If every document arrived with an expiry date then this would be fine.
Unfortunately precious few do and so other heuristic rules are often
used to guess when a conditional GET is required. The rule most
commonly used is based on a proportion of the time since the document
was Last-Modified. This assumes that a recently modified document is
likely to change again soon, while a document that has not been touched
in ages will remain fairly static. A maximum upper bound is often used
to make sure that conditional GETs happen on a regular basis even if
the Last-Modified date was a long time ago.
>I assume that there have been discussions of this: is there a reference to
>a summary anywhere?
>
>Colman
Discussion of these issues used to happen on www-proxy@w3.org, but the
list of Mailing Lists at W3C [1] suggests that www-proxy is `previous'
rather than `current'. Archives of the list only exist to January 1995.
Has the list been discontinued?
Neil Smith
HENSA Unix
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[1] http://www.w3.org/hypertext/WWW/Mail/Lists.html