RE: a=control in SDP

Magnus:

The extra slash does not hurt anything, does it?
Many people in 3GPP interop do use this approach (as suggested by Aaron).

Regards,
Thomas

-----Original Message-----
From: Magnus Westerlund [mailto:magnus.westerlund <at> era.ericsson.se]
Sent: Wednesday, September 11, 2002 12:21 AM
To: acolwell <at> real.com
Cc: IETF MMUSIC WG
Subject: Re: [MMUSIC] a=control in SDP

Hi Aaron,

Aaron wrote:

>I was under the impression that you could support this functionallity by 
>doing the following
>
>In the DESCRIBE response
>Content-Base: rtsp://example.com/donald.mp4/
>On session level:
>a=control:*   
>On media level
>a=control:trackID=2
>
>Is that not the case?
>
No, It would solve the problem at media level and that URL would resolve 
to "rtsp://example.com/donald.mp4/trackID=2" which is what I want. 
However the aggregated control URL on the session level would become 
"rtsp://example.com/donald.mp4/". And in this URL that extra slash is 
not desired.

Regards

Magnus

>
>Aaron
>
>On Tue, 10 Sep 2002, Magnus Westerlund wrote:
>
>>Hi,
>>
>>I have found a problem when using relative URLs in a=control: lines. One 
>>would like for on-demand stored files to not have to edit any of the 
>>control URLs when responding on a DESCRIBE request. This would be 
>>extremely good to have for container files like MP4 files. All media is 
>>part of the file and can be referenced within the file with just a track 
>>id. But as the relative URL scheme is defined in RFC 2396 which updates 
>>RFC 1808 referenced in the RTSP RFC, any relative URL must at least 
>>contain the filename of the container file. This results in that a SDP 
>>document delivered to a client may look like this.
>>
>>For a container file that has the URL: rtsp://example.com/donald.mp4
>>On session level:
>>a=control:*
>>On media level
>>a=control:donald.mp4/trackID=2
>>
>>Container files that stores SDPs would not like to have the filename of 
>>the file in there. Having the filename in the files creates problem when 
>>copying or renaming the file. A server may update the filename but this 
>>puts more load on the server when the goal of these container files it 
>>to put minimal load on the server and be media independent. Therefore it 
>>would be good if one could make SDP a=control lines like this:
>>
>>On session level:
>>a=control:*
>>On media level
>>a=control:*/trackID=2
>>
>>So does any see problems with extending the semantics of "*" in 
>>a=control URLs to allow the above media level usage?
>>
>>Then I have a second question. Is the use of "/" before "trackID=" the 
>>most appropriate way of referencing into a container file? Or would 
>>parameter or any other separator be better? At least "#" seems to be a 
>>bad choice as it refers to actions for the user agent (client).
>>
>>Regards
>>
>>Magnus Westerlund
>>
>>
>> From RFC 2326:
>>----------------------------------------
>>C.1.1 Control URL
>>
>>   The "a=control:" attribute is used to convey the control URL. This
>>   attribute is used both for the session and media descriptions. If
>>   used for individual media, it indicates the URL to be used for
>>   controlling that particular media stream. If found at the session
>>   level, the attribute indicates the URL for aggregate control.
>>
>>   Example:
>>     a=control:rtsp://example.com/foo
>>
>>   This attribute may contain either relative and absolute URLs,
>>   following the rules and conventions set out in RFC 1808 [25].
>>   Implementations should look for a base URL in the following order:
>>
>>   1.     The RTSP Content-Base field
>>   2.     The RTSP Content-Location field
>>   3.     The RTSP request URL
>>
>>   If this attribute contains only an asterisk (*), then the URL is
>>   treated as if it were an empty embedded URL, and thus inherits the
>>   entire base URL.
>>----------- end of quote -----------
>>
>>
>

--

-- 

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: magnus.westerlund <at> era.ericsson.se

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