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

Magnus Westerlund <magnus.westerlund@ericsson.com> Tue, 04 December 2018 10:52 UTC

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From: Magnus Westerlund <magnus.westerlund@ericsson.com>
To: Sergio Garcia Murillo <sergio.garcia.murillo@gmail.com>, =?iso-8859-1?Q?I=F1aki_Baz_Castillo?= <ibc@aliax.net>
CC: "rtcweb@ietf.org" <rtcweb@ietf.org>
Thread-Topic: [rtcweb] Clarification on simulcast and RID and RepairedRtpStreamId
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Subject: Re: [rtcweb] Clarification on simulcast and RID and RepairedRtpStreamId
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On 2018-12-04 11:01, Sergio Garcia Murillo wrote:
>> However, an RTP middlebox has the freedom to introduce those header
>> extensions when switching. They do not need to be sent end-to-end all
>> the time. So after the initial establishment of them, I see it as the
>> central middlebox responsibility to ensure that they are included in the
>> down stream legs towards the conference participants. Also at least for
>> video introducing them only on reception of a FIR also makes sense.
> Same could be said about changing the ssrc to avoid collisions and we 
> would have avoided our selves to introduced all this new ids.

I think you are significantly misrepresenting the reasons why RID's are
needed. The fact that we have multiple models for how RTP middleboxes
works is a significant contributing factor. The next is in RTP ancestry
as a non-centralized multi-party protocol, i.e. intended to work over
any source multicast. And as simulcast do implies a selection by the
middlebox that aspect do needs to be handled. A clean-slate approach
could possibly select another set of constraints and maybe be somewhat
simpler but not a lot.

I do note that also the MIDs and RIDs only have per signalling context
and may require translation between the various legs of the conference
in a multi-party one.

Cheers 

Magnus Westerlund 

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