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

Magnus Westerlund <magnus.westerlund@ericsson.com> Tue, 04 December 2018 09:04 UTC

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From: Magnus Westerlund <magnus.westerlund@ericsson.com>
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Thread-Topic: [rtcweb] Clarification on simulcast and RID and RepairedRtpStreamId
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Date: Tue, 4 Dec 2018 09:04:18 +0000
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Subject: Re: [rtcweb] Clarification on simulcast and RID and RepairedRtpStreamId
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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:
>
>> 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.
> So, you say that the receiver would receive both media and RTX streams
> with same RID values, and then the receiver would decide whether it's
> a media or a rtx packet based on PT, right?

Yes, but for the Repair stream I think the right thing would be to
replace the RtpStreamID with a RepairedStreamId SDES Item and header
extension as appropriate but with the same RID ID.


>
>
>> 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.

>
> Why is there any problem with 4588? This is just about the browser
> adding RID to RTX packets. That breaks nothing in RFC 4588 (IMHO).

With the exception for the issue with the header extension related to
RepairedRTPStreamID above I agree nothing breaks. However, the late
binding mechanism in RFC4588 is actually problematic in some cases.
Where the RepairedRTPStreamId pointing to the relevant RID actually
supports dynamic in session rebinding and have no issues with multiple
outstanding NACK requests prior to learnt the binding. So this is a
superior mechanism that should be recommended for usage with RFC4588 RTX
in all new applications and possibly some old ones.

The legacy fallback story appears reasonably clear as there are several
signalling attributes one can key on that wouldn't be there unless one
supports this mechanism.


Cheers

Magnus Westerlund 

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