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

From: Jean-Philippe Martin-Flatin <syj@ecmwf.int>
Date: Fri, 18 Aug 1995 11:40:41 +0100
Message-Id: <9508181140.ZM3008@helena>
To: Koen Holtman <koen@win.tue.nl>
Cc: fielding@beach.w3.org, http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
On Aug 18, 12:21, Koen Holtman wrote:
>
> Jean-Philippe Martin-Flatin:
> >What do we gain by having both a 'no-cache' and a 'private' Pragma
> >in terms of functionality ? A 'shared response cache' is basically
> >a proxy/cache, and Pragma is meaningful to proxies only, not to user
> >agents. So both headers really mean "don't cache this response in a
> >proxy/cache", and both let a user-agent local cache free to cache
> >the response or not.
>
> Pragma: private would instruct a user agent not to cache the response
> if its cache memory (say part of a harddisk in an MS-DOS pc in a
> university PC lab) is publicly accessible.  This is particularly
> important for user agents that do not clear their caches at the end
> of the session.

Pragma is transparent to a user agent. From
<URL:http://www.w3.org/hypertext/WWW/Protocols/HTTP1.0/draft-ietf-http-spec.html#Pragma>:

    Pragma directives do not apply to the end-points of a request/response
    chain. For example, a user agent's internal (non-shared) cache and/or
    history mechanism should ignore all pragma directives in received
    messages. Similarly, pragma directives are not applicable to the origin
    of a resource, though they may be applicable to a server's internal
    response cache.

You have a point though, and maybe there ought to be another header telling
the user agent that this information is private, and that it should make all
possible efforts (very platform specific) to keep it private. But that
wouldn't be achieved with a Pragma header field.

Jean-Philippe
Received on Friday, 18 August 1995 03:43:06 EDT

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