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Re: Age calculation in HTTP: two Internet Drafts

From: Roy T. Fielding <fielding@kiwi.ics.uci.edu>
Date: Thu, 25 Sep 1997 18:47:04 -0700
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Message-Id: <9709251900.aa08279@paris.ics.uci.edu>
I have been reticent to discuss this issue further because I already
discussed it to death, twice.  Nevertheless, since Jeff spent some
very valuable time generating a draft, I will make another attempt to
explain the problem.

>   Roy Fielding has taken issue with the statement ``HTTP/1.1 caches
>   MUST send an Age header in every response.'' [2].  Fielding correctly
>   points out that the use of the word ``caches'' in this sentence is
>   incorrect.  He lists two possible rewordings for this sentence:
>      a) An HTTP/1.1 server that includes a cache MUST send an Age
>         header field in every response.
>      b) An HTTP/1.1 server that includes a cache MUST include an Age
>         header field in every response generated from its own cache.
>   There is, however, a third possible rewording, if one realizes that
>   the original drafting error was to sloppily substitute the word
>   ``cache'' where ``proxy'' was meant (mea culpa):
>      c) An HTTP/1.1 proxy MUST send an Age header field in every
>         response.
>   Note that an HTTP proxy does not necessarily include a cache.  The
>   other sentences referring to ``caches'' in that paragraph reflect the
>   same drafting error.

I excluded that possibility because it would require that all proxies
implement complete compliance with the Cache-Control and Expires header
fields, including those that do not and never will cache a message.
Such a requirement cannot be justified.

>3.1 Quantifying the error of the Age estimate: Interpretation C
>   If interpretation (c) is used (i.e., the change proposed above in
>   section 2), then the value of the Age header field might overestimate
>   the actual amount of time since the response was generated at the
>   origin server.  One can calculate the size of the estimation error
>   for a path containing N HTTP/1.1 proxies:
>       Error_C = Mean_RTT * N * (N + 1)/2
>   where Mean_RTT is the mean round-trip time (RTT) between neighboring
>   pairs of proxies.  Note that this measures the error in the Age value
>   at the final recipient client, not the error at the last HTTP/1.1
>   proxy on the path.

That is entirely bogus.  I gave a complete summary of the age
calculation, as defined in the RFC 2068, in my draft (Section 2).
Because the age calculation involves the maximum of the clock difference
between the proxy and the origin or the value of the received Age,
the estimation error is unbounded with interpretation (c).  For N=3,
it is in fact


where only the variables b,c,d represent round-trip time.  skewUA, skewA,
skewB, and skewC, are all unbounded clock skew time errors.  It is those
values that dominate the equation -- Mean_RTT is insignificant and
irrelevant to this argument, aside from pointing out that interpretation (c)
results in overestimation even when clock synchronization is perfect.

What I find bizarre is that Jeff spent a great deal of effort demonstrating
that the above is unbounded with present HTTP servers.  Clock skew is
the enemy of the age calculation, which is why the Age header field must
not have an additive dependence on the clock skew of every recipient.

>3.2 Quantifying the error of the Age estimate: Interpretation B
>   If interpretation (b) is used, then the value of the Age header field
>   might underestimate the actual amount of time since the response was
>   generated at the origin server.

That is also true of interpretation (c).  The only difference is that
a proxy is likely to have a better clock than a UA.

>   The scenario that generates the
>   greatest underestimate is when the path between the origin server and
>   the HTTP/1.1 client's cache includes an HTTP/1.0 proxy cache, and the
>   client has a skewed clock.  In particular, the client's clock is set
>   into the past by some amount.

Correction: The only scenario in which interpretation (b) *might* result
in an underestimate of age which is worse than that of (c) is where one
or more HTTP/1.0 caches are in the request chain AND the response came
from one of those HTTP/1.0 caches in which it resided for some time AND
the user agent's system clock is running behind the origin server's clock.
In this one case, (c) would compensate for the clock skew if there existed
an HTTP/1.1 proxy between the user agent and the HTTP/1.0 cache generating
the response AND the HTTP/1.1 proxy is better-synchronized to the origin
server clock.  All of these coincidences combined would still have NO
impact on the correctness of the response unless the application required
the freshness criteria to be strictly enforced for that response AND
the current response would be different.

>3.4 Implications of overestimating the Age value: Interpretation C
>   What happens if the Age value is overestimated?  If this happens,
>   some "fresh" responses appear to be "stale", and so unnecessary cache
>   misses may be generated.  Except in the case where the network is
>   partitioned, this is a performance problem, but does not lead to the
>   delivery of responses with the wrong body or headers.  (When the
>   network is partitioned, caches are allowed to return stale values
>   with an appropriate Warning, so this is irrelevant to the current
>   discussion).

If the network is already congested, and caching is being used to reduce
that congestion, then failure to cache will result in denial of service.
That is denial of service for every protocol using the same network wire,
not just HTTP traffic, and many corporations (and several nations) install
proxy caches specifically to prevent this scenario.

If inaccurate Age values cause such congestion, cache administrators
will be compelled to ignore Age altogether.  That is why following (c)
will result in less semantic transparency in the long run.  In order
to be trusted, Age must be independent of clock skew.

Other benefits of (b) that were not mentioned include that (b) is a more
accurate (hence trusted) estimate for all other cases, and that lack of
an Age header field in a response from an all-HTTP/1.1 chain would
indicate that the response has been obtained from the origin.

>3.5 Implications of underestimating the Age value: Interpretation B
>   What happens if the Age value is underestimated?  If this happens,
>   some "stale" responses appear to be "fresh", and are returned to the
>   client without any Warning.  The client, in this case, naively
>   obtains the wrong response value.

No, it retains the stale value.  There is no implication that, just because
a response is stale, that it necessarily is invalid or even different from
what the client would have received directly from the origin.  The maximum
length of time the response would remain stale is (Expires - Date) or
the value of the max-age cache-directive, both of which are defined by
the origin server and not subject to UA clock-skew errors.

Furthermore, we can classify all applications of HTTP as either

  1) requires freshness preservation to preserve correctness
  2) does not require freshness preservation to preserve correctness

For the first case to work in practice, the origin server MUST prevent
an HTTP/1.0 proxy from caching the response.  We even provided special
means for an origin server to do so using max-age overriding of Expires,
and using Via to indicate the presence of an HTTP/1.0 application.
If such prevention is not implemented, then the HTTP/1.0 cache will
provide stale responses to clients, thus contradicting the requirement
of freshness preservation.  For the second case, underestimation of age
is irrelevant.

Since the above fact eliminates the only scenario in which (c) is useful,
and in all other scenarios (c) is detrimental to the HTTP system,
interpretation (b) is the only rational choice.

>   In other fields of computer science (for example, compilers or
>   multiprocessor caches), aggressive optimization always depends on a
>   reliable understanding of the situation at hand.  That is, if one
>   cannot be sure that the transformation done by the optimization
>   preserves the semantics of the system, one cannot safely do that
>   optimization (and, consequently, one may be prevented from doing a
>   lot of other optimizations that are enabled by the first one).  Even
>   when optimizations are done "speculatively", this always involves
>   being able to check the results for semantic correctness before
>   committing them.

There are a lot of optimizations that one cannot do with HTTP.  Those
optimizations were sacrificed for the sake of a system that does not
depend on immediate connectivity and low latency, as do all of the systems
to which you refer.  Sacrificing the benefits of HTTP in order to
obtain the benefits of other existing protocols or non-HTTP applications
is a mistake.  HTTP has very different design constraints, and should
be judged according to those constraints, or replaced outright by one
of those other protocols that better meets the application's needs.

Received on Thursday, 25 September 1997 19:12:44 EDT

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