Subject: [AVTCORE] Input on draft-alvestrand-rtp-sess-neutral-00

From: "Roni Even" <[email-masked]>
Date: Thu, 15 Mar 2012 13:46:46 +0200

Hi,

I am sending this review to the AVTcore list since it will be on the AVTcore
agenda for Paris.

My input is as an individual.

I think the review given in the document provides a good discussion point on
the multiplexing issue with regards to RTP sessions.

Section 5 discuss section 5.2 of RFC 3550 which lists 5 problems why not to
multiplex different RTP media types in one RTP session.

Problems 1-3 are addressed by using different SSRCs while for point 5 my
understanding is that the draft  suggests using BUNDLE to signal the
different priority partitions. 

This leaves problem 4 (the mixer). Section 7 of RFC  3550 does not mandate
any specific mixing, I assume that it was meant that since an RTP session is
composed of the same media type it will allow the mixer to combine all input
RTP streams using his own (Single?) SSRC. This is probably already broken
for the examples listed in the draft (FEC,  T140) if the RTP mixer does not
know how it can mix the subtypes only based on the RTP header information
(for dynamic payload type numbers) without having the SDP information.

I think that the issue of the RTP mixer behavior should also be discussed
and some text is needed to explain what is needed for it to work. I think
this is the motivation for signaling the RTP session as part of the RTP
information or we can say that RTP mixer will not work for different media
type or even for the cases in the examples (FEC, T140). This can be an
update to the topology draft (RFC5117).

Roni Even