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Re: #29: correcting corrected_initial_age

From: David Morris <dwm@xpasc.com>
Date: Mon, 8 Mar 2010 11:56:39 -0800 (PST)
To: HTTP Working Group <ietf-http-wg@w3.org>
Message-ID: <Pine.LNX.4.64.1003081153420.31841@egate.xpasc.com>


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
> 
Received on Monday, 8 March 2010 19:57:16 GMT

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