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Re: i37 - Vary and non-existant headers

From: Adrien de Croy <adrien@qbik.com>
Date: Fri, 16 Nov 2007 09:46:52 +1300
Message-ID: <473CB03C.5000703@qbik.com>
To: Henrik Nordstrom <henrik@henriknordstrom.net>
CC: Jamie Lokier <jamie@shareable.org>, 'HTTP Working Group' <ietf-http-wg@w3.org>

Henrik Nordstrom wrote:
> On tor, 2007-11-15 at 08:51 +0000, Jamie Lokier wrote:
>> What about whitespace differences (e.g. leading/trailing whitespace,
>> whitespace following the header name colon), multi-line breaks, and
>> multiple occurrences of the same header?
>> Does exact means _exactly_ in all those respects, or is it acceptable
>> for a cache to canonicalise certain things, like converting multiple
>> occurences of a header to a single header by joining with ", " (or
>> ","), converting multi-line headers to single-line, and stripping
>> leading/trailing whitespace from the header value?
> Ofcourse there is some canonisation taking place, you can't compare two
> sets of headers otherwise as the LWS and list rules allows the exact
> same thing to be transmitted in many ways.. But there is no knowledge of
> the meaning of any of the headers involved in the process.

If we don't take into account any meaning of headers, we lose many 
opportunities to cache.

E.g. if an original request for something contained an Accept-Language 
header specifying many acceptable languages, the response comes back 
with one of those languages.  If a subsequent request comes in which 
will accept the language that is cached, should that be served back or not?

I guess there are difficulties there where for one request it was the 
preferred language and for another it was a less-preferred language.

In which case how would you compare Accept-Language tags?  You'd need to 
compare them as a list with q values.  Or would you do it so that if the 
q value was 1 in the original request and subsequent request then its ok 
to return that one?:

> The process is spelled out in quite detail in 13.2 Caching Negotiated
> Documents, with very little room for error other than implementation
> bugs. That's provided one cares to read 13.2 before jumping to
> conclusions about how this should work.
> Please note that in case of Vary request headers compare it's safe to
> err on the cautious side and get false "Vary request" misses. So if you
> optimize by doing a string literal compare instead of exact value then
> things will still work, just not optimally so..
> Regards
> Henrik

Adrien de Croy - WinGate Proxy Server - http://www.wingate.com
Received on Thursday, 15 November 2007 20:46:21 UTC

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