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The problem with proxy-revalidate, and a proposed solution

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Thu, 19 Dec 96 16:36:14 PST
Message-Id: <9612200036.AA18024@acetes.pa.dec.com>
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
X-Mailing-List: <http-wg@cuckoo.hpl.hp.com> archive/latest/2126
In the HTTP-WG session at the IETF meeting last week, we briefly
discussed the problem proxy-revalidate.  Paul Leach and I volunteered
to take this as an action item.

Here are two excerpts from the current HTTP/1.1 spec:
    When the must-revalidate directive is present in a response
    received by a cache, that cache MUST NOT use the entry after it
    becomes stale to respond to a subsequent request without first
    revalidating it with the origin server. (I.e., the cache must do an
    end- to-end revalidation every time, if, based solely on the origin
    server's Expires or max-age value, the cached response is stale.)


    The proxy-revalidate directive has the same meaning as the must-
    revalidate directive, except that it does not apply to non-shared
    user agent caches. It can be used on a response to an authenticated
    request to permit the user's cache to store and later return the
    response without needing to revalidate it (since it has already
    been authenticated once by that user), while still requiring
    proxies that service many users to revalidate each time (in order
    to make sure that each user has been authenticated).

Note that the only reasonable reading of this definition of
proxy-revalidate is that

	if (acting as as shared cache) then
	    if (proxy-revalidate) and (entry age < max-age) then
		revalidate before using this entry
		use this entry without revalidating

However, the rest of the second paragraph doesn't make sense
with this reading, if one assumes that most end-client caches will
obey the max-age directive.  That's because if the origin server
wants such a client to be able to use an authenticated response
from its cache without revalidating it, but wants a shared proxy
cache to revalidate the response (to make sure that the new client
has been authenticated), then there is no way to express this.
I.e., the origin server would like to set max-age > 0 to allow
the end-client to cache without revalidating, but would have to set
max-age=0 to force the proxy to revalidate.

This is my fault, I suspect, because I wrote this part of the
spec.  I was trying to solve two problems with one feature, and
I don't think that was possible.  The first problem was the
one described above, which was described to me by the authentication
people (primarily Paul Leach), and which Paul has now made me
understand again.  The second problem was that we introduced
"must-revalidate" to deal with caches that might be loose about
observing max-age, and several people observed that this is somewhat
too strict for many applications, where it's OK for the end-client
cache to be "loose", but it might not be OK for a proxy cache to be
"loose".  So the current specification of proxy-revalidate actually
does solve that problem.

Note that "Cache-control: private" is very inefficient, because
it forces a proxy to fully reload the entity body each time; it
cannot do a conditional GET in this case.  So that's not really
a solution.

The problem with proxy-revalidate also makes it much harder to
do efficient hit-counting, *with or without* our proposed hit-metering

After thinking about it for a while, and making up a table of all
the possible combinations, I've come to the conclusion that there
is no way to express all of the possibly desirable policies with
the current set of cache-control directives, even if we tried to
redefine the meaning of proxy-revalidate.  In order to make
authentication work without defeating caching entirely, we either
need to add at least one cache-control directive, or we would have
to redefine proxy-revalidate in a way that would make it much
less efficient.

Paul and I believe that adding one more directive is the right
solution.  There are several possibilities that would work.
One would be something like
	Cache-control: proxy-mustcheck
which would be ignored by non-shared caches, but would mean that
a shared cache would have to revalidate entry regardless of its
age (i.e., even if apparently "fresh").  In typical use, one might
	Cache-control: max-age=30, proxy-mustcheck
which allows all caches to store the response, and an end-client
to use its cache entry for ages up to 30 seconds, but requires
a proxy to always revalidate.

The other approach, which Paul favors (and I think I do, too) is
define something like
	Cache-control: proxy-maxage=NNN
which supplies a separate age limit that applies only to shared
caches.  For example, a server doing authentication or hit-metering
might send:
	Cache-control: max-age=30, proxy-maxage=0
which means the same thing as the previous example (but actually
takes one less byte to encode!).

It probably makes sense to define proxy-maxage as implying
"proxy-revalidate", since otherwise one would have to send both
directives, and it's likely that this would be the normal case.

The nice thing about proxy-maxage is that it allows other things,
too, such as
	Cache-control: max-age=30, proxy-maxage=10
or even
	Cache-control: max-age=10, proxy-maxage=30
to provide finer control over the action of caches.  It's not
immediately clear to me that this is useful, but it seems to
be no more expensive to implement than "proxy-mustcheck", and
it might turn out to be useful later on.

I believe that Larry wants us to issue an Internet-Draft on
this topic, and not to discuss it to death on the mailing list.
But because there are several possible choices, I wanted to
see what people think before getting started on the Internet-Draft.


P.S.: Regarding IETF process: we discussed this at the meeting
in San Jose, with the Area Director present.  If the consensus
of the working group is that (1) this is truly a bug, and (2)
that the proposed fix is both small and not controversial, then
the IESG will probably allow us to make this kind of change
without restarting at Proposed Standard.  So please don't get
started now on a discussion about process; we can have that
later on, if there is still disagreement on the technical issues
once specific proposal(s) have been documented.
Received on Thursday, 19 December 1996 16:49:11 UTC

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