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Re: #29: correcting corrected_initial_age

From: Mark Nottingham <mnot@mnot.net>
Date: Thu, 11 Mar 2010 11:16:32 +1100
Cc: HTTP Working Group <ietf-http-wg@w3.org>
Message-Id: <F85A12FF-7595-43FA-938F-114ACE480557@mnot.net>
To: Julian Reschke <julian.reschke@gmx.de>
Looks good. The algorithm changes still needs to be applied, however.

On 11/03/2010, at 2:41 AM, Julian Reschke wrote:

> On 10.03.2010 09:50, Julian Reschke wrote:
>> ...
> In the meantime I realized that the current text is totally confusing because it first uses "now" as "time the response is received", and then later as "time the age is calculated". This is also reflected in pseudo code statements that are repeated, and in which "now" is replaced by "response_time".
> I just fixed this by using "now", "request_time" and "response_time" consistently, which also allowed getting rid of duplicated pseudo-code. I think this improves readability a lot. See <http://trac.tools.ietf.org/wg/httpbis/trac/changeset/786>.
> New text:
> -- 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.  age_value, if all of the caches along the response path implement
>       HTTP/1.1.
>   These are combined as
>     apparent_age = max(0, response_time - date_value);
>     corrected_received_age = max(apparent_age, age_value);
>   When an Age value is received, it MUST be interpreted relative to the
>   time the request was initiated, not the time that the response was
>   received.
>     response_delay = response_time - request_time;
>     corrected_initial_age = corrected_received_age + response_delay;
>   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 --
> Best regards, Julian

Mark Nottingham     http://www.mnot.net/
Received on Thursday, 11 March 2010 00:17:05 UTC

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