Subject: [AVT] RTP collision resolution

From: Aaron Colwell <[email-masked]>
Date: Sun, 27 Apr 2003 13:31:07 -0700 (PDT)

I've been reading through the RFC 1889 and draft-ietf-avt-rtp-new-12.txt
and I still can't quite figure out a few things about the SSRC collision
resolution algorithm.

I will first start with the simple case where there is one sender in the
RTP session. In this case there is only one sending SSRC and the "send a
BYE for the old SSRC and pick a new SSRC" is pretty clear and easy for the
client to figure out what is happening. Does the sender keep the same
sequence number and timestamp offset across this switch? My current
assumption is that it does, but I could not find that explicitly stated
anywhere. Is that assumption correct? I hope so since it might cause all
sorts of problems for certain payload types like interleaved AMR.

The second case that I am confused about is when there are more than one
sender in an RTP session. For this example I'll use the case where we are
doing SSRC multiplexing of a stream and a retransmission stream. In this
case both SSRCs are associated with the same CNAME. In the highly unlikely
chance that both SSRCs conflict with existing SSRCs in the session, how
will the client resolve the proper transition of both of these SSRCs to
new SSRCs? The only way that I can think of to resolve this would be to
use the payload type. Is that how the resolution should happen?

My third case is similar to my second case. I have seen examples in the
archives where people talk about using SSRC multiplexing for a lecture
type environment where there is a SSRC assigned to a lecturer, and a
SSRC assigned to a question microphone. In this scenario, the two SSRCs
likely have the same payload type so the resolution mechanism from case 2
would not work. How is the client supposed to resolve the transitions in
this case? Do all of these sources have the same CNAME? If not, then the
resolution can be done based on the CNAME. I was under the impression
that the CNAME would be the same for all the SSRCs in this scenario
though.

One final question about SSRC multiplexing in general. Is it assumed that
all SSRCs associated with the same CNAME should be mixed together on the
client side?

Any clarification people can provide on these questions would be great.

Thanks,

Aaron
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