Re: [rtcweb] Clarification on simulcast and RID and RepairedRtpStreamId

Magnus Westerlund <magnus.westerlund@ericsson.com> Mon, 03 December 2018 10:02 UTC

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From: Magnus Westerlund <magnus.westerlund@ericsson.com>
To: =?iso-8859-1?Q?I=F1aki_Baz_Castillo?= <ibc@aliax.net>, "rtcweb@ietf.org" <rtcweb@ietf.org>
Thread-Topic: [rtcweb] Clarification on simulcast and RID and RepairedRtpStreamId
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Date: Mon, 3 Dec 2018 10:02:05 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/rtcweb/Upav6OfdyVMYQ4Df33rnvARP30A>
Subject: Re: [rtcweb] Clarification on simulcast and RID and RepairedRtpStreamId
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Hi,

I think what should really be done is to update RFC 4588 to recommend
that one uses RepairedStreamID to indicate the retransmission stream's
relation to the source stream for the same RTP session multiplexing
case. I also think there is a point to discuss the separate RTP session
multiplexing case and its SSRC usage.  This will however require one to
have a legacy discussion section for both these cases.




On 2018-12-01 13:04, Iñaki Baz Castillo wrote:
> Q1: Is there any spec defining the SDP syntax to indicate
> RepairedRtpStreamId values in the SDP?


As Sergio stated, no needed for that as the ID value referenced are the
RID ID for the source stream.

>
> Q2: Is THIS (RID + RepairedRtpStreamId) the proper way to go for
> simulcast + RTX?

Yes, if you would use the RFC 4588 format.

However, for FLEXFEC one doesn't need that at all as the repair packets
are self-describing. I have raised an question in PAYLOAD WG if flexfec
should be explicit about its non usage of RepairedStreamId. 


>
> Also, when it comes to receive media with RTX it's clear that WebRTC
> does not define how to receive simulcast streams for WebRTC clients
> (browsers), so browsers must receive a single stream plus an optional
> RTX stream.
>
> In this case, as far as a=mid plus RTP MID extension is used (or
> a=ssrc) signaled, there is zero need for RID. However, how is RTX
> supposed to be signaled in this case? Chrome expects ""a=ssrc-group:
> FID MEDIA_SSRC RTX_SSRC" in the remote SDP to be able to receive an
> additional RTX stream.
>
> Q3: Is this the way to go for receiving a single stream with RTX?
>
To attempt to restate your question. For RTP sessions, where there are a
single source media stream per Media Description in SDP (m= block) and
where there is explicit MID signalling and SSRC signalling, then one
will not need RepairedStreamId and can instead rely on explicit MID
signalling for the RTX stream?

I think no. I think using RepairedStreamId independent if there is
multiple source RTP streams or not per media description provides a more
consistent experience.

However, as noted at the start. The use of RepairedStreamId for RFC4588
RTX format is not well defined and how to handle and detect legacy. So,
I think an update should be done.

Cheers

Magnus Westerlund 

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