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Re: The problem of having multiple Content-Range headers in HTTP response

From: Conrad Parker <conrad@metadecks.org>
Date: Fri, 5 Mar 2010 09:36:40 +0900
Message-ID: <dba6c0831003041636u4282fdb1rf90ad449418b7e4e@mail.gmail.com>
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Cc: raphael.troncy@eurecom.fr, Media Fragment <public-media-fragment@w3.org>
On 5 March 2010 06:07, Silvia Pfeiffer <silviapfeiffer1@gmail.com> wrote:
> On Fri, Mar 5, 2010 at 12:11 AM, Conrad Parker <conrad@metadecks.org> wrote:
>> On 4 March 2010 17:14, Silvia Pfeiffer <silviapfeiffer1@gmail.com> wrote:
>>> On Thu, Mar 4, 2010 at 4:59 PM, Conrad Parker <conrad@metadecks.org> wrote:
>>>> On 4 March 2010 11:55, Silvia Pfeiffer <silviapfeiffer1@gmail.com> wrote:
>>>>> On Thu, Mar 4, 2010 at 1:14 PM, Conrad Parker <conrad@metadecks.org> wrote:
>>>>>> On 4 March 2010 10:25, Silvia Pfeiffer <silviapfeiffer1@gmail.com> wrote:
>>>>>>> On Thu, Mar 4, 2010 at 12:00 AM, Silvia Pfeiffer
>>>>>>> <silviapfeiffer1@gmail.com> wrote:
>>>>>>>> 2010/3/3 RaphaŽl Troncy <raphael.troncy@cwi.nl>:
>>>>>>>>
>>>>>>>>> Another problem is how should we express that when 2 tracks have been
>>>>>>>>> requested?
>>>>>>>>
>>>>>>>> The background here is that using a comma as in track=audio,subtitle
>>>>>>>> will not work in the HTTP headers, since the comma is used to separate
>>>>>>>> headers from each other. As such, something like:
>>>>>>>> † †Content-Range: track audio,subtitle/653.791
>>>>>>>> would be parsed to
>>>>>>>> † †Content-Range: track audio
>>>>>>>> † †Content-Range: subtitle/653.791
>>>>>>>> which is obviously incorrect.
>>>>>>
>>>>>> why?
>>>>>>
>>>>>> the comma thing is not about "will be parsed to", but "is equivalent
>>>>>> to". So, an application could add the two headers in order:
>>>>>>
>>>>>> Content-Range: track audio
>>>>>> Content-Range: subtitle/653.791
>>>>>>
>>>>>> and this pair would be equivalent to, and could be merged by an
>>>>>> intermediary proxy into,:
>>>>>>
>>>>>> Content-Range: track audio, subtitle/653.791
>>>>>>
>>>>>> So the comma thing is more about allowing the infrastructure to be
>>>>>> more flexible (ie. different parts of an application stack can append
>>>>>> headers without rewriting existing ones) while still being cacheable
>>>>>> (the two header representations are equivalent).
>>>>>>
>>>>>> Conrad.
>>>>>>
>>>>>
>>>>> I am not even sure that comma thing actually works in this way.
>>>>>
>>>>
>>>> RFC2616, Section 4.2 Message Headers:
>>>>
>>>> Multiple message-header fields with the same field-name MAY be present
>>>> in a message if and only if the entire field-value for that header
>>>> field is defined as a comma-separated list [i.e., #(values)]. It MUST
>>>> be possible to combine the multiple header fields into one
>>>> "field-name: field-value" pair, without changing the semantics of the
>>>> message, by appending each subsequent field-value to the first, each
>>>> separated by a comma. The order in which header fields with the same
>>>> field-name are received is therefore significant to the interpretation
>>>> of the combined field value, and thus a proxy MUST NOT change the
>>>> order of these field values when a message is forwarded.
>>>
>>> Ah thanks. I wasn't able to find this. I wonder if this implies that a
>>> proxy can split a field-value at a comma as it likes.
>>
>> no, as far as I understand it only the other way (combining) can be
>> done automatically.
>>
>> This is becaues it's possible to define a header that does use comma
>> for something else, but in that case it's not allowed to have more
>> than one instance of that field -- ie. if you violate the "if and only
>> if" requirement of the paragraph I posted above. I think it's best to
>> avoid that when defining new headers, so that we allow the combining
>> behavior etc. In particular for our case we are defining a kind of
>> list, and the standard comma mechanism is useful.
>>
>
> So if I understand you correctly, you'd also suggest using something
> else than a comma to separate the list of tracks in a track fragment
> URI?

If you want to express that the content contains tracks A and C, using
header Foo, then you could say:

Foo: track A, track C

but not:

Foo: tracks A,C

If you really wanted that form then you would have to replace the
comma with something else.

On the other hand, perhaps the first form is more flexible; an
application (say. some simple CGI script) that adds subtitle track C
could simply add the "Foo: track C" header.

Conrad.

>
> Thanks for clarifying - it's good to read and understand these things
> thoroughly. I was indeed wondering how Microsoft's scheme could work
> when they in fact used commas.
>
> Cheers,
> Silvia.
>
>
Received on Friday, 5 March 2010 00:37:17 GMT

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