Subject: Re: [AVT] CNAME

From: Stephen Casner <[email-masked]>
Date: Mon, 6 Aug 2001 07:13:22 -0700 (PDT)

On Thu, 26 Jul 2001, Jori Liesenborgs wrote:

> In draft-ietf-avt-rtp-new-09, it says that a CNAME item must be included
> in each compound RTCP packet. If several consecutive RTCP compound packets
> have to be sent, for example because there are many RR blocks, does each
> of these compound packets need to have a CNAME? Or just one of them?

Thanks for asking this question because it points out a weakness in
the spec.  The answer is that only one compound RTCP packet should be
sent per interval, so the CNAME question becomes moot.  The reason why
there should be only one is that the interval calculation will not
work correctly if more than one is sent.  If multiple compound RTCP
packets were sent per interval, the avg_rtcp_size calculation would
have to take as input the sum of the lengths of all the packets sent
together.  That's hard because there isn't any way to tell when you
receive the first of these packets how many more there might be.
There is also a problem that there would be a significant risk that
the total RTCP bandwidth would be exceeded if a large number of
sources suddenly started transmitting and a large number of receivers
started reporting about them.

Instead, a receiver should build multiple RR's (and at least a CNAME)
in one compound RTCP packet up to the MTU limit.  If there are more
sources than will fit, then the receiver should round-robin through
them across multiple intervals.  I admit that this does complicate the
database a little bit.  I the implementation I am familiar with, which
used a hashed list with a linear enumeration function, we did not
worry about the fact that new sources could hash in before the current
value of the enumeration pointer.  We could just continue with the
enumeration and get them the next time around.

This increases the interval between a pair of reports for a given
source, but the design of RTCP using cumulative counters means that is
not a problem (other than getting information more slowly).

I think we've talked about this a few times, but there was a good deal
of hand waving going on.  I would be interested in hearing comments on
what I've said above, because I should add something to the spec about
this.

BTW, the current revision (as of a couple of weeks ago) is
draft-ietf-avt-rtp-new-10, but there was not change in this regard.

							-- Steve

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