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Behaviour of Smart UAs vs. UAs that need server help in error cases

From: Davy Van Deursen <davy.vandeursen@ugent.be>
Date: Tue, 03 May 2011 13:12:39 +0200
Message-ID: <4DBFE327.1010401@ugent.be>
To: Media Fragment <public-media-fragment@w3.org>
Dear all,

another topic related to the test case discussions ...

The issue is about the case where we have an invalid media fragment URI, 
but that invalidity is only detectable by the UA if it has information 
regarding the source media (see [1] in the spec). For example: 
media.webm#t=15 and the duration of media.webm is only 10s.

Suppose we have a smart UA (i.e., scenario described in [2], only byte 
range requests are used). The smart UA is able to determine the duration 
of the media, interprets the media fragment URI, and seeks to the end of 
the media (as discussed in [3]).

So far so good, but what happens when we have a UA using a media 
fragments-enabled server (i.e., scenario described in [4], where time 
ranges are used)? The UA does not know the duration of the source media. 
Therefore, it just sends an HTTP Range request to the server:

GET media.webm HTTP/1.1
Range: t:npt=15-

Since the requested time range is invalid, the server answers with a 416 
(Requested Range Not Satisfiable), and the UA still doesn't have a clue 
about the duration of the media.

So the question is, what is the behaviour of a UA getting a 416 from a 
Media Fragments-enabled server (especially in the case when the decoding 
pipeline is not setup)?

I think this situation must be clarified in the spec.

Best regards,

Davy

[1] 
http://www.w3.org/2008/WebVideo/Fragments/WD-media-fragments-spec/#error-media
[2] 
http://www.w3.org/2008/WebVideo/Fragments/WD-media-fragments-spec/#processing-protocol-UA-mapped
[3] 
http://lists.w3.org/Archives/Public/public-media-fragment/2011Apr/0054.html
[4] 
http://www.w3.org/2008/WebVideo/Fragments/WD-media-fragments-spec/#processing-protocol-Server-mapped

-- 
Davy Van Deursen

Ghent University - IBBT
Department of Electronics and Information Systems - Multimedia Lab
URL: http://multimedialab.elis.ugent.be/dvdeurse
Received on Tuesday, 3 May 2011 11:13:11 UTC

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