W3C home > Mailing lists > Public > public-media-fragment@w3.org > October 2008

Re: video use-case

From: Dave Singer <singer@apple.com>
Date: Mon, 6 Oct 2008 09:03:59 -0700
Message-Id: <p06240800c50fe47a600e@[10.0.1.8]>
To: Media Fragment <public-media-fragment@w3.org>

I rather suspect that these points have been covered before, but in 
case not, here they are:

As I understand it, fragment identifiers follow a "#" in URIs, and 
they are interpreted by the user-agent.  The syntax and semantics of 
the fragment identifier are defined by the resource type.  That is, 
what constitutes a legal fragment for say HTML may look very 
different from a legal fragment for say MP4.  The fact that they are 
interpreted by the UA does NOT mean that the entire resource is 
automatically downloaded and then the fragment interpreted, however. 
For example, in a media file that has an index and is in time-order, 
a user-agent wanting a time-based subset may be able to use 
byte-range requests to get the index, and then the part(s) of the 
file containing the desired media. (We do this today for MP4 and MOV 
files).

Query identifiers follow a "?" and are interpreted by the server. 
The syntax and semantics are defined by the server you are using.  To 
the UA, the result is "the resource".  I'm not aware of servers that 
can do time-based trimming of media files, but it's certainly 
possible that they exist.

I think it would be fairly easy, however, to do server-based 
time-trimming of, for example, RTSP/RTP-based streams.  I rather 
think the same would be true for mpeg-2 streams.  (This would be 
query-based).  It would also be easy for a client to interpret 
fragments and do the corresponding seek request(s) over RTSP.  I am 
unclear on the ownership of fragment identifiers in RTSP, however.

This 'ownership' of fragment and query identifiers rather suggests to 
me that the best we can do is a 'best practices' document.  It would 
be great if a whole variety of media files were to adopt the same 
fragment syntax, and a whole variety of HTTP servers were to adopt 
the same query syntax, but I doubt that we can mandate it.

This also raises the question of whether the work will be 
HTTP-specific, or whether other delivery protocols will be 
considered.  (e.g. RTSP/RTP, shoutcast, ultravox, mpeg-2 transport, 
and so on).
-- 
David Singer
Apple/QuickTime
Received on Monday, 6 October 2008 16:04:41 GMT

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