Re: Symmetrical streams

On 2011-07-12 13:48, Harald Alvestrand wrote:
> On 07/11/11 22:18, Magnus Westerlund wrote:
>>> It's logical for these RTP sessions to be bidirectional.
>> RTCP within an given RTP session should always be bi-directional for
>> efficiency in number of transport flows needing to be established and
>> NAT traversal. Some application usages will result in that some RTP
>> session will only be unidirectional when it comes to actual media
>> streams. And uni-directional media streams of different purposes should
>> not be group together into a common RTP session over the same
>> bi-directional transport just to create bi-directional flows as that
>> messes with RTCP behavior.
> To be clear about the word "purpose" here, since it's gotten me into 
> trouble several times:
> Sending two video streams from one sender (for instance a MUC) to one 
> recipient (for instance an user) counts as one "purpose", right? Even 
> though one may be a high-res document-sharing screencast and the other 
> is a thumbnail-appropriate talking head?

I would say that document sharing and a view of the presenter are
actually two different purposes. The reasons is that if one would have a
central node performing switching operation on these streams, they would
likely be different behaviors. If the usage scenario is that a group
discusses a set of slides coming from one end-point. Then the slides
should be distributed to all unconditionally. However, which speaker
video stream to forward by the central unit is the one that is currently
the most active speaker. Thus different behaviors.

I use the word purpose, in the meaning the streams purpose in the
application, but it might be easier to think of this as different
application behavior around the streams.

In the end it is the application needs and behaviors that truly will
separate the streams into different type of sessions.

> While sending an audio stream and a video stream, even though they come 
> from the same source and are intended to be presented together, counts 
> as two different "purposes", right?

To clarify in the case of two different media types, they always have
different purposes/behaviors.


Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVM
Ericsson AB                | Phone  +46 10 7148287
Färögatan 6                | Mobile +46 73 0949079
SE-164 80 Stockholm, Sweden| mailto:

Received on Tuesday, 12 July 2011 14:24:44 UTC