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

[Magnus Westerlund <[e-mail]>]

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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