Re: [MMUSIC] BUNDLE TEXT: De-mux procedures (June 19th)

Paul Kyzivat <pkyzivat@alum.mit.edu> Wed, 19 June 2013 19:37 UTC

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Date: Wed, 19 Jun 2013 15:35:13 -0400
From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Subject: Re: [MMUSIC] BUNDLE TEXT: De-mux procedures (June 19th)
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On 6/19/13 1:09 PM, Emil Ivov wrote:
>
>
> On 19.06.13, 18:47, Christer Holmberg wrote:
>> Hi,
>>
>>>> The question is whether we shall specify exactly how entities
>>>> associate RTP packets with the correct m= lines, or whether we shall
>>>> simply give some guidelines and hints.
>>>>
>>>> For example, a client can choose to use unique PT values per m-
>>>> values, within a given BUNDLE group, and everything should work
>>>> fine. And, if the endpoint runs out of PT values, the create a new
>>>> BUNDLE group. Now, whether it's religiously correct, I don't know :)
>>>>
>>>> OR, if the endpoints are able to signal which SSRCs they are going
>>>> to use, they can use that.
>>>>
>>>> OR, if the endpoints support some kind of RTP header extension for
>>>> this purpose, they can use that.
>>>>
>>> OR, if they don't care which M-line the stream is associated with,
>>> the question may be moot.
>>
>> Correct.
>>
>>> In all cases, the payload type MUST clearly identify the codec
>>> configuration it's associated with.
>>
>> *IF* you have a way to find the right m= line, couldn't PT X have
>> different codec configurations in different m= lines?

Yes, I think so.

> Let's assume that it could ... but then how would you know that your
> peer also supports the same demuxing technique as yourself?

Muxing/demuxing (in the sense of associating to a particular m-line) 
MUST be consistent on both ends. (The sender knows which m-line he 
associates a packet with, and expects that the receiver will make the 
same association.)

There must be enough signaling to get the two sides in sync about this.

> In SIP for example, when constructing a first offer, you may want to map
> both Opus and VP8 to 96, then add SSRCs for demuxing. This means that
> you expect your peer to support SSRC demuxing and you can't expect that
> unless we mandate it.

In this case there will be two m-lines, one audio and one video, each 
using PT 96. And the offer specifies a specific ssrc for each m-line.

If the answerer does't understand or support muxing by ssrc it will then 
note that if it accepted both m-lines it would not be able to associate 
packets with m-lines. So it then has two choices in constructing its answer:

- reject the bundle, but accept both m-lines
- accept the bundle, but reject one of the m-lines.

This works even for hypothetical new demux techniques that might not be 
supported by all.

	Thanks,
	Paul

> Or am I missing something?
>
> Emil
>
>