Re: [MMUSIC] Simulcast: Inclusion of the same SCID in multiple Simulcast streams

["Roni Even" <[e-mail]>]

Hi Magnus,
Looking more at your question and since I do not see so much usage of
multicast in multipoint conferencing applications, this looks OK. Section
6.4 almost addresses my concern about usage of simulcast with multicast in
SDP for streaming like applications. Will send a separate email
Roni

> -----Original Message-----
> From: Roni Even [mailto:[e-mail]]
> Sent: Wednesday, November 02, 2016 1:34 PM
> To: 'Magnus Westerlund'; 'mmusic (E-mail)'; 'draft-ietf-mmusic-sdp-
> [e-mail]'
> Subject: RE: [MMUSIC] Simulcast: Inclusion of the same SCID in multiple
> Simulcast streams
> 
> Hi Magnus,
> I would like to verify that I understand this correctly.
> 
> The case is about having a media source that sends simulcast streams in
two
> RTP sessions represented in SDP by two m-lines. The first one will have,
for
> example, HD resolution and CIF while the second will have SD and same CIF
> as the first one. You suggest to not allow having the CIF in both RTP
sessions
> but still allow having, for example, one with HD and VGA and a second with
> SD and CIF?
> 
> I see value when using multicast with simulcast to have one multicast
group
> with HD and a second multicast with SD and maybe a third multicast with
CIF
> from the same source (example case is streaming live events (sports, ...)
> using simulcast and multicast for distribution)
> 
> Roni
> 
> > -----Original Message-----
> > From: mmusic [mailto:[e-mail]] On Behalf Of Magnus
> > Westerlund
> > Sent: Tuesday, November 01, 2016 11:46 AM
> > To: mmusic (E-mail); [e-mail]
> > Subject: [MMUSIC] Simulcast: Inclusion of the same SCID in multiple
> > Simulcast streams
> >
> > WG,
> >
> > During the reviews of the edits yesterday I did find one more issue
> > that should be resolved to avoid interoperability issues. There is
> > currently no text making it explicit if it is allowed to have the same
> > simulcast format (and thus
> > SCID) appear in multiple simulcast streams. I believe that unless we
> > either explicit allow this or explicitly disallow it implementations
> > will make different interpretations and thus we end up with
interoperability
> issues.
> >
> > So the reason Bo and I didn't conclude yesterday that this is a bad
> > idea and we should just explicitly disallow it because it would
> > simplify things are two reasons.
> >
> > The first is that as we have attempted to create the best
> > possibilities for future extensions, e.g. for multicast or at least
> > spread one media source over multiple RTP sessions. Thus, forbidding
> > things can cause issues for the ones defining such extensions, unless
> > we see it necessary to ensure the base mechanism working.
> >
> > Secondly, there are actually usage benefits for allowing a format to
> > be included in multiple simulcast streams. A not unusual way to
> > consider simulcast streams is that they fulfill a purpose in the
> > application. For example one simulcast is the video suitable to
> > present the current speaker in a better fidelity version with higher
> > resolution. One may also have a simulcast stream suitable for the
> > recent speakers that is an intermediate format between high resolution
> > and low resolution thumbnails. In such a case the simulcast stream for
> > the high resolution could include a least preferred simulcast format
> > (SCID=Y) of a rather low resolution version (for being the high
> > resolution) as the last fallback for this streams purpose. At the same
> > time the intermediate simulcast stream could include the same format
> > SCID=Y as suitable format for this medium resolution usage, possible
> > with a much higher preference as it would provide good quality for
> > this streams purpose. So by including the same format in two streams,
> > in situation where the media sender becomes resource restrained, for
> > example in bandwidth, the media source sender could make the decision
> > that the best way of providing both the high and intermediate
> > simulcast streams are to select to send a single RTP stream according to
> SCID=Y that matches both.
> >
> > If one would not allow an SCID to be in multiple simulcast streams,
> > then the likely result in the above case is that one have to turn of
> > the high resolution simulcast stream due to insufficient resources.
> > This may or may not mean that the application logic chooses to use the
> > intermediate stream as stand in for the high, given that the stream
> > fits the restriction of any of the high resolution simulcast stream's
formats.
> >
> > So, this example is not particular convincing in regards to why one
> > should allow this, as the application logic and with some care when
> > negotiating the simulcast formats for the different streams still can
> > allow a simulcast using middlebox to utilize the simulcast stream as
> > stand in for another simulcast stream if that one is not available.
> >
> > So, I currently don't have any better example of why one would allow
> > it, but I think it is important that the WG gets to think about this.
> > So please consider if you have usage that would benefit from allowing
> > multiple simulcast streams to include the same format (with the same
> > SCID)? If no good usage arises then I suggest we explicitly disallow it.
> > Please provide your feedback by the end of the IETF meeting (18th of
> > November).
> >
> > Cheers
> >
> > Magnus Westerlund
> >
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