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Re: #29: correcting corrected_initial_age

From: Mark Nottingham <mnot@mnot.net>
Date: Tue, 9 Mar 2010 20:22:43 +1100
Cc: "Julian F. Reschke" <julian.reschke@gmx.de>
Message-Id: <FEAD54AA-D80B-4CE3-8CD0-5F94794A477D@mnot.net>
To: HTTP Working Group <ietf-http-wg@w3.org>
Agreed. Julian, since you did the patch, could you update?

Cheers,


On 09/03/2010, at 6:56 AM, David Morris wrote:

> 
> 
> On Tue, 9 Mar 2010, Adrien de Croy wrote:
> 
>> 
>> 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.
> 
> Removal doesn't seem like the right solution ... we need to clarify that
> we me the time as known to the local system.
> 
> Perhaps replace: "local time" with "UTC time known to the local system"
> 
> Or some other phrasing which removes the ambiguity.
> 
> Dave Morris
> 
>> 
>> 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
>> 
> 


--
Mark Nottingham     http://www.mnot.net/
Received on Tuesday, 9 March 2010 09:23:18 GMT

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