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Format of RTSP URLs

From: Rob Lanphier <robla@prognet.com>
Date: Mon, 14 Jul 1997 20:07:26 -0700
Message-Id: <3.0.32.19970714200724.011aeb24@mail.prognet.com>
To: www-talk@w3.org, uri@bunyip.com
Cc: confctrl@isi.edu
Members of www-talk and uri,

First, an introduction:  the confctrl group is currently working on the
Real Time Streaming Protocol (RTSP), which is for control of on-demand data
with real-time properties (usually multimedia).  See
http://www.real.com/prognet/rt for more details on this.

Without getting into too much depth about the protocol itself, we are
trying to sort out how the protocol should handle multi-stream container
files (ala Quicktime, ASF, RealMedia File Format, which often contain both
audio and video streams, and may contain many other types).  It is the
consensus that it is important for the protocol to deal in a distinct
manner with the different streams.  The controversy stems from if/how the
URL is used to achieve this.

A promenent proposal for achieving this is as follows:
Full Container file:
rtsp://foo.com/example.mov

Individual Track within container file:
rtsp://foo.com/example.mov?track=1
(the "track=1" portion is file format specific, the "?" is the consistant
part).

The issues we are debating are:
1.  Whether the protocol should make special accomodations for multistream
container files
2.  If yes, if/how should that be expressed in the URL

I'm going to switch here to representing myself and Progressive Networks.
Here's the concern that we have.  "?" is used in HTTP as the query
delimiter, and the "?track=1" use isn't consistant with that.  We want to
make sure that things are as consistant as possible between RTSP and HTTP
because:

*  We want the authoring scenarios to be protocol independent.  If someone
goes through the trouble of writing the media metafile and web page which has
different streams from the same container showing up on different parts of
the page, we want that metafile to be the same whether or not the content
is actaully being streamed via RTSP or HTTP.
*  One day, given the similar appearence of RTSP and HTTP, there may be an
effort to merge RTSP and HTTP, or to define some common framework that both
share.  We want to avoid pain when we can.
*  Frankly, we want to make sure that someone could implement a "cgi"
filesystem using the RealMedia Architecture that works as similarly as
possible to HTTP.  Any binding of "?" to protocol operation makes this
difficult/impossible.

The URL scheme, taken from Roy Fielding's draft on the subject
(draft-fielding-url-syntax-05.txt) is something we'll have to consider very
seriously in all of this.  The URL syntax there is:
<scheme>://<site>/<path>?<query>#<fragmentid>

The problem with that scheme is that "fragmentid" is really "client-side
fragment id".  What we really need is a server side fragment id as well.

<scheme>://<site>/<path>?<query>:<ssfrag>#<fragmentid>

This server-side fragment id would allow the client to play around with the
URL without messing up the query portion.  ":" is presented only as a
strawman.  I'm not sure what the correct character would be here.

The point here is to make it as simple as possible for a server to add and
subtract fragments from the server-side fragment portion.  If this is
buried in the query, it's very difficult.  If it is clearly delimited and
hanging off of the end, it's really straightforward.

I don't want to drag anyone here into a debate about whether or not
container file accomodations are needed (though I would welcome those who
want to dive in further and comment on confctrl), but I'm mainly interested
in is what the feeling is with regards to new URL schemes and consistancy
with HTTP.

Background information on this issue can be found in the following mail
archive:
http://www.mbone.com/lists/confctrl.1997/

Of particular interest will be:
"Container files and various methods of dealing with them" thread (the
inaugural message)
"Solving 1-1-n" thread
"Format of RTSP URLs" thread
"Sessions, presentations and URLs in RTSP" message

Many thanks in advance for your insight on this.

Rob


---
Rob Lanphier               Voice: (206)674-2322         Fax: (206)674-2699
Program Manager-Protocols                         Email: robla@prognet.com
Progressive Networks-Home of RealAudio            Web: http://www.real.com
For more information on firewalls:       http://www.real.com/firewall.html
For more information on RTSP:               http://www.real.com/prognet/rt
Received on Monday, 14 July 1997 23:09:22 GMT

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