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Re: #29: correcting corrected_initial_age

From: Mark Nottingham <mnot@mnot.net>
Date: Thu, 4 Mar 2010 15:39:59 +1100
Cc: Alex Rousskov <rousskov@measurement-factory.com>, Roy Fielding <fielding@gbiv.com>
Message-Id: <A7653F30-5A27-492D-B950-472F48FF1894@mnot.net>
To: HTTP Working Group <ietf-http-wg@w3.org>
We haven't heard back from Alex, and the other issue I mentioned didn't seem to get enough support to move on. So, I suggest we do the conservative thing:

Current text:
>   age_value     - Age header field-value received with the response
>   date_value    - Date header field-value received with the response
>   request_time  - local time when the cache made the request 
>                  resulting in the stored response
>   response_time - local time when the cache received the response
>   now           - current local time
>   
>   apparent_age = max(0, response_time - date_value);
>   corrected_received_age = max(apparent_age, age_value);
>   response_delay = response_time - request_time;
>   corrected_initial_age = corrected_received_age + response_delay;
>   resident_time = now - response_time;
>   current_age   = corrected_initial_age + resident_time;

Replacement text:
>   age_value     - Age header field-value received with the response;
>                                0 if not available.
>   date_value    - Date header field-value received with the response;
>                                see [ref] for requirements regarding responses
>                                without a date_value.
>   request_time  - local time when the cache made the request 
>                  resulting in the stored response
>   response_time - local time when the cache received the response
>   now           - current local time
>   
>   apparent_age = max(0, response_time - date_value);
>   response_delay = response_time - request_time;
>   corrected_initial_age = max(apparent_age, age_value + response_delay)
>   resident_time = now - response_time;
>   current_age   = corrected_initial_age + resident_time;

Comments?



On 14/10/2009, at 8:31 PM, Mark Nottingham wrote:

> Hi Alex,
> 
> We put this on the HTTPbis issues list a while ago <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/29>, and I've discussed it with a few folks F2F, but AFAICT it hasn't been discussed on-list.
> 
> In a nutshell, I think you're correct that there's a problem here, but your proposal:
> 
>> creation_time = min(date_value, request_time - age_value);
>> current_age = now - creation_time;
> 
> has a few (small-ish) issues.
> 
> 1) The corrected_received_age's subtraction of the date_value from now has the (intended, I assume) effect of accounting for upstream HTTP/1.0 caches that don't append an Age header. Your proposal doesn't do this.
> 
> This is already being diccussed on-list (see recent thread "cache freshness / age calcs"), and may go away anyway. Your input there would be appreciated.
> 
> 2) The behaviour when date_value isn't present isn't specified; we could address this in prose, but it would be awkward.
> 
> This could probably be worked around by either specifying a slightly more complex formula, or specifying that when the Date header isn't present, a completely separate (and presumably much simpler) formula is to be used.
> 
> Cheers,
> 
> 
> 
> On 31/08/2002, at 2:59 AM, Alex Rousskov wrote:
> 
>> 
>> Hi there,
>> 
>> 	We are testing a couple of RFC 2616 MUSTs related to
>> current_age calculation. Many proxies violate a subset of test cases
>> that includes an artificial proxy-to-server delay. Looking at the
>> results, I think that the proxies are doing the "right thing" and the
>> RFC has a problem.
>> 
>> 	I will start with a specific example when current_age formula
>> from the RFC yields a way-too-conservative and unnatural result (100%
>> error). I will then describe the problem and suggest a fix.
>> 
>> 	I understand that a way-too-conservative age does not lead to
>> stale documents being returned. However, if we want proxies to be
>> compliant, we may want to fix/mention the problem in the errata or
>> elsewhere. Otherwise, the more problems like that are left unaddressed
>> (ignored), the more difficult it would be to convince implementors to
>> pay attention to the RFC.
>> 
>> 	Perhaps I got it all wrong, please check!
>> 
>> 
>> A simple example
>> ----------------
>> 
>> Here is a real and simple example that detected the problem with the
>> original current_age formula from "13.2.3 Age Calculations". The
>> absolute values of timestamps below ("0" and "7") have no
>> significance.
>> 
>>  time event
>>  ---- ------------------------------------------------------------
>>   0.0 client request generated
>>   0.0 client request reached the proxy, it is a MISS
>>   0.0 proxy request to origin server is generated
>>   0.0 proxy request reached the origin server
>>   0.0 server response generated with Date correctly set to 0, no Age header
>>   -- a network delay of 7 seconds --
>>   7.0 server response reached the proxy
>>   7.0 proxy cached the response
>>   7.0 proxy forwarded the response
>>   7.0 the response reached the client
>>   7.0 another client request for the same URL generated
>>   7.0 client request reached the proxy, it is a HIT
>>   7.0 proxy must compute Age header value, see math below
>> 
>> Following RFC 2616:
>> 
>>   age_value = 0             (the cached response has no Age header)
>>   date_value = 0            (the cached response has Date set to 0)
>>   request_time = 0          (the proxy generated request at time 0)
>>   response_time = 7         (the proxy received response at time 7)
>>   now = 7                   (the current time is 7)
>> 
>>   apparent_age = max(0, response_time - date_value) = 7
>>   corrected_received_age = max(apparent_age, age_value) = 7
>>   response_delay = response_time - request_time = 7
>>   corrected_initial_age = corrected_received_age + response_delay = 14
>>   resident_time = now - response_time = 0
>>   current_age   = corrected_initial_age + resident_time = 14
>> 
>> The true age is, of course, 7 and not 14. The above formulas just double true
>> current age in the case of a network delay between the proxy and the origin
>> server. The fixed formula (see below for the discussion) does not:
>> 
>> current_age = now - min(date_value, request_time - age_value) =
>>            = 7 - max(0, 0 - 0) = 7
>> 
>> N.B. If the proxy computes Age header for misses and uses that as
>> age_value when serving hits, the formulas yield the same result.
>> 
>> 
>> The Problem
>> -----------
>> 
>> RFC 2616 says:
>> 
>>  Because the request that resulted in the returned Age value must have
>>  been initiated prior to that Age value's generation, we can correct
>>  for delays imposed by the network by recording the time at which the
>>  request was initiated. Then, when an Age value is received, it MUST
>>  be interpreted relative to the time the request was initiated...
>>  So, we compute:
>> 
>>     corrected_initial_age = corrected_received_age
>>                           + (now - request_time)
>> 
>> I suspect the formula does not match the true intent of the RFC
>> authors. I believe that corrected_initial_age formula counts
>> server-to-client delays twice. It does that because the
>> corrected_received_age component already accounts for one
>> server-to-client delay. Here is an annotated definition from the RFC:
>> 
>>  corrected_received_age = max(
>>    now - date_value, # trust the clock (includes server-to-client delay!)
>>    age_value)        # all-HTTP/1.1 paths (no server-to-client delay)
>> 
>> I think it is possible to fix the corrected_initial_age formula to
>> match the intent (note this is the *initial* not *received* age):
>> 
>>  corrected_initial_age = max(
>>    now - date_value,                # trust the clock (includes delays)
>>    age_value + now - request_time)  # trust Age, add network delays
>> 
>> There is no need for corrected_received_age.
>> 
>> 
>> Moreover, it looks ALL the formulas computing current_age go away with
>> the above new corrected_initial_age definition as long as "now" is
>> still defined as "the current time" (i.e., the time when current_age
>> is calculated):
>> 
>>  current_age = corrected_initial_age
>> 
>> So, we end up with a single formula for all cases and all times:
>> 
>> current_age = max(now - date_value, age_value + now - request_time) =
>>            = now - min(date_value, request_time - age_value)
>> 
>> It even has a clear physical meaning -- the min() part is the conservative
>> estimate of object creation time. We could rewrite for clarity:
>> 
>> creation_time = min(date_value, request_time - age_value);
>> current_age = now - creation_time;
>> 
>> 
>> Am I missing something important here? If I am right, and the current
>> formulas count server-to-client delays twice, is it worth mentioning
>> in the errata or elsewhere as a bug? Or should we insist that
>> implementations use current_age calculation from the RFC anyway?
>> 
>> Thank you,
>> 
>> Alex.
>> 
>> -- 
>>                           | HTTP performance - Web Polygraph benchmark
>> www.measurement-factory.com | HTTP compliance+ - Co-Advisor test suite
>>                           | all of the above - PolyBox appliance
>> 
> 
> 
> --
> Mark Nottingham     http://www.mnot.net/
> 
> 


--
Mark Nottingham     http://www.mnot.net/
Received on Thursday, 4 March 2010 04:40:33 GMT

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