Re: WGLC issue: 'no-transform'

On Mon, Nov 26, 2012 at 04:58:46PM +1100, Mark Nottingham 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 of [Part2])
> > 	? Content-Range (Section 5.2 of [Part5])
> > 	? Content-Type (Section 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: 
> > 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.

I agree. Also, I think we should take the intent from 2616 which clearly
explains *why* proxies must absolutely respect this rule. I have already
seen some proxies recompress whatever they could without any consideration
for the importance of original data, so having this would be really nice.

> 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.

+1 for me too.


Received on Monday, 26 November 2012 07:15:35 UTC