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Re: WGLC issue: 'no-transform'

From: Mark Nottingham <mnot@mnot.net>
Date: Tue, 27 Nov 2012 13:37:20 +1100
Message-Id: <5C4DDBA2-B50D-43AA-A3BB-ECAF10C4FB03@mnot.net>
To: HTTP Working Group <ietf-http-wg@w3.org>
... now <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/418>.


On 26/11/2012, at 4:58 PM, Mark Nottingham <mnot@mnot.net> wrote:

> p1 says:
> 
>> A proxy must not modify or add any of the following fields in a message that contains the no-transform cache-control directive:
>> 
>> 	 Content-Encoding (Section 3.1.2.2 of [Part2])
>> 	 Content-Range (Section 5.2 of [Part5])
>> 	 Content-Type (Section 3.1.1.5 of [Part2])
>> A transforming proxy may modify or add these fields to a message that does not include no-transform, but if it does so, it must add a Warning 214 (Transformation applied) if one does not already appear in the message (see Section 7.5 of [Part6]).
> 
> p6 says: 
> 
>> 7.2.2.9 no-transform
>> 
>> The "no-transform" response directive indicates that an intermediary (regardless of whether it implements a cache) must not change the Content-Encoding, Content-Range orContent-Type response header fields, nor the response representation.
> 
> 2616 was a bit more wordy:
> 
>> 14.9.5 No-Transform Directive
>> 
>>   no-transform
>>      Implementors of intermediate caches (proxies) have found it useful
>>      to convert the media type of certain entity bodies. A non-
>>      transparent proxy might, for example, convert between image
>>      formats in order to save cache space or to reduce the amount of
>>      traffic on a slow link.
>> 
>>      Serious operational problems occur, however, when these
>>      transformations are applied to entity bodies intended for certain
>>      kinds of applications. For example, applications for medical
>>      imaging, scientific data analysis and those using end-to-end
>>      authentication, all depend on receiving an entity body that is bit
>>      for bit identical to the original entity-body.
>> 
>>      Therefore, if a message includes the no-transform directive, an
>>      intermediate cache or proxy MUST NOT change those headers that are
>>      listed in section 13.5.2 as being subject to the no-transform
>>      directive. This implies that the cache or proxy MUST NOT change
>>      any aspect of the entity-body that is specified by these headers,
>>      including the value of the entity-body itself.
> 
> 
> (the text currently in p1 closely corresponds to that in 2616's 13.5.2).
> 
> The issue here is that the intent in 2616 is clear -- the body shouldn't be modified if no-transform is present -- but since we split it up, a reader of p1 doesn't have any indication of this unless they chase up in p6 (note there isn't any reference, beyond the unrelated one to the Warning header).
> 
> I'd suggest we need p1 to explicitly say the body can't be changed, as p6 does.
> 
> While we're at it, changing "A proxy must not modify or add any of the following fields..." to "A proxy (transparent or non-transparent) must not modify or add any of the following fields " would make what's going on a lot clearer.
> 
> --
> Mark Nottingham   http://www.mnot.net/
> 
> 
> 
> 

--
Mark Nottingham   http://www.mnot.net/
Received on Tuesday, 27 November 2012 02:37:47 GMT

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