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

[Harald Alvestrand <[e-mail]>]

On 10/11/2012 04:55 PM, Stach, Thomas wrote:
> inline
>
>> -----Ursprüngliche Nachricht-----
>> Von: Harald Alvestrand [mailto:[e-mail]]
>> Gesendet: Donnerstag, 11. Oktober 2012 16:45
>> An: Stach, Thomas
>> Cc: Christer Holmberg; Jonathan Lennox; mmusic WG
>> Betreff: Re: AW: How does mmt grouping work with multiple
>> streams of the same type?
>>
>> On 10/11/2012 04:13 PM, Stach, Thomas wrote:
>>> Harald,
>>>
>>> thanks for the reply.
>>> It helps for my second question, but not for my main question.
>>> How can the answerer recognize (from the m=anymedia line
>> alone) that multiple video streams need to be set up in the
>> same RTP session?
>> If each video stream is signalled by the presence of SSRC-associated
>> signalling for that video stream, you count the number of
>> video streams signalled.
> Could that be done by e.g. enhancing the new a=mmtype with an ssrc element?
> E.g.
>
>         a=mmtype: 0 audio ssrc=1
>         a=mmtype: 8 audio ssrc=1
>         a=mmtype: 97 audio ssrc=2
>         a=mmtype: 31 video ssrc=3
>         a=mmtype: 32 video ssrc=4
>
> would mean 2 audio streams and 2 video streams, whereas
>
>         a=mmtype: 0 audio ssrc=1
>         a=mmtype: 8 audio ssrc=1
>         a=mmtype: 97 audio ssrc=1
>         a=mmtype: 31 video ssrc=2
>         a=mmtype: 32 video ssrc=2
>
> means 1 audio and 1 video stream based on the number of different SSRCs?

On the principle of "don't invent new stuff if you already have stuff", 
I think we should continue to use RFC 5576 for this purpose:

a=ssrc:1 <something follows>

The suggestion above wouldn't work for the case of two SSRCs both using 
the same payload type.

>
>>> Regards
>>> Thomas
>>>
>>>> -----Ursprüngliche Nachricht-----
>>>> Von: Harald Alvestrand [mailto:[e-mail]]
>>>> Gesendet: Donnerstag, 11. Oktober 2012 15:41
>>>> An: Stach, Thomas
>>>> Cc: Christer Holmberg; Jonathan Lennox; mmusic WG
>>>> Betreff: Re: How does mmt grouping work with multiple streams
>>>> of the same type?
>>>>
>>>> On 10/11/2012 03:12 PM, Stach, Thomas wrote:
>>>>> Christer, Harald, Jonathan,
>>>>>
>>>>> I have the impression that the
>>>> draft-holmberg-mmusic-sdp-mmt-negotiation is based on
>>>> Jonathan's proposal
>>>>> to have an explicit desrciption for the bundled streams as
>>>> proposed in
>>>>> http://www.ietf.org/mail-archive/web/mmusic/current/msg09454.html
>>>>>
>>>>> Among other points the following is stated:
>>>>> * The m=bundle is a complete description of an entirely
>>>> separate m-line; if it's used, the other m-lines in the group
>>>> are rejected and the information they carry is ignored.
>>>>> I think this a desirable property
>>>>>
>>>>> Now, consider a case were you have 4 streams, i.e. 4
>>>> m-lines (e.g. 2 audio and 2 video).
>>>>> How would that be mapped to the m=anymedia line?
>>>>> How does mmt grouping work with multiple streams of the same type?
>>>>>
>>>>>    From the currently proposed syntax for the m=anymedia line
>>>> I cannot deduce that multiple audio or video streams are offered.
>>>>> I would have to look into the "legacy" m-lines to recognize this.
>>>>> In case that e.g. the video stream use different codecs, I
>>>> could also learn that only from the legacy m-lines.
>>>>
>>>> In the case where it's desirable to carry multiple video
>>>> streams in one
>>>> RTP session, and have different descriptions in SDP for
>> the different
>>>> video streams, there has to be some signalling that tells the
>>>> recipient
>>>> which description applies to which video stream.
>>>>
>>>> The video streams will have different SSRCSs. Therefore, the only
>>>> reasonable answer in the MMT grouping case seems to be
>>>> SSRC-associated
>>>> signalling.
>>>>
>>>> In the other variants, it's actually unclear to me how the
>>>> video streams
>>>> described by the different m= lines would be detected by the
>>>> recipients;
>>>> I'd been assuming SSRC-associated signalling for other
>>>> reasons, so only
>>>> when this discussion resurfaced, I started thinking about
>> this again,
>>>> and discovered that I didn't have an answer.
>>>>
>>>> That is, in the description
>>>>
>>>> a=group:bundle foo bar baz
>>>> m=video 4270
>>>> a=mid:foo
>>>> a=property-x:1
>>>> m=video
>>>> a=mid:bar
>>>> a=property-x:2
>>>> m=video
>>>> a=mid:baz
>>>> a=property-x:3
>>>>
>>>> one would expect a single RTP stream to be set up; when
>> the recipient
>>>> gets 3 video streams with SSRCS 234, 235 and 236, how does he
>>>> know which
>>>> stream should have which value for "property-x"?
>>>> One can solve it with SSRC-associated signalling in this case too:
>>>>
>>>> a=ssrc:234 property-x:1
>>>>
>>>> but then you're back to doing SSRC-associated signalling, and
>>>> if you're
>>>> already doing that, we're not very different from the MMT
>> description
>>>> anyway.
>>>>
>>>> That's how I think of it at the moment, anyway - YMMV.
>>>>
>>>>
>>>>