Re: [Sframe] Partial decodability and IDUs

Bernard Aboba <bernard.aboba@gmail.com> Sun, 22 November 2020 23:10 UTC

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From: Bernard Aboba <bernard.aboba@gmail.com>
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Date: Sun, 22 Nov 2020 15:10:52 -0800
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Cc: Magnus Westerlund <magnus.westerlund@ericsson.com>, rlb@ipv.sx, sframe@ietf.org
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To: Sergio Garcia Murillo <sergio.garcia.murillo@cosmosoftware.io>
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Subject: Re: [Sframe] Partial decodability and IDUs
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On Nov 22, 2020, at 2:48 PM, Sergio Garcia Murillo <sergio.garcia.murillo@cosmosoftware.io> wrote:
> 
> The only RTCP recovery I am aware of that could apply for IDUs is the slice loss indication which requires macroblock information, which is a way lower information layer than the one SFUs works with (it may not even be available if payload is encrypted).
> 
> Moreover, the SFU will not rely the SLIs received from the receiving endpoint as it has no way of tracking SLIs requests from endpoints and map them to already sent ones in order to avoiding overflowing the encoder with SLIs requests.
> 
> Also, before adding support from it, I would like to get real implementation feedback as it is currently not implemented in (lib)webrtc and I have not heard anynone asking for it ever.


[BA] Although RPSI and SLI Sections are included in several codec documents, I have not seen them implemented. We do not support SLI or RPSI in our SFU. The reasoning is that with RTX and FEC and other messages supported in WebRTC (NAK, PLI, FIR), then SLI and RPSI do not provide incremental value. 


>> So in the later cases you might want to distribute the layers on different SSRCs so that one can immediately determine how important the
>> information is. I think that possibility will exist if this RTP payload format is done correctly.

[BA] We do support MRST with H.264/SVC, and found some advantages in doing so (not having to rewrite sequence numbers in an SFU). However MRST is not supported in VP8, VP9, AV1 and most H.264/SVC implementations, and the performance cost of translation outweighed the forwarding performance advantages.