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Re: Calculation of age headers

From: Koen Holtman <koen@win.tue.nl>
Date: Wed, 1 Oct 1997 21:24:38 +0200 (MET DST)
Message-Id: <199710011924.VAA10011@wsooti08.win.tue.nl>
To: Larry Masinter <masinter@parc.xerox.com>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com

Here are my comments on the Age issue.

1. First, the ambiguity of the words `1.1 cache' in 2086 is more than
just an editorial error, and it should not be resolved by just
replacing the words with `1.1 proxy', which is what the original
editor of that text meant to write.  If Jeff had originally written
`1.1 proxy', I would have complained in the last call of 2068.  See
the end of
 http://www.ics.uci.edu/pub/ietf/http/hypermail/1996q3/0450.html 
for some remarks on the history of these words.

2. Like Roy and Ari, I am against requiring that all proxies, even
non-caching ones, always add Age.  A firewall proxy (which may not
even have an internal clock!) should not be burdened by such a
requirement.

3. The age header value calculation in the 1.1 spec has three parts,
which I label as follows:

  LOCAL_CLOCK: correct for a too low age value in the received Age
               header (which can be too low due to storage in an
               intermediate 1.0 cache) by calculating the age of the
               response as the difference between the Date header and
               the local clock, and using this value for the age if it
               is higher
  the calculation is:
      apparent_age = max(0, response_time - date_value);
      corrected_received_age = max(apparent_age, age_value);


  ADD_DELAY: add the delay between the sending of the request message
             and the receipt of the response message to the age.
  the calculation is:
      response_delay = response_time - request_time;
      corrected_initial_age = corrected_received_age + response_delay;

  ADD_RESIDENT: add the time the response was resident in local cache
                memory
  the calculation is:
      resident_time = now - response_time;
      current_age   = corrected_initial_age + resident_time;

As far as I can see, Jeff and Roy take the following positions:

Jeff: every proxy MUST do LOCAL_CLOCK and ADD_DELAY, and ADD_RESIDENT
if applicable.

Roy: every proxy serving a response from its local cache MUST do
LOCAL_CLOCK, ADD_DELAY, and ADD_RESIDENT.

My own position is:

Koen: every proxy MAY do LOCAL_CLOCK.  Every proxy serving a response
from its local cache MUST do ADD_DELAY and ADD_RESIDENT.

Considering the three different actions in isolation:

4. There is no controversy over when a proxy should do ADD_RESIDENT.

5. As far as ADD_DELAY is concerned:
  - I think there is agreement among Jeff, Roy, and me that
    a proxy, when, serving a response from cache memory, MUST do
    ADD_DELAY, because not doing so may lead to underestimating the
    age.
  - Jeff's draft has a calculation which shows that always doing
    ADD_DELAY, even when not serving from cache memory, does not
    contribute that much to overestimating the age.  I agree with the
    conclusion, though I have some doubts about th calculation (it
    fails to take into account a delay due to the DNS lookup time in
    the final proxy, for example, and don't know right away if it is
    also applicable if you have a transatlantic link in between.)
  - Though doing ADD_DELAY in every proxy is not that bad, Jeff's
    draft does not have any argument or scenario which shows that
    doing ADD_DELAY in every proxy is actually good for something.
  - My main concern is that doing ADD_DELAY in every proxy is
    unnecessary for correctness, and therefore confusing for
    implementers.  I also find that aesthetically, it is worse than
    standardising a header name with a spelling error.
  - In my opinion, the spec should require only proxies serving a
    response from cache memory to do ADD_DELAY, but I could live with
    a spec which requires all proxies to do it, _IF_ a large note for
    implementers is added which confirms that this requirement is
    actually silly.

6. As far as LOCAL_CLOCK is concerned:
  - This is the most controversial issue
  - Jeff argues that every proxy should do LOCAL_CLOCK
  - He backs this up with a complicated scenario involving a 1.0 cache
    and a bad user agent clock.
  - I agree this scenario has a point, but Jeff's inference that,
    therefore, every proxy should do LOCAL_CLOCK goes to far for
    me.  I would conclude from this scenario that it is _sometimes_ a
    good idea for a proxy to do LOCAL_CLOCK.
  - Roy says that always doing LOCAL_CLOCK in every proxy is too
    expensive (in terms of overestimating ages which lead  to loss of
    cache efficiency), and he prefers a less safer, but cheaper system
    in which only some proxies do LOCAL_CLOCK.
  - In the second paragraph of section 3.2 of his draft, Jeff seems to
    argue that it is not possible to have a system which is cheaper,
    but as safe as, doing LOCAL_CLOCK in every proxy.  But this is
    wrong. It is possible to be cheaper, and still as safe.  For
    example:
       - if a proxy only does LOCAL_CLOCK when detecting (using the
         via header) that the upstream chain is not a pure 1.1 chain,
         then this will be cheaper (less change of overestimating the
         age) but still as safe.
       - it is possible to for a proxy or user agent to make a
         conservative measurement of the delta-time between its own
         clock and the clock of the origin server (by, for example,
         doing an OPTIONS request on the origin server and looking at
         the Date header in the response).  By using this delta-time:

           apparent_age = max(0, response_time - date_value + delta-time);

         the calculation which corrects for possible storage in a 1.0
         cache could be made more accurate, while still as safe
  - I conclude that the LOCAL_CLOCK correction is a hack to correct
    for breakage due to 1.0 caches in the chain.  This hack is neither
    optimal, not will it be necessary anymore in pure 1.1 chains, as
    Roy has already pointed out.  In pure 1.1 chains it is only a
    potential source of unnecessary performance degradation.
  - The spec should not require any proxy to ever do LOCAL_CLOCK.  It
    should merely encourage a proxy to do LOCAL_CLOCK if:
      - the proxy knows or suspects that there is a 1.0 proxy cache
        upstream
      - and if it knows or suspects that the user agent has a very bad
        clock, and no sophisticated mechanism to correct for its bad
        clock
    Proxies should also be encouraged to do something more
    sophisticated than LOCAL_CLOCK.

In summary, I like Roy's proposal better than Jeff's proposal, but I
would like it best if the age header value calculation gets rewritten
as:

      age = age_value;

      // A proxy MAY use a better way of calculating apparent_age
      // than the formulae below
      apparent_age = max(0, response_time - date_value);

      // A proxy MAY use any heuristic to compute the 
         correct_age_for_1.0_cache boolean
      correct_age_for_1.0_cache = 
            <any heuristic, or the value of a configuration option>;

      if(correct_age_for_1.0_cache) age = max(age, apparent_age);

      if( taking_response_from_cache_memory || I_am_a_user_agent )
      {
        response_delay = response_time - request_time;
        age = age + response_delay;
      }

      if( taking_response_from_cache_memory )
      {
        resident_time = now - response_time;
        age = age + resident_time;
      }

      new_age_value = age;

and I would like the Age header part of the spec to read:

A HTTP/1.1 caching proxy MUST send an Age header in every response
taken from its cache memory.  A HTTP/1.1 non-caching proxy, and a
HTTP/1.1 caching proxy which is relating an upstream response rather
than taking it from cache memory, MAY add and Age header to the
response if non is present, and MAY update the Age header which is
present when the age adjustment calculation in the proxy yields a
changed value.


Koen.
Received on Wednesday, 1 October 1997 12:28:26 EDT

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