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

Iñaki Baz Castillo <ibc@aliax.net> Tue, 08 March 2016 13:23 UTC

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From: =?UTF-8?Q?I=C3=B1aki_Baz_Castillo?= <ibc@aliax.net>
Date: Tue, 8 Mar 2016 14:23:30 +0100
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Subject: [MMUSIC] BUNDLE: no reason por payload type to be unique within bundled m= lines
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draft-ietf-mmusic-sdp-bundle-negotiation-27 in section 10.1.2 states:

   Multiple bundled "m=" lines might represent RTP based media.  As all
   RTP based media associated with a BUNDLE group belong to the same RTP
   session, in order for a given payload type value to be used inside
   more than one bundled "m=" line, all codecs associated with the
   payload type number MUST share an identical codec configuration.
   This means that the codecs MUST share the same media type, encoding
   name, clock rate and any parameter that can affect the codec
   configuration and packetization.
   [I-D.ietf-mmusic-sdp-mux-attributes] lists SDP attributes, whose
   attribute values must be identical for all codecs that use the same
   payload type value.

This basically states that there cannot be two different codecs with
same payloadType (or same codec with different codec parameters)
within two m= sections sharing the same "transport".

Let me please describe a "real" use-case:

* SFU server.
* Alice wants to send audio "opus" with payload 100 and SSRC 1111.
* Bob wants to send audio "g722" with payload 100 and SSRC 2222.
* Carol just wants to receive audios from Alice and Bob over a single
transport (BUNDLE).

When Carol connects with the SFU she negotiates a single ICE/DTLS
transport, and the SFU notifies Carol about two receiving audio tracks
(from Alice and Bob):

1) mid: alice-audio,  codec: opus,  payload: 100, ssrc: 1111
2) mid: bob-audio,   codec: g722,  payload: 100, ssrc: 2222

This would mean that Carol receives an SDP with two "m=audio"
sections using BUNDLE:

  m=audio [...] 100
  a=rtpmap:100 opus/48000/2
  a=ssrc:1111 cname:foo
  m=audio [...] 100
  a=rtpmap:100 g722/48000
  a=ssrc:2222 cname:bar

What is the problem here? Why is having the same payload type (100) a problem?

To be clear, Carol can demux the incoming RTP:

* By matching the RTP MID header (if present).
* By matching the RTP SSRC (1111 => 'alice-audio', 2222 => 'bob-audio').

There is no need to check the RTP payload in order to retrieve the
appropriate m= section. And worse: this constraint forces the SFU
developer to rewrite the payload type sent by the participants in a
SFU room so all of them are different.

IMHO there is no real justification for this constraint, it provides
nothing useful, and it limits and makes worse some common scenarios.
So I ask for such a constraint to be removed from the draft.

Thanks a lot.

Iñaki Baz Castillo