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Re: 24-hour old misses

From: Jeffrey Mogul <Jeff.Mogul@hp.com>
Date: Fri, 27 Sep 2002 17:48:54 -0700
Message-Id: <200209280048.g8S0msYj028932@wera.hpl.hp.com>
To: Alex Rousskov <rousskov@measurement-factory.com>
cc: ietf-http-wg@w3.org

Catching up (slowly!) on old email:

Alex writes:

    RFC 2616, Section 13.2.4 "Expiration Calculations" contains
    the following MUST:
    
       The cache MUST attach Warning
       113 to any response whose age is more than 24 hours if such warning
       has not already been added.
    
    The context of that MUST is expiration calculation and, hence, cache
    hits. On the other hand, attaching a Warning to misses may help in
    case there is a non-1.1-compliant cache upstream.
    
    Did the author(s) intend for the above MUST to apply to HIT responses
    only?

Again, I believe I wrote this part.

Take a look at section 14.46 (Warning) which defines the specific
warning-value mentioned above:

   113 Heuristic expiration
     MUST be included if the cache heuristically chose a freshness
     lifetime greater than 24 hours and the response's age is greater
     than 24 hours.

True, it's not explicit here that this applies only to cache
hits.  But it would only be interesting on a miss if the cache's
heuristic choice for a freshness lifetime were to be inserted
into the forwarded response header.  In other words, the origin
server sent no Expires header (or something equivalent, such
as a max-age directive) and the proxy guessed at one and then
added an Expires header to the response.

But look at section 13.5.2 (Non-modifiable Headers), which says
that

   A transparent proxy MUST NOT modify [the Expires field in
   a response, but MAY add one] if not already present. If an
   Expires header is added, it MUST be given a field-value identical to
   that of the Date header in that response.

so (by that MUST) the cache isn't allowed to propagate its
heuristically-chosen freshness lifetime to anyone else.

Since section 13.5.2 doesn't discuss Cache-Control, this looks
like a loophole in the spec (e.g., the cache could perhaps
add "Cache-control: max-age=1000000000") but I would hope that
proxy cache implementors aren't actively looking for loopholes
of this sort.

Probably, therefore, you should read the relevant part of 14.46
as if it said:

   113 Heuristic expiration
     MUST be included if the cache heuristically chose a freshness
     lifetime greater than 24 hours for a cache entry, the response
     is taken from that cache entry, and the age of the response is
     greater than 24 hours.

I can't see any reason to Warn anyone on a miss that creates
a new cache entry with a heuristic expiration lifetime, given
that the cache shouldn't be propagating these heuristic values
towards the client.

-Jeff
Received on Friday, 27 September 2002 20:49:01 GMT

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