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Re: Improving If-Modified-Since

From: Lou Montulli <montulli@mozilla.com>
Date: Wed, 16 Aug 95 23:52:19 -0700
Message-Id: <3032E723.41C6@mozilla.com>
To: Mike Meyer <mwm@contessa.phone.net>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
In article <19950816.75DE920.13DA5@contessa.phone.net> mwm@contessa.phone.net
(Mike Meyer) wrote:
> > It is not at all a fatal flaw.  The size returned by the client needs
> > to be specified to be the same as the "content-length" returned by
> > the server during the request.  If line-feed conversion is being
> > done consistantly the size can be compared accurately.
> This makes the "content-length" an arbitrary value that must be stored
> out of band in the cache, and returned to the server as part of a
> "version check" request.

the IMS last modified date is already a value that must be stored
out of band in the cache.  The client local time can not be
relied upon so it can not be used to compute a last modified date.
Only in cases where you can guarentee reliable dates (i.e not
PC's and not Macs) can you use the local date as a value, I
suspect some UNIX proxies do this.

> If we're going to change the spec for this, and are requiring that
> clients keep out of band data to comply, why not provide versioning of
> the kind the server (which has to do the work) wants to support?
> Observation: the version string has no meaning as far as the client is
>         concerned.
> Proposal:
> A new header from servers: Version: Version-string
> A new header from clients: If-Not-Version: Version-string
> Version-string is a string of characters other than ";" and whitespace.
> Since version-string only has meaning to the server, it can be
> whatever you want. The file size; a VMS file version number; an MD5
> digest; a CRC, etc.

This would work pretty well but has the disadvantage of not being
reproducable on the client end.  A standardized checksum method
would remove the need for out of band data to be stored, or if
the out of band data was lost, it could be regenerated.

Lou Montulli                 http://www.mcom.com/people/montulli/
       Netscape Communications Corp.
Received on Wednesday, 16 August 1995 23:52:33 UTC

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