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Re: Meaning of header fields Date: and Age:

From: Alex Rousskov <rousskov@measurement-factory.com>
Date: Tue, 6 Jul 2004 10:40:57 -0600 (MDT)
To: Greg Robson-Garth <gregrg@optusnet.com.au>
Cc: ietf-http-wg@w3.org
Message-ID: <Pine.BSF.4.58.0407061018190.85419@measurement-factory.com>

On Tue, 6 Jul 2004, Greg Robson-Garth wrote:

> 1. Date
>
> In a cache server, assuming the message is now resident in the cache
> and has been requested by a HTTP client. Should the cache server
> send with message with:
>
> a)       the Date header as it received it from the origin server

Yes, if there was one. Otherwise, "A received message that does not
have a Date header field MUST be assigned one by the recipient if the
message will be cached by that recipient"

> 2. Age
>
> Does the Age header represent the seconds relative to the date/time in the
> date header.

Not always.

   "The Age field value is the cache's estimate of the amount of time
   since the response was generated or revalidated 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."

> So if the Age is say Age: 60 and the time is 03:00:00, then its
> birth time (for a want of a better word) is 02:59:00. If this is the
> case, then surely the correct calculation of corrected_received_age
> in 13.2.3 should be
>
>             now - date_value + age_value

The (now - date_value) part is usually the age we want. "Age_value"
from the Age: header is usually a little smaller than (now -
date_value) age because it does not account for network delays.

The age calculation algorithm in RFC 2616 is indeed buggy, but in a
different, more subtle, way:
http://lists.w3.org/Archives/Public/ietf-http-wg/2002JulSep/0048.html

IIRC, nobody commented on that age calculation bug and its proposed
fix. It is not mentioned in the RFC 2616 errata. I suspect it will
remain with us forever.

Alex.
Received on Tuesday, 6 July 2004 12:41:01 GMT

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