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Re: Minor fixes to draft 05

From: Roy T. Fielding <fielding@liege.ICS.UCI.EDU>
Date: Wed, 03 Jul 1996 16:07:51 -0700
To: Jeffrey Mogul <mogul@pa.dec.com>
Cc: jg@w3.org, http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Message-Id: <9607031607.aa15061@paris.ics.uci.edu>
> This change (only requiring Age on responses taken from a cache)
> breaks the clock-deskewing properties of the original Age design.
> The problem comes when a pre-1.1 cache is in the path and has
> held onto the response for a while.  It's safest to add the Age
> header as soon as possible, since it reduces the chances that
> a skewed clock could result in incorrect comparisons.

That is incorrect -- in fact, adding an Age header onto a response
received from somewhere else can only make the algorithm less accurate.
The reason is because the outbound requester is already going to add
the amount of time it takes for the response to arrive, including any
time spent by the cache in question.  Therefore, adding an Age header
onto a response retrieved by a further network request will double
the amount of Age that the outbound requester will observe,
for no reason whatsoever, and guaranteeing that the Age calculation
will be wrong.

 ...Roy T. Fielding
    Department of Information & Computer Science    (fielding@ics.uci.edu)
    University of California, Irvine, CA 92717-3425    fax:+1(714)824-4056
    http://www.ics.uci.edu/~fielding/
Received on Wednesday, 3 July 1996 17:01:10 EDT

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