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From Ben Laurie <>
Subject Re: mod_proxy changes
Date Tue, 28 May 1996 18:32:03 GMT
Roy T. Fielding wrote:
> > This seems perverse ... if the canonicalization is valid for the cache, surely
> > one must assume it is valid for the URI? [yeah, I know, I should be on HTTP-WG
> > for this kind of argument ... but can I take the mail load?]
> > 
> > That is, surely:
> > 
> > cache(a) ~ cache(b) => a ~ b [Axiom 1 (required for correct function of cache)]
> > cache(a) ~ cache(canon(a)) [Axiom 2 (stated above)]
> > 
> > where ~ means "is equivalent to"
> > 
> > so,
> > 
> > canon(a) ~ a [by axioms 1 and 2].
> Nice theory -- it just doesn't work in practice.  The problem is that
> not all caches and origin servers share the same canonicalization algorithm.

Why not? If the canonicalization is not determined how can its use be
sanctioned in a proxy?

> A cache's storage handling algorithm is not part of the HTTP protocol
> (and thus not part of the normative requirements [when done correctly]),
> which is why internal canonicalization is allowed.  However, the cache
> (more accurately, a proxy) is not allowed to screw up the user's request
> by mis-canonicalizing the user's request-URI when it is forwarded, and
> thus the first response will be correct (in the user's view) even if the
> cache is incompetent.  In practice, it turns out that incorrect hits on
> correct responses are always preferable to incorrect responses due to
> broken requests, and thus the problem of a later request getting a
> cache hit after canonicalization (but not before canonicalization) becomes
> a non-problem.

I'm sorry, I got lost in the tangle there. Could you explain in words of one
syllable, please? With diagrams? ;-)

> Of course, it is never a problem if the origin server avoids using
> non-reserved characters for reserved purposes (which is when canonicalization
> breaks), but then the protocol doesn't prevent people from being stupid.

Ermm .. doesn't that make the server broken? [And therefore not a cause for
concern within the scope of the spec].



BTW, I'll move this to private mail if I'm boring people.

> ......Roy

Ben Laurie                  Phone: +44 (181) 994 6435
Freelance Consultant and    Fax:   +44 (181) 994 6472
Technical Director          Email:
A.L. Digital Ltd,           URL:
London, England.

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