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RE: New document on "Simple hit-metering for HTTP"

From: Paul Leach <paulle@microsoft.com>
Date: Mon, 5 Aug 1996 16:09:05 -0700
Message-Id: <c=US%a=_%p=msft%l=RED-77-MSG-960805230905Z-34657@INET-01-IMC.itg.microsoft.com>
To: "'mogul@pa.dec.com'" <mogul@pa.dec.com>, "'koen@win.tue.nl'" <koen@win.tue.nl>
Cc: "'http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com'" <http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com>


>----------
>From: 	koen@win.tue.nl[SMTP:koen@win.tue.nl]
>Sent: 	Sunday, August 04, 1996 6:11 AM
>To: 	mogul@pa.dec.com
>Cc: 	http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
>Subject: 	Re: New document on "Simple hit-metering for HTTP"
>
>Jeffrey Mogul:
>>
>[...]
>>Our goal was NOT to solve the general problem of collecting demographic
>>information; it was to reduce the incentive for origin servers to
>>defeat caching merely so that they could collect simple hit-counts, of
>>a sort that the caches could just as easily collect for them.
>[...]
>>You can find a copy at
>>
>>    http://ftp.digital.com/~mogul/draft-ietf-http-hit-metering-00.txt
>[...]
>
>I have several comments.
>
>1. Cascaded proxy caches.
>
>At first glance, there seem to be counting problems in a cascaded
>proxy cache situation.  If we have the arrangement
>
>   origin server ---- proxy 1 ------ proxy 2 ---- user agent
>
>and the user agent requests and uncached page, section 5.1 seems to
>say that both proxy 1 and proxy 2 must set the use count to 1 when
>relaying the page.  This results in a count of 2 being reported in the
>end, though the page is only viewed once.  It seems like there needs
>to be a special case for proxy 1: a proxy should not count if it is
>relaying the response to another proxy.  (Under HTTP/1.1, the Via:
>header in the request would tell you that you are talking to another
>proxy.)

Sounds like a bug. The fix sounds yucky, though. WE'll try to dream up a
cleaner one.

>2. Number of unconditional GETs = number of times read???
>
>You argue that the number of unconditional GETs, rather than the total
>number of GETs, more accurately reflects the number of times a page is
>read.  I don't know if this is true; I would like to see section 4
>discuss user agents on shared machines, and situations in which user
>agent disk caches are disabled entirely because there is a central
>proxy (like on our local sun cluster).

>We didn't really intend the claim to be that strong. The "number of
unconditional GETs = number of times read" only holds (approximately)
true for interior, shared caches. (By interior, I mean that the client
of the cache itself has a cache.) A single user leaf cache doesn't have
to do anything in the hit-metering design (it can ignore max-uses and
not send use-count) but a shared leaf cache would need to remember
requestors' From or IP address to try to relay a meaningful use-count.
>
>3. A `hit' being an *un*conditional GET
>
>In the current (classic) meaning of the word,
>
>  1 hit-classic = 1 request on an origin server.
>
>Your draft defines a new kind of hit:
>
>  1 hit-new = 1 200/203/206 response returned to a user agent.
>
>Now, if I am an origin server which uses cache busting, and if most
>caches play by the rules, then for my server I will measure:
>
>  hit-new < hit-classic .
>
>Assuming that I get payed by the hit, I have absolutely no incentive
>to start measuring hit-news instead of hit-classics.  To stop using
>the cache-busting based hit-classics would be economic suicide.

No, the  payment per hit-new would just be higher than for per
hit-classic.
>Maybe the content providers only switch to hit-new when an ad contract runs
out and the advertiser demands the better counting method, but other
>than that I see no real barrier.
>
>So even if hit-new is a better metric than hit-classic, I fear it
>won't be effective at reducing cache busting.
>
>The nicest solution to this problem seems to be for proxies to count
>both hit-new and a second metric:
>
>  1 touch-new = 1 response returned to a user agent
>
>for which it is guaranteed that
>
>   hit-classic <= touch-new .
>
>(Note: `hit' and `touch' would *not* be my proposals for adequate
>names for these metrics.)


There is no need to have a new way count hit-classic -- people already
do that today.

>4. Interaction with Vary
>
>I don't like the extra complexity and inefficiency introduced by the
>Vary counting rules in section 3. (See second-to-last sentence of
>Section 5.1.)
>
>I think the proposal would be better if the Vary special case were
>removed entirely.

Don't you think that providers of multilingual content want to count the
hits of the French, German, Dutch, and English (etc.) version
separately? And that, if they insist on not losing counts when a page is
flushed from the cache, they'll want to do that on multilingual pages
too?

However, I think I could be convinced that there only needs to be one
count per Etag. The idea was that even if there were only an English
version of a page, having the server set "Vary: Accept-Language" would
cause separate counting of hits for users with "fr" "de", etc, so that
the potential for these languages could be gauged without creating pages
for all of them. Being able to count them is, I think, a GOOD THING; but
maybe one could create a separate Etag without having to create a
separate physical copy of the page. We'll think about it.

>5. Overhead in proxy efficiency
>
>I'm wondering if the counting mechanisms in the draft won't cause an
>unacceptable overhead for high-performance cache implementations.  I
>think we definitely need the opinions of proxy cache implementers on
>this issue.

The design is supposed to just require addition of a counter to a data
structure that needs to be around anyway, and to add a few bytes to a
message you needed to send anyway.  The alternative is that pages for
which demographic info is required aren't cached at all, which is
plainly MUCH worse.
>
>One possible hit counting alternative, post-processing proxy logfiles
>and delivering the results to the servers, seems to have less
>overhead.

But way more complicated to spec and implement.
>
More replies to the other comments later.

>Paul 
Received on Monday, 5 August 1996 16:14:29 EDT

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