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cache freshness / age calcs

From: Adrien de Croy <adrien@qbik.com>
Date: Mon, 12 Oct 2009 16:12:01 +1300
Message-ID: <4AD29E81.3000801@qbik.com>
To: HTTP Working Group <ietf-http-wg@w3.org>

Hi all

I've been poring through draft-ietf-httpbis-p6-cache-07 trying to figure 
out what to do in a particular case.

This case is where the origin server has a clock that is way out of 
whack, and specifies expiry times close to the level of error.  No Age 
header, so presumably not from some intermediary cache.

The documentation for calculation of the apparent age states:

"A response's age can be calculated in two entirely independent ways:

   1.  now minus date_value, if the local clock is reasonably well
       synchronized to the origin server's clock.  If the result is
       negative, the result is replaced by zero.

   2.  age_value, if all of the caches along the response path implement

The obvious question being what if the clocks are not reasonably well 
synchronised, and 2 doesn't hold either (either not all HTTP/1.1 or no 
Age header)?  How can you even tell if a clock is synchronised or not?  
In that case does the spec not attempt to specify how to calculate age?

In my problem case, using now - date_value places a huge skew on the 
time at which the stored response can no longer be considered fresh.  It 
significantly reduces the effectiveness of the cache.  Obviously the 
resolution is to get the server admin to fix their clock, but that's an 
uphill battle.

I would presume that if you get a response with a Date header, and no 
Age header from a 1.1 O-S, then you should presume that the Date header 
is an indication of the local clock at that server (ignoring RTT and 
time to generate).  If there is an age header, you should consider the 
Date header to be some date in the past from which the Age was 
subsequently calculated (e.g. caches don't update Date headers to their 
own value when serving from cache - or do they?).   Is this why the age 
is calculated as the larger of the received age value vs the apparent 
age rather than the sum of the 2?

If a response didn't come from a cache, it cannot have been generated 
before it was requested.  Therefore calculating an apparent age is 
misguided if there is no age header.  A conservative view of the age of 
a response not from cache should therefore be bounded by the age of the 
request, rather than the difference in clocks (which can be large).

What do browsers commonly do in this case?



Adrien de Croy - WinGate Proxy Server - http://www.wingate.com
Received on Monday, 12 October 2009 03:08:45 UTC

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