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Re: Notify user agent available fragment

From: Jeroen Wijering <jeroen@longtailvideo.com>
Date: Wed, 5 May 2010 18:00:26 +0200
Cc: RaphaŽl Troncy <raphael.troncy@eurecom.fr>, Eric Carlson <eric.carlson@apple.com>, public-media-fragment@w3.org
Message-Id: <3D49CA87-C262-4B68-A2AC-3627FA772338@longtailvideo.com>
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Dear all,

>>> Incidentally, reaching the HAVE_METADATA state will also be a
>>> precursor to some of the cases that the MF spec identifies, such as:
>>>
>>> http://www.w3.org/2008/WebVideo/Fragments/WD-media-fragments-spec/#processing-protocol-UA-mapped
>>>
>>> orhttp://www.w3.org/2008/WebVideo/Fragments/WD-media-fragments-spec/#processing-protocol-Server-mapped
>>> . Thus it's not unreasonable to deal with this condition for certain
>>> usage cases.
>>
>> Indeed! And I think we should write this explicitly in the spec as  
>> each time
>> I explained the work of the Media Fragments WG in a presentation, I  
>> get this
>> as a question. Who wants to edit the spec to mention this?
>
> 5.2.1 states:
> "This is the case typically where a user agent has already downloaded
> those parts of a media resource that allow it to do or guess the
> mapping, e.g. headers or a resource, or an index of a resource."
>
> If we want to stay independent of the HTML5 specification, this is an
> acceptable description of the condition, IMHO. If we want to use HTML5
> as an example, we can certainly add the note on HAVE_METADATA.
>
> Raphael, I think no matter whether it is written in the spec or not,
> you will always get this questions, since it is a core issue to
> understand. ;-)

Allow me to elaborate a little more on the HAVE_METADATA state as  
provided in HTML5, since this touches the overall use case I had in  
mind for my initial question: streaming. Today's main streaming  
platforms (Flash, Silverlight, Quicktime) converge towards the  
practice of requesting small fragments of a video over HTTP,  
seamlessly concatenating them in the player to a full video. This  
practice, as you know, allows for functionalities such as live  
streaming and multi-bitrate delivery leveraging existing HTTP  
infrastructure.

On a request level, a standardization of this functionality seems  
perfectly covered using Media Fragments. On a discovery level,  
however, relying on the user-agent to retrieve the metadata part of a  
video causes some practical issues:

*) The main reason for multi-bitrate delivery (and streaming in  
general) is bandwidth conservation. If a user-agent has to retrieve  
part of the mediafile in order to extract metadata, the opposite would  
actually be the case. Especially with long-form content, metadata  
headers could grow as large as several megabytes. On top of this, the  
user-agent would actually have to request the metadata for every  
single bitrate in a multi-bitrate scenario.
*) In case of live streaming, technical metadata of the resource may  
not exist in the resource itself. It is typical for live HTTP  
streaming solution to maintain metadata externally, so that the  
resource itself doesn't need updating at two points (head and tail)  
while the live event is in progress.

All three aforementioned platforms rely on external (XML, M3U8)  
descriptor files that inform the user-agent of the exact file and  
fragment availability, to work around both issues. It seems a  
standardization of such functionality could be a use case for the  
Media Multitrack API (although it currently focusses on accessiblity  
tracks). I presume my best bet would be to raise this question with  
the team working on the Media Multitrack API? Or am I off track here  
and should such functionality not be considered part of the Video on  
the Web efforts? I could imagine it has previously been dismissed as  
either too specific or unwieldy...

Kind regards,

Jeroen
Received on Wednesday, 5 May 2010 16:01:01 GMT

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