

Re: Proposal: Pragma min-age (Was:Re: A modest proposal)

From: Roy Fielding <fielding@beach.w3.org>
Date: Fri, 18 Aug 1995 13:47:22 -0400
Message-Id: <199508181747.NAA12183@beach.w3.org>
To: Koen Holtman <koen@win.tue.nl>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
>As discussed earlier on www-talk, if a sufficient number of service authors
>keeps putting Expires: <yesterday> or Pragma: no-cache headers in responses
>for frivolous reasons, a web cache administrator may want to (selectivelty)
>`tune' the cache to ignore these headers (even if this means that the cache
>does not conform to http 1.x anymore).
>Such a `tuned' cache would introduce incorrectness

No, it *might* introduce incorrectness.  I claim that over 99% of
today's non-cacheable pages are still "correct" after being cached,
where correctness is defined as containing the same substantive and
qualitative information content as would be obtained directly from
the origin.  [Note: my claim is based on personal observation, not
controlled experimentation]

That is why a cache administrator is willing to "tune" the cache,
and it is not something that can be fixed within the protocol.
Caches will do what providers want them to do when providers stop
marking cacheable pages as non-cacheable.

>Pragma: min-age=<delta-seconds>

I am opposed to this change.  First, any sensible cache administrator
would never send such a header, since it effectively changes the request
profile and therby lowers the quality of the response.

Second, the cache is tuned based on each individual response, not based
on every request.  If the response looks like it shouldn't be cached,
a tuned cache will not cache it.  If it says "no-cache" and is coming
from a reputable provider, a tuned cache will not cache it.  If it
comes from a notoriously wasteful provider, the cache will cache it
and there is no incentive whatsoever for the cache administrator to
warn the wasteful provider ahead of time.

BTW folks, lack of a Last-Modified date does not imply no-cache.
That is only an optimization used by Ari's two proxies and Navigator;
it does not hold true in general.

                      Visiting Scholar, MIT/LCS + World-Wide Web Consortium
                      (fielding@w3.org)                (fielding@ics.uci.edu)
Received on Friday, 18 August 1995 10:52:23 EDT
