Subject: Re: [MMUSIC] Relative control attributes in SDP

From: [email-masked]
Date: Fri, 18 Oct 2002 12:12:55 +0200

Magnus, you wrote:

>I don't find it reasonable that the HTTP client has to be able to parse
>SDP and update it based on the Content-Location/Content-Base header.

I completely aggree it would be very bad,
it breaks the modularity i.e.
a HTTP browser, when it sees a SDP file should be able to
simply look up which "helper" application to invoke ...

the question is, where can this be normatively documented ?

regards,

Philippe Gentric
Software Architect
Philips MP4Net
[email-masked]
http://www.mpeg-4.philips.com

                      Magnus Westerlund                                                                                                        
                      <[email-masked]             To:  Tom Marshall <[email-masked]>                                              
                      ricsson.se>                          cc:  [email-masked]                                                                
                      Sent by:                              (bcc: Philippe Gentric/MP4-SUR/CE/PHILIPS)                                         
                      [email-masked]                Subject:   Re: [MMUSIC] Relative control attributes in SDP                          
                                                                                                                                               
                                                           Classification:                                                                     
                      18/10/2002 09:35                                                                                                         

Hi Tom,

In my opinion all SDPs received with other means than a RTSP DESCRIBE
should have absolute control URLs. Then one avoids all these problems.
But In your case the HTTP client has to change the SDP and create
absolute URLs from the relative and the Content-base/Content-Location
header. But one has to be aware that Content-Base header does not exist
in the draft standard version of HTTP 1.1 (RFC 2616) and that is the
header with the best semantics.

Content-Location does not only work as base URL for the entity (SDP). It
also has the role of pointing out the location if available elsewhere.
This should be true in most cases that the SDP can be received by an
RTSP DESCRIBE from the Content-Location URL. But it is possible to have
media structures that does not correspond to that and then
Content-Location can't be used.

I don't find it reasonable that the HTTP client has to be able to parse
SDP and update it based on the Content-Location/Content-Base header.

So it is simplest to serve SDP's for HTTP download with absolute control
URLs.

Regards

Magnus

Tom Marshall wrote:

>I have a question regarding the proper handling of the Content-Base
>header
>in an SDP for an RTSP presentation that is fetched via HTTP.
>
>RFC2326 specifies the format of the "control" attribute in SDP.  It
>provides
>for a completely relative syntax with the base url being provided in the
>Content-Base and Content-Location headers.  RFC2068 (section 14.11) also
>has
>an explanation of how these headers are to be used, which matches up
>nicely
>with RTSP.
>
>Suppose an SDP is downloaded via HTTP.  This could be downloaded by a
>browser on behalf of a media player, for example.  The SDP has no
>absolute
>urls and the HTTP response has a Content-Base header that provides the
>base
>url.  How should the control attributes be handled after the SDP has
>been
>downloaded?  Should the HTTP client use the Content-Base header and
>modify
>the control attribute, or should the SDP be left as-is?
>
>Example:
>
>GET /video.sdp HTTP/1.0
>Host: media.example.com
>
>HTTP/1.0 200 OK
>Content-Base: rtsp://media.example.com/video.sdp/
>...
>
>v=0
>...
>m=audio
>a=control:trackID=0
>...
>m=video
>a=control:trackID=1
>
>If the SDP is passed around unmodified, there is no absolute url and
>it's
>pretty useless to the media player.  However, if the Content-Base header
>is
>used, the media player will see this, and it will be happy:
>
>v=0
>...
>m=audio
>a=control:rtsp://media.example.com/video.sdp/trackID=0
>...
>m=video
>a=control:rtsp://media.example.com/video.sdp/trackID=1
>
>

--

Magnus Westerlund

Multimedia Technologies, Ericsson Research ERA/TVA/A
----------------------------------------------------------------------
Ericsson AB                | Phone +46 8 4048287
Torshamsgatan 23           | Fax   +46 8 7575550
S-164 80 Stockholm, Sweden | mailto: [email-masked]

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