Subject: Single RTSP session to control multiple RTP streams

From: Eric Fleischman <[email-masked]>
Date: Tue, 24 Jun 1997 09:37:47 -0700

Anup Rao wrote:

>Whats seems to be the issue of debate:
>"A single URL and single RTSP session to control multiple streams/RTP
>streams. "
>Is this something the group feels strongly for ?As pointed out(I
>think),  this is possible only when tied intricately with the session
>description itself - ie the interpretation of a control message
>referencing the URL is tied in with the session desc. This will also
>involve(as rightly pointed out by Eric and others a modified SETUP
>message).

Yes, I concur that this is the core issue of this debate.

Until last week, I had not seen any posting on this list questioning
that an original intent of this WG was to define a single RTSP session
which can control multiple (RTP) streams. Back in February (or was it
March??) when Henning described the RTSP protocol to the W3C
Synchronization Working group (in the presence of the other co-authors)
this usage was explicitly highlighted. Early list discussions also
confirmed this orientation and goal and I participated in more than one
discussion about this very topic.

Thus, the view from my knot-hole is that this goal is indeed a
historical target of this working group. However, I suspect that the
authors of the spec have been slowly drifting from this target as they
have considered the difficult synchronization problems which can occur
if the controlled multiple (RTP) streams are not bounded within the
constraints provided by SDP or SDP-like protocols (e.g., the
full-fledged synchronization problems addressed by approaches such as
RTSL). 

I also suspect that the conflict identified by Anup between the
recipient selecting the port address (a truly good idea) and the SDP
approach where the conference originator selects the port address (also
a good idea), coupled with belatedly discovered weaknesses in SETUP,
have encouraged this drift.

My suggestion to remedy this is to require that either SETUP or DESCRIBE
must be used. SETUP uses will permit the recipient to select the port
address. DESCRIBE uses will permit the conference originator to select
the port address(es). If both SETUP and DESCRIBE are used in the same
RTSP session then a potential conflict may occur if both are indicating
different addresses. This potential conflict needs to be handled in the
spec.