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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 GMT

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