title="Selective Forwarding Middlebox">SFM</a> may not be determined until
by a peer. For an <a title="Selective Forwarding Middlebox">SFM</a> the supported
{{scalabilityModes}} may depend on the negotiated RTP header extensions. For example,
if the <a title="Selective Forwarding Middlebox">SFM</a> cannot parse codec payloads
(either because it is not designed to do so, or because the payloads are encrypted),
then negotiation of an RTP header extension (such as the AV1 Dependency Descriptor
defined in Appendix A of [[AV1-RTP]]) could be a prerequisite for the
<a title="Selective Forwarding Middlebox">SFM</a> to forward {{scalabilityModes}}.
As a result, the {{scalabilityModes}} supported by an
<a title="Selective Forwarding Middlebox">SFM</a> may not be determined until
completion of the Offer/Answer negotiation.
</p>
</dd>
Expand Down