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

Sergio Garcia Murillo <sergio.garcia.murillo@gmail.com> Tue, 04 December 2018 09:26 UTC

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From: Sergio Garcia Murillo <sergio.garcia.murillo@gmail.com>
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Date: Tue, 4 Dec 2018 10:30:21 +0100
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Subject: Re: [rtcweb] Clarification on simulcast and RID and RepairedRtpStreamId
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On 04/12/2018 10:04, Magnus Westerlund wrote:
> On 2018-12-03 22:31, Iñaki Baz Castillo wrote:
>> On Mon, 3 Dec 2018 at 11:02, Magnus Westerlund
>> <magnus.westerlund@ericsson.com> wrote:
>>
>>> 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.
>> Is that issue related to the fact that RTX packets are supposed to
>> also carry original packet header extensions?
> That implications I actually hadn't thought about. But you are right
> there is an issue here.
>
> So RFC 4588 says this:
>
>
>     If the original RTP header carried an RTP header extension, the
>     retransmission packet SHOULD carry the same header extension.  This
>     header extension MUST be placed right after the fixed RTP header, as
>     specified in RTP [3].  In this case, the retransmission payload
>     header MUST be placed after the header extension.
>
>
> So following that an implementation SHOULD include the RtpStreamID
> header extension and the RFC is silent about adding additional ones. So
> the reconstructed original packet needs to have the RtpStreamId header
> extensio. However, some text on the use of RepairedRtpStreamId appears
> required. Like that it should be added at certain cases, like first
> sendings of an repair RTP stream (until know it be delivered) as well as
> when endpoints join the session. The receiver also needs to know to
> strip this particular header and not include it in those that are
> attached to the reconstructed packet.


Given that RFC 4588 already covers the inclusion of the original 
RTPStreamID header into the RTX packet and that FlexFEC will not use 
RapairedStreamId mechanism (as a single flex fec stream will be used for 
all protected media), I see no use at all for the RepairedStreamId 
header extension and related mechanisms at all.


Best regards

Sergio