Re: [MMUSIC] How does mmt grouping work with multiple streams of the same type?

["Stach, Thomas" <[e-mail]>]

Harald Alvestrand wrote:
> On 10/12/2012 09:58 AM, Stach, Thomas wrote:
>>>> I think we should continue to use RFC 5576 for this purpose:
>>>> 
>>>> a=ssrc:1 <something follows>
>> I haven't thought about that, but yes it would work for as well.
>> For two audio streams (audio1 w/ PCMU or PCMA, audio2 w/ iLBC) it
>> would look like this? 
>> 
>>     a=rtpmap:0 PCMU/8000
>>     a=rtpmap:8 PCMA/8000
>>     a=rtpmap:97 iLBC/8000
>>     a=mmtype: 0 audio
>>     a=mmtype: 8 audio
>>     a=mmtype: 97 audio
>>     a=ssrc:12345 fmtp:0
>>     a=ssrc:12345 fmtp:8
>>     a=ssrc:67890 fmtp:97
> Yes, except that as far as I can tell from RFC 5576, you
> can't use fmtp:
> without an actual parameter value: "This parameter MUST only
> be used for
> media
>     types for which source-level format parameters have
> explicitly been
>     specified". (section 6.3).
> 
> If you need to use a field that's always present, use CNAME.
> 
> Note(1): If you only use one payload type for all your audio,
> you have
> no need to signal which format they're using.
> 
> Note(2): In the above, you have ssrc 12345 using both PCMU/8000 and
> PCMA/8000. I don't know if it's intentional or not.
> 
Yes, this was intentional, 
I used two different ssrc (12345, 67890) in order to indicate that I want to establish two audio streams. 
I used ssrc=12345 twice in order to give a hint that the first audio stream should either use PCMU or PCMA (and ssrc=12345).
The second audio stream would use ssrc=67890 and iLBC.

However from your responses I'm not sure if we are on the same page with respect to what I want to achieve.
Let me tray to clarify.

My understanding is that 
- the m=anymedia gives a complete description of what is offered,
- if the answerer understands m=anymedia, it accepts the m=anymedia line and rejects all other m-lines
- if the answerer understands m=anymedia, it can ignore all other m-lines in the SDP, it doesn't even have to look at them in order to build its answer.

If this is correct, consider an SDP offer with e.g. 4 m-lines (2 audio+2 video)

       v=0
       o=alice 2890844526 2890844526 IN IP4 host.atlanta.com
       s=
       c=IN IP4 host.atlanta.com
       t=0 0
       m=audio 10000 RTP/AVP 0 8
       a=rtpmap:0 PCMU/8000
       a=rtpmap:8 PCMA/8000
       m=audio 10002 RTP/AVP 97
       a=rtpmap:97 iLBC/8000
       m=video 20000 RTP/AVP 31 
       a=rtpmap:31 H261/90000
       m=video 20002 RTP/AVP 32
       a=rtpmap:32 MPV/90000

How would the associated m=anymedia line look like in this case, such that the answerer can see 
that 2 audio and 2 video streams are offered without having to look into the other m-lines?

How would the answer look like?

Thanks
Thomas


>>>> 
>>>> The suggestion above wouldn't work for the case of two SSRCs
>>>> both using the same payload type.
>> Well it would if you duplicated the a=mmtype attribute with
> different ssrc elements in that case.
>>       a=mmtype: 0 audio ssrc=1
>>       a=mmtype: 0 audio ssrc=2
>> Would you do something else for a=ssrc?
> 
> Well, a=ssrc *is* something else. The above mixes info about payload
> types and info about SSRCs; I think that's a bad design.