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Add WebRTC-specific interactions with capture/receive/RTP timestamps #224

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Add WebRTC-specific interactions with capture/receive/RTP timestamps #224

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1acc485
add WebRTC-specific interactions with capture/receive/RTP timestamps
handellm Sep 20, 2024
3ee4f43
Rebase and update
handellm Dec 18, 2024
92a79db
Fix some style stuff & undo webrtc-extensions.js changes.
handellm Dec 18, 2024
18a0c80
Address reviewer comments. Use SHOULD/MUST instead of is. Remove info…
handellm Jan 10, 2025
427dec6
Offline review with henbos@: removed ambiguous text.
handellm Jan 10, 2025
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Expand Up @@ -72,6 +72,19 @@ <h2>Terminology</h2>
<p>
The process of <dfn data-lt="free|freed|freeing">freeing</dfn> a candidate is defined in [[RFC8445]] Section 8.3.
</p>

<p>
<!-- TODO: use the xref mechanism when mediacapture-extensions spec status is cleared up.
See https://github.com/w3c/mediacapture-extensions/issues/132 -->
The following terms are defined in <a href="https://w3c.github.io/mediacapture-extensions">mediacapture-extensions</a>
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Due to the unclear state of the mediacapture-extensions spec (not listed in browser-specs) I had to make some temporary reference structure as per below, when referring to concepts defined there.

<a href="https://w3c.github.io/mediacapture-extensions/#video-timestamp-concepts/">video timestamp concepts</a>
</p>
<ul>
<li><dfn>presentation timestamp</dfn></li>
<li><dfn>capture timestamp</dfn></li>
<li><dfn>receive timestamp</dfn></li>
<li><dfn>RTP timestamp</dfn></li>
</ul>
</section>
<section id="ice-csp">
<h3>
Expand Down Expand Up @@ -1540,6 +1553,43 @@ <h2>Event summary</h2>
</tbody>
</table>
</section>
<section>
<h2>Timestamp behavior</h2>
<h2>RTCRtpSender video track frame timestamp effects on outgoing packet NTP and RTP timestamps</h2>
<p>
The user agent defines a <dfn class="export">frame timestamp</dfn> being the same as the [=capture timestamp=]
of the frame being produced on the {{RTCRtpSender}} {{MediaStreamTrack}} if it is set.
Comment on lines +1558 to +1561
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Sorry, I'm not following what this timestamp is by the name "frame timestamp". Not all track sources will insert capture timestamps in the tracks they produce. Is this what you mean by "if it is set"?

If it is unset the user agent MUST estimate a timestamp from the sent frame's [=presentation timestamp=] together with the
time it was received by the {{RTCRtpSender}}. This estimate MUST NOT be based on [= RTP timestamp =].
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What does "received by the RTCRtpSender" mean? Do you mean RTCRtpReceiver?

</p>
<p>
The NTP and RTP timestamps of the encoded frame corresponding to the frame being produced on the {{RTCRtpSender}}
{{MediaStreamTrack}} MUST be sourced from the [=frame timestamp=].
</p>
<h2>RTCRtpReceiver timestamps</h2>
<h3>Remote capture timestamp</h3>
<p>
<!-- TODO: How this is estimated is under specified and should be fixed, but the section holds the requestVideoFrameCallback text for now -->
For a frame produced in a {{RTCRtpReceiver}} track, the user agent computes a
<dfn class="export">remote capture timestamp</dfn>. It is a best-effort estimate of the local capture
time on the sender translated to the receiver clock, and can use methods like using RTCP SR
as specified in [[?RFC3550]] Section 6.4.1, or by other alternative means if use by RTCP SR
isn't feasible.
</p><p>
Each frame's [=capture timestamp=] is set to the [=remote capture timestamp=], if available.
</p>
<h3>Received RTP timestamp</h3>
<p>
For a video frame produced in a {{RTCRtpReceiver}} track, the frame's [=RTP timestamp=] MUST be set
from the RTP timestamp of constituent packets of the corresponding received encoded frame.
</p>
<h3>Receive timestamp</h3>
<p>
For a frame produced in a {{RTCRtpReceiver}} track, the [=receive timestamp=] MUST be set
as the time the corresponding encoded frame was received by the platform, i.e. the time at which the
last packet belonging to this frame was received over the network.
</p>
</section>
<section class="informative" id="security-considerations">
<h2>
Security Considerations
Expand Down