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Re: #29: correcting corrected_initial_age

From: Julian Reschke <julian.reschke@gmx.de>
Date: Thu, 11 Mar 2010 20:29:07 +0100
Message-ID: <4B994483.5070404@gmx.de>
To: Mark Nottingham <mnot@mnot.net>
CC: HTTP Working Group <ietf-http-wg@w3.org>
On 11.03.2010 01:16, Mark Nottingham wrote:
> Looks good. The algorithm changes still needs to be applied, however.
> ...

Here we go:

-- snip --
2.3.2.  Calculating Age

    HTTP/1.1 uses the Age response-header to convey the estimated age of
    the response message when obtained from a cache.  The Age field value
    is the cache's estimate of the amount of time since the response was
    generated or validated by the origin server.  In essence, the Age
    value is the sum of the time that the response has been resident in
    each of the caches along the path from the origin server, plus the
    amount of time it has been in transit along network paths.

    The following data is used for the age calculation:

    age_value

       The term "age_value" denotes the value of the Age header
       (Section 3.1), in a form appropriate for arithmetic operation; or
       0, if not available.

    date_value

       HTTP/1.1 requires origin servers to send a Date header, if
       possible, with every response, giving the time at which the
       response was generated.  The term "date_value" denotes the value
       of the Date header, in a form appropriate for arithmetic
       operations.  See Section 9.3 of [Part1] for the definition of the
       Date header, and for requirements regarding responses without a
       Date response header.

    now

       The term "now" means "the current value of the clock at the host
       performing the calculation".  Hosts that use HTTP, but especially
       hosts running origin servers and caches, SHOULD use NTP
       ([RFC1305]) or some similar protocol to synchronize their clocks
       to a globally accurate time standard.

    request_time

       The current value of the clock at the host at the time the request
       resulting in the stored response was made.

    response_time

       The current value of the clock at the host at the time the
       response was received.

    A response's age can be calculated in two entirely independent ways:

    1.  the "apparent_age": response_time minus date_value, if the local
        clock is reasonably well synchronized to the origin server's
        clock.  If the result is negative, the result is replaced by
        zero.

    2.  the "corrected_age_value", if all of the caches along the
        response path implement HTTP/1.1; note this value MUST be
        interpreted relative to the time the request was initiated, not
        the time that the response was received.


      apparent_age = max(0, response_time - date_value);

      response_delay = response_time - request_time;
      corrected_age_value = age_value + response_delay;

    These are combined as

      corrected_initial_age = max(apparent_age, corrected_age_value);

    The current_age of a stored response can then be calculated by adding
    the amount of time (in seconds) since the stored response was last
    validated by the origin server to the corrected_initial_age.

      resident_time = now - response_time;
      current_age = corrected_initial_age + resident_time;

-- snip --

(see 
<http://trac.tools.ietf.org/wg/httpbis/trac/attachment/ticket/29/i29.diff>)

Best regards, Julian
Received on Thursday, 11 March 2010 19:30:13 GMT

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