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Re: #29: correcting corrected_initial_age

From: Adrien de Croy <adrien@qbik.com>
Date: Tue, 09 Mar 2010 00:50:36 +1300
Message-ID: <4B94E48C.4030406@qbik.com>
To: Mark Nottingham <mnot@mnot.net>
CC: HTTP Working Group <ietf-http-wg@w3.org>, Alex Rousskov <rousskov@measurement-factory.com>, Roy Fielding <fielding@gbiv.com>

I'd suggest removing the word "local" in all cases referring to time.

Since all date/times are in UTC (absolute time), the word local may 
potentially confuse implementors into using a local time (non UTC time) 
which breaks things like apparent_age.

Regards

Adrien

Mark Nottingham wrote:
> 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/
>
>
>   

-- 
Adrien de Croy - WinGate Proxy Server - http://www.wingate.com
Received on Monday, 8 March 2010 11:51:17 GMT

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