This has been discussed before on www-talk, e.g.
The relevent bit being a new header --
should reflect the date/timestamp of last comparison with the original
(i.e. the Date: header returned by the source document's HTTP server
in that comparison). Note that this should not be changed by cache
managers more than once-removed from the origin. This is because the
act of checking the cache consistency with the original document is
equivalent to getting a new copy of that document, but the act of
checking cache consistency with a higher-level cache is only equivalent
to copying that cache.
Has anyone implemented this?
...Roy Fielding ICS Grad Student, University of California, Irvine USA
<A HREF="http://www.ics.uci.edu/dir/grad/Software/fielding">About Roy</A>