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Improving If-Modified-Since

From: Lou Montulli <montulli@mozilla.com>
Date: Mon, 14 Aug 95 19:03:48 -0700
Message-Id: <30300084.167E@mozilla.com>
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
I've been getting lots of bug reports due to corrupted or out dated
caches.  I would like to propose an extension to the If-modified-since
header to improve the situation.  I'd like to start sending

If-modified-since: DATE; size=SIZE

The addition of size=SIZE informs the server of the current size of
the document cached by the client.  The size acts as a checksum,
if the size of the file to be served is different than the
size given in the If-modified-since header than a 304 should
not be returned.  

The advantage of size over other checksums is that it is highly efficient.
Clients and servers can obtain the information at little or no cost.

The disadvantage of size is that it is not completely accurate as a checksum.
An MD5 hash or some other content based checksum would be far more accurate
but would require lots of additional overhead.

In the future, if a stronger checksum is deemed necessary it can be
added as another part of the If-modified-since header.  Perhaps:
 If-modified-since: DATE; size=SIZE; md5=SIGNATURE 

I have tested this change against the Netscape, NCSA, CERN and Apache
servers and all of them ignore the addition of size=SIZE, so we
can add this addition without fear of backwards compatibility concerns.


Lou Montulli                 http://www.mcom.com/people/montulli/
       Netscape Communications Corp.
Received on Monday, 14 August 1995 19:04:53 UTC

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