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

[Iñaki Baz Castillo <[e-mail]>]

2016-03-08 20:11 GMT+01:00 Christer Holmberg <[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.
>
> Why do you think this is BUNDLE specific? If you think it should be allowed to use the same PT value for different codecs within a BUNDLE group, shouldn't it then also be allowed to use the same PT value for different codecs within a single m- line? In both cases we are talking about a single RTP session, carried over a single transport.

There is no way in SDP-land to specify which payload will be carried
over which SSRC within a m= section. So SSRC is the only way to
properly demux it. In fact, when we see something like:

a=rtmap:100 opus/48000/2
a=rtmap:96 rtx/9000
a=fmtp:96 apt=100
a=ssrc-group:FID 111111 222222
a=ssrc:111111 cname foo
a=ssrc:222222 cname foo

That means: opus will be sent over the SSRC 111111 or 222222, and rtx
will be sent over SSRC 222222 or 111111, but we don't even know which
SSRC will be used until the first RTP packet is received.

So, SDP does not allow sending different codecs with same PT over the
same m= section given that all the "audio/video" codecs will be
carried over the *same* SSRC stream.




-- 
Iñaki Baz Castillo
<[e-mail]>