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

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

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?

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.
> 
> 
> 
>