Subject: [AVT] RTCP handling in mixers

From: Magnus Westerlund <[email-masked]>
Date: Wed, 01 Feb 2006 16:59:10 +0100

Hi,

We are working on updating the Codec Control messages in AVPF. In that 
draft we have been working on the topologies that may occur in 
conferencing. In that discussion I are a bit uncertain on how the RTP 
mixer model is supposed to handle RTCP in a specific case:

               +-----+
    +---+     /       \     +-----------+      +---+
    | A |<---/         \    |           |<---->| B |
    +---+   /   Multi-  \   |           |      +---+
           +    Cast     +->|   Mixer   |
    +---+   \  Network  /   |           |      +---+
    | C |<---\         /    |           |<---->| D |
    +---+     \       /     +-----------+      +---+
               +-----+

In this case the mixer mixes the streams from the left domain to the two 
mixed domains B and D. They are different as any transmission from B 
will be mixed to D and vice versa. But there are no reason for the mixer 
to mix media from B or D to the multicast domain. I assume that B's and 
D's SSRCs will be used to indicate the source of the media.

However the RFC 3550 rules for mixers is that they should not forward 
reception reports from the left domain to either domain on the right. 
Also B's or D's reception reports should not be forwarded out of 
respective domains. That would result that for A, B will only be visible 
when transmitting as media and SR would be forwarded then to A.

Is this a case not envisioned? Or is the intention that the mixer have 
one SSRC for each participant in domains it mixes to and sends out 
traffic using those, and transmitting RRs also from them to indicate the 
presence of B?

Help with this would be greatly appreciated.

Cheers

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
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From: Stephen Casner <[email-masked]>
Date: Wed, 1 Feb 2006 23:42:49 -0800 (PST)

Magnus,

> mixer model is supposed to handle RTCP in a specific case:

There is not a requirement to mix the streams from the left domain to
B or D, either.  That is, you should use a translator if the goal is
to effectively broaden the multicast domain to B and D that have only
unicast service available.

If mixing is desired, then the mixer certainly could deliver one mixed
stream to the multicast network.

> when transmitting as media and SR would be forwarded then to A.

That is the limitation of a mixer.  It has other advantages, but is
not always the best choice.

> traffic using those, and transmitting RRs also from them to indicate the

I tried in Section 7 to convey the notion that "there may be many
varieties of translators and mixers designed for different purposes
and applications", and to state the rules as generically as possible.
You might be able to craft a hybrid mixer/translator for this
application, but you need to keep straight whether you have one SSRC
space or multiple spaces.  In the latter case, you can't pass SSRC ids
among them.

                                                        -- Steve