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Re: Comments on draft-mogul-http-hit-metering-01.txt

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Tue, 04 Mar 97 17:57:57 PST
Message-Id: <9703050157.AA13230@acetes.pa.dec.com>
To: Koen Holtman <koen@win.tue.nl>
Cc: http-wg@cuckoo.hpl.hp.com
Catching up on my old email ...

Koen seems to have two major objections to the current
draft of the hit-metering document:
	(1) he believes that the introduction of hit-metering
	will decrease, not increase, the amount of caching that
	takes place.  I.e., it will increase, not decrease,
	total traffic.
	(2) he objects to "_any_ positive claims about the
	relation between hits and users".

Koen also has some minor objections, most of which I am happy
to resolve.

I'll address these objections point-by-point, but first I want
to make one thing clear: where the difference between our
positions is a matter of opinion which cannot be resolved
by existing data, then a negative opinion about a proposed
protocol specification (especially a fully optional extension)
is not sufficient reason to kill the specification.

To give an example from outside the HTTP-WG: suppose I were to
state an opinion that the use of MD5 is a bad for the default
authentication algorithm in IPv6, because it is too slow for
some environments ... and suppose someone else stated that MD5 is the
right choice for a default, because it is fast enough for almost
all environments, and there is no widely available alternative.
No matter how much we debate this, we could not resolve it by
debate, because the hypothesis (that MD5 is "too slow" for "too
many" aplications) can only be tested by trying it.

Getting back to the case at hand: Koen's says his "main efficiency
worry" is:
     If many people, who do not use cache busting now, start using hit
     metering, then caches _outside_ of the metering subtree will have
     to do much more revalidation requests, which means more RTT
     overheads and a slower web.
The reality of this worry depends on several sub-hypotheses:
    (1) The availability of hit-metering will induce a large number
    of sites that do not now do cache-busting to use hit-metering.
    [Otherwise, hit-metering could not increase the amount of
    cache-busting in any subtree, metering or otherwise.]

    (2) Hit-metering will be deployed in proxies in a pattern
    that will lead, in many cases, to a metering subtree that does
    not extend to, or near, the last hop to the clients.  [Otherwise,
    there won't be many "caches outside the metering subtree(s)".]

    (3) If both #1 and #2 hold, any improvement in caching within
    the metering subtree (which would be due to the use of
    hit-metering by servers that would otherwise use cache-busting
    anyway) is not enough to offset the new cache misses outside
    the metering subtree.  In order to quantify this, one has to
    consider a lot of parameters, such as the fanout of responses
    within and outside the metering subtree, the network delays
    (bandwidth, speed-of-light, and queueing) within and outside
    the metering subtree, the sizes of caches inside and outside
    the subtree, and assumptions about the use of other protocol
    features (such as persistent connections).  [In other words,
    this is pretty hard to quantify using a model.]

    (4) If #1, #2, and #3 all hold, cache operators won't react to
    the increased revalidation traffic by dropping out of the metering
    subtree (i.e., by disabling hit-metering), ultimately pushing
    the burden of handling the revalidations back to the origin
    servers (which would provide an incentive for them to stop
    the use of hit-metering).

I will happily grant that if these four hypotheses are all true
(and more than just slightly true), then hit-metering would be
a bad thing for the network.  But it should also follow that Koen
must admit that if some or all of these sub-hypotheses (especially
#1 and #2) are false, then hit-metering would not cause performance
problems, and (if they are false in a big way) then hit-metering
would provide a definite improvement.

Further, these sub-hypotheses are all beliefs about how large
numbers of humans (server operators and proxy operators) will
react in a complex and evolving environment.  It would be foolish
of me to assert that I could prove or disprove any of them with
the information we have available today, and I frankly don't
expect anyone else to be able to prove or disprove them (based
on current information).

So it basically comes down to making guess about these hypotheses,
about the worst-case, best-case, and likeliest scenarios, and
(whether we act or not) taking a risk that we're making the wrong
choice.  While Koen writes "Not doing anything is sometimes
the most logical course of action", I don't think he has made
a strong case that "not doing anything" about cache-busting is
actually our best bet.

On to Koen's other major complaint.

Section 4 of the hit-metering draft is labelled "Analysis",
and starts:
   We recognize that, for many service operators, the single most
   important aspect of the request stream is the number of distinct
   users who have retrieved a particular entity. We believe that our
   design provides adequate support for user-counting, based on the
   following analysis.
After a complaint from Koen, I revised the second sentence so that
this now reads:
   We recognize that, for many service operators, the single most
   important aspect of the request stream is the number of distinct
   users who have retrieved a particular entity. We believe that our
   design provides adequate support for user-counting, within the
   constraints of what is feasible in the current Internet, based on 
   the following analysis.

Note that this language is NOT a part of the specification per se,
and is heavily qualified; we use phrases like "for many service
operators", "we believe", "based on the following analysis".  This
is NOT a statement of fact, it's an opinion, and clearly labelled
as such.

Koen responds:
    [I] can think of several currently possible techniques, most of
    them involving actual statistical methods, which would would be
    more accurate.
    Bottom line: I want you to stop making _any_ positive claims about the
    relation between hits and users.

I'd surely like to see a well-defined description, including some
analysis, of these other possible techniques, and perhaps James
Pitkow's paper (when it becomes available) will shed some light.
But I'm not interested in continuing a debate of the form "I
know a better way to do this, but I'm not going to provide a
detailed argument about why it is better."

As for making "positive claims" about the relation between hits
and users: It would be ludicrous to suggest that there is no
correlation between the number of hits seen when using hit-metering
and the number of distinct users, and so I am not even going to
accuse Koen of suggesting that.

It is debatable whether the correlation is exact (i.e., that
hit-metering gives a user-count with 0% error); in fact, as we
state later in the analysis, hit-metering gives an approximation,
and we clearly state that "there are some circumstances under which
this approximation can break down."  In other words, we are clearly
making a claim about the quality of the approximation, nothing more.

We also observe that existing techniques (either cache-busting or
full caching) can, and usually do, give much worse approximations
than hit-metering would.  Koen has not argued this point.

So, in a last attempt to satisfy Koen, I'll rewrite this again
to make it clear that we are talking about an approximation:

   We recognize that, for many service operators, the single most
   important aspect of the request stream is the number of distinct
   users who have retrieved a particular entity. We believe that our
   design provides adequate support for approximate user-counting,
   within the constraints of what is feasible in the current Internet,
   based on the following analysis.

Beyond that, it would be pointless to continue subjecting the
working group to this debate, so I won't.  If anyone wants to
discuss this offline, I'm willing to continue it that way.

On to some minor objections:

   Please rename the proposal `Simple Hit-Metering and Usage-Limiting for

That makes sense; done.

    >As for section 8, "Interactions with varying resources": this simply
    >states the bare minimum necessary to make sensible use of the Vary
    >mechanism as it is currently defined in the HTTP/1.1 RFC.
    Section 8 is not minimalist, it maximises the info!
    Smaller solutions, which still make sense to me, are:
     1) not counting for each combination of request headers, but for each
	entity tag only
     2) counting for each content-location only
     3) only one count for the entire resource

Our section 8 is minimal in the complexity it imposes on the
proxy implementation; we made no claims about how much (or how
little) information it provides to the origin server, although
it manifestly provides no more information than cache-busting

It seems to me that counting hits per-entity-tag is pretty
much equivalent to counting them per Vary-defined variant.
After all, if the server is using strong entity tags, then
there is a one-to-one mapping between variant responses and
entity-tags.  And it seems like a bad idea to use the same
weak entity tag for two different variants of the same resource,
since this makes it impossible for a proxy cache to reliably
do conditional GETs.

An HTTP/1.1 cache must either not cache a response with a
Vary header (in which case hit-metering does not apply), or it
must comply with Vary, which means keeping at least a minimal
amount of per-entry data.  It does not require keeping
per-content-location data, so basing hit-metering on content-location
seems like a burden on cache implementors.

Keeping one count for the entire resource would be easily
to implement, but it would probably encourage servers providing
variant responses to use cache-busting instead of hit-metering.
Which gets back to the original argument about whether or not
they would use cache-busting, and so "see above".

    Like Ted, I _am_ concerned that introducing a single sticky header
    will reduce the usable lifetime of HTTP/1.x as a protocol suite.
    You can only add so much features before you drown in feature
    interactions.  Some of this is already visible in your section

Well, there's one simple requirement in section 5.5 (which 
describes how a non-caching proxy can forward Meter headers in
both directions without creating inaccuracy) that depends on
this issue.  And if the protocol did have a general-purpose
sticky-header mechanism, it would also create interactions
in section 5.5 (additional ones, in fact).

But the stickiness of the Meter request header is, admittedly, a fairly
small optimization.  The statistics from your 1996 message show a mean
request header size of 200 bytes (if I interpret that message
correctly).  I looked at a recent proxy trace with several hundred
thousand references, and found a mean request header size of 305.5
bytes.  Your analysis also suggested that, using persistent
connections, the mean client-server connection would carry 9.2
requests.  It's reasonable to assume that a proxy-to-server
persistent connection, which might be reused before it closes,
would carry at least that many.

Making the Meter header per-request (rather than per-connection)
would, in simplest (and hopefully, most common) case, involve
sending 8 bytes per request.  This is a 4% overhead using your
200-byte mean, or a 2.6% overhead using the mean that I measured.
If the Meter request header is sticky, and assuming 9.2 requests
per connection, then the overheads would drop to 0.43% or 0.29%,
depending on which mean header size one uses.

Whether or not the Meter header is sticky, the "Connection:Meter"
header is per-connection, so that's a constant in this analysis;
about 2 bytes per mean request, or a 1% overhead.

A 4% "extra" overhead is probably just below the threshold where
we should begin to be concerned.  This is a dangerous argument
to make, though, if we make it 10 times independently.  It might
be OK to drop the sticky-header mechanism from the hit-metering
draft, but I would caution against using this analysis to justify
not adopting a general sticky-header mechanism for HTTP.

Anyway, I'll drop the sticky-header stuff from the hit-metering
proposal IF nobody objects to the extra overhead in the requests.
And if it leads to at least one of the current critics turning
into a supporter.

Regarding prefetching, Koen wrote:
    I believe your system will always count a page _pre_fetch done by a
    proxy as a hit, no matter whether the page is actually seen by a user
    later.  You need to fix this.
I responded:
    [The] entire issue of prefetching in HTTP needs some more thought.
    [...] The hit-metering proposal might need to be fixed, but the
    general     problem is broader.
to which Koen wrote:
    I am aware that there is a broad problem, but you need to fix this
    sub-problem nevertheless.  Just define some flag that the cache can
    include if it does a prefetch.  I'm aware that this does not solve
    the problem of prefetching outside of the metering subtree.
After giving this "more thought" (but not nearly enough), I think we
probably *will* have to define some way to flag a request as a prefetch
... but I do NOT think that this flag should be specified in the
hit-metering proposal.  It would be a bad situation to end up with
several different ways of flagging prefetches.  I will add something to
the hit-metering proposal to say that once a prefetch-flag is defined,
then a proxy that both uses hit-metering and generates prefetches MUST
use that flag.
I'll address TCN in a separate message, since this isn't
an issue where there is a real difference of opinion.

Received on Tuesday, 4 March 1997 18:12:55 EST

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