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Re: Cache-Control and Pragma

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Thu, 10 Dec 98 11:54:03 PST
Message-Id: <9812101954.AA04951@acetes.pa.dec.com>
To: "Nottingham, Mark (Australia)" <mark_nottingham@exchange.au.ml.com>
Cc: http-wg@hplb.hpl.hp.com
    I can't find anything in the Draft that addresses the situation
    when both Cache-Control and Pragma: no-cache headers are set, and
    they are in conflict.

    For instance, if I use
    
    Cache-Control: must-revalidate
    Pragma: no-cache
    
    on a HTTP response, it would be desireable to have it cached (and
    always revalidated) by HTTP 1.1 caches, and not cached by HTTP 1.0
    caches.  However, I can see nothing in the Draft about this, and
    some implementations, upon reading 14.32 Pragma, might never cache
    the object. While the action is safe, it's not ideal.
     
    Am I missing something somewhere? Thanks.
    
Larry Masinter (who writes that this is "not specified") and Koen (who
points out that what you probably want to do is to send an Expires
header) are both right.

However, there is a subtle point that you and they both missed: the
formal specification for "Pragma: no-cache" applies ONLY to request
messages (in both HTTP/1.1 and in RFC1945); it has no formal definition
for response messages.

While it is possible that some caches do look at "Pragma" headers in
responses, I don't know of any specific proxy software that does.  I
checked both Squid 1.1.20 and an ancient copy of the CERN server/proxy
code; both would ignore "Pragma" headers in responses.  So sending
"Pragma: no-cache" in a response would probably be pointless,
regardless of what the HTTP/1.1 specification says about its
interaction with other headers.

In retrospect, I think the wording of the specification of
Pragma should have included a Note to this effect, because
lots of people seem to be confused by this (even me, at times).
But let's not delay the standardization process over this (non)issue!

-Jeff
Received on Thursday, 10 December 1998 19:54:19 EST

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