Re: [AVT] Final edits to RTP spec and A/V profile

[Stephen Casner <casner@acm.org>]

Attempt number three on the definition of an RTP session.  David Leon
made some suggestions, and I fixed some typo problems.  Is this clear?
And even more important, do you agree with what it specifies?

        RTP session: An association among a set of participants
             communicating with RTP.  A participant may be involved in
             multiple RTP sessions at the same time.  In a multimedia
             session, each medium is carried in a separate RTP session
             with its own RTCP packets.  A participant distinguishes
             multiple RTP sessions by reception of different sessions
             using different pairs of destination transport addresses,
             where a pair of transport addresses comprises one network
             address plus a pair of ports for RTP and RTCP.  All
             participants in an RTP session may share a common
             destination transport address pair, as in the case of IP
             multicast, or the pairs may be different for each
             participant, as in the case of individual unicast network
             addresses and port pairs.  In the unicast case, a
             participant may receive from all other participants using
             the same pair of ports, or may use a distinct pair of ports
             for each.

             The distinguishing feature of an RTP session is that each
             defines a distinct space of SSRC identifiers (defined
             next).  The set of participants included in one RTP session
             consists of those that can receive an SSRC identifier
             transmitted by any one of the participants either in RTP as
             the SSRC or a CSRC (also defined below) or in RTCP.  For
             example, consider a three-party conference implemented
             using unicast UDP with each participant receiving from the
             other two on separate port pairs.  If each participant
             sends RTCP feedback about data received from one other
             participant only back to that participant, then the
             conference is composed of three separate point-to-point RTP
             sessions.  If each participant provides RTCP feedback about
             its reception of one other participant to both of the other
             participants, then the conference is composed of one
             multi-party RTP session.  The latter case simulates the
             behavior that would occur with IP multicast communication
             among the three participants.

                                                        -- Steve

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