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Re: #29: correcting corrected_initial_age

From: Mark Nottingham <mnot@mnot.net>
Date: Wed, 17 Mar 2010 20:57:52 +1100
Cc: HTTP Working Group <ietf-http-wg@w3.org>
Message-Id: <BDDE69B4-A77F-45E1-A37E-BD10B96C102A@mnot.net>
To: Julian Reschke <julian.reschke@gmx.de>
Looks good to me.


On 12/03/2010, at 6:29 AM, Julian Reschke wrote:

> 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


--
Mark Nottingham     http://www.mnot.net/
Received on Wednesday, 17 March 2010 09:58:26 GMT

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