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

[Stephen Casner <casner@acm.org>]

On Wed, 15 Jan 2003, Pekka Pessi wrote:

> 	What is the difference between an RTP cloud (a la section 7) and
> 	an RTP session?

An RTP cloud is a portion of an RTP session.  As section 7 says,
typically, each cloud is defined by a common network and transport
protocol (e.g., IP/UDP) plus a multicast address and transport level
destination port or a pair of unicast addresses and ports.  In other
words, the clouds are separated from each other by being on different
networks or using different transport protocols or addresses.  The
translator or mixer joins together the clouds into one RTP session by
being connected to each cloud.

> >        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.
>
> 	Don't we have multiple mediums in a single RTP session, for
> 	example, with some MPEG4 packetization? So, s/medium/RTP media
> 	type/, or is it then a circular definition?

Well, yes.  How about:

            In a multimedia session, each medium is typically carried
            in a separate RTP session with its own RTCP packets unless
            the encoding itself multiplexes multiple media into a
            single data stream.

> >             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.
>
> 	I'd drop this text away, as a RTP session can consist of two
> 	multicast group connected via a translator or a mixer.

I don't think it is sufficient to just drop this text.  I could add a
sentence to say that the addressing situation may be more complicated
with translators and mixers in section 7.

> 	I also think translators and mixers that forward CSRCs and SDESs
> 	should be mentioned in the definition.

A simple mention is appropriate, but I think any discussion is better
deferred to section 7.  I think the handling of CSRCs and SDESs is
specified clearly there.

                                                        -- Steve


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