Re: [MMUSIC] BUNDLE: no reason por payload type to be unique within bundled m= lines

[Magnus Westerlund <[e-mail]>]

Den 2016-03-09 kl. 12:00, skrev Iñaki Baz Castillo:
> 2016-03-09 11:25 GMT+01:00 Magnus Westerlund <[e-mail]>:
>>> If what I say in my previous email is true (maximum of 16 audio+video
>>> participants in a WebRTC multi-stream SFU conference) I think there is
>>> a big problem in BUNDLE, and hence in MMUSIC.
>>
>> <
>>
>> My understanding is that this is the error in your understanding of BUNDLE.
>> As long as the PT=100 all have the identical configuration they can share
>> the PT over multiple m= blocks in the same BUNDLE.
>>
>> Section 10.1.1 of Bundle-27 draft:
>>
>>     o  A specific payload type value can be used in multiple bundled "m="
>>        lines if each codec associated with the payload type number shares
>>        an identical codec configuration [Section 10.1.2].
>
> I know that Magnus, but what you stated above basically means that
> some RTP middleboxes just use the PT to match streams (ignoring the
> SSRC) and also that some RTCP matching rules use the PT as well.
>
> So, the fact that we have 20 bundled m=lines with same PT and codec
> configuration still worries me, because each of those PT belongs to a
> separate stream. How will the "legacy" RTP middlebox identify the
> proper stream by just checking the PT if there are multiple streams
> using the same PT? Why is not that a problem just because all the
> codec sharing PT have the same configuration? That's what I don't yet
> understand.
>

Your middlebox will have to do rewrite of the PT numbers to align the PT 
usage between one particular conference participant (endpoint) and the 
middlebox. You have to provide a consistent configuration between all 
RTP streams in the RTP session's leg between that endpoint and the 
middlebox. Yes, this is not optimal, but SDP in Offer/Answer context is 
lacking features for forcing the different endpoints to use a common 
configuration.

Cheers

Magnus Westerlund

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