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Re: Improving If-Modified-Since

From: Alex Hopmann <hopmann@holonet.net>
Date: Mon, 14 Aug 1995 20:00:07 -0700
Message-Id: <199508150300.UAA25771@holonet.net>
To: Lou Montulli <montulli@mozilla.com>, http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
I agree with Lou's proposal. Regarding Simon's suggestion that we also
provide a simple checksum, I don't think that would really be a great
benifit. The overhead of simply opening a file and scanning through its
contents strikes me as greater than that of the MD5 ifself. So personally I
would either want an extremely fast mechanism such as the file size & date,
or else go for the real MD5.

>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.
>
>Comments?
>
>:lou
>-- 
>Lou Montulli                 http://www.mcom.com/people/montulli/
>       Netscape Communications Corp.
>
>
Alex Hopmann
ResNova Software, Inc.
hopmann@holonet.net
Received on Monday, 14 August 1995 20:06:58 EDT

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