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Re: Expires, Last-Modified, Pragma: no-cache etc.

From: Lou Montulli <montulli@mozilla.com>
Date: Tue, 15 Aug 95 20:19:42 -0700
Message-Id: <303163CE.ABD@mozilla.com>
To: John Franks <john@math.nwu.edu>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
> It seems to me that much (but not all) of the discussion over the last
> few months on the Expires, Last-Modified headers is really due to the
> desire of server administrators to control whether a document is cached.
> One of the big improvements in the current HTTP spec is the "Pragma:
> no-cache" header.  This provides a way to say, "Don't cache this,"
> without the need for fictitious expiration or last-modified dates.
> This is important.  There are often good reasons for saying that a
> document should not be cached, but having a future expiration date
> (for example, if the validity of the document doesn't, in fact *expire*
> until that date).
> The only problem is that server admins often want to prevent local disk
> caching as well as proxy caching and the Pragma: no-cache applies only
> to the latter.  Perhaps we also need a "Pragma: no-local-cache".
> One thing of which I am thoroughly convinced is that if a clean mechanism
> for giving administrators this capablility is not made available then
> every conceivable ugly hack which has the desired effect will gain
> widespread use.  We will see 1970 expiration dates (always a lie) and
> future last-modified dates (also a lie).  These hacks will likely work
> with some clients/proxies and fail with others. There will, no doubt,
> even be those who use different hacks based on the User-Agent header
> of the request!  Why not make life easier for everyone and allow a server
> to cleanly request that a document not be cached to local disk.
> This should be a separate request from the Pragma: no-cache request to
> prevent proxy caching.

I recently changed netscape to interpret "Pragma: no-cache" and not
cache the object.  This is slightly different than a "Expires" header
because the object will not even be cached for history navigation.
(Documents that are expired are still shown when traversing the session
history).  Haveing the client interpret "Pragma: no-cache" lets servers
tell the client that this information is highly sensitive or volitile
and should not be cached in any way.

Lou Montulli                 http://www.mcom.com/people/montulli/
       Netscape Communications Corp.
Received on Tuesday, 15 August 1995 20:21:52 UTC

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