I'm glad to see that there is a way for a client to express
its need for the latest-and-greatest version of the referenced
document, or to force any caches to be cleared.
But a simple change could add a lot of power.
Many clients might need not have the absolute latest version
of a document, but might be happy with one that is X time-units old.
For example, the client might want to say, in effect, "give me the
document referenced by this URL. I am willing to accept a copy that
might be 2 hours old."
(When I say "might be X hours old", I'm referring not to the age of
the document since it last changed, but the time since it may have
been copied from the authoritative server.)
So... It would be useful to have the (optional) ability to give a
number with the "no-cache" pragma to express a time (or date?). If a
cache has a copy which is younger than the specified time, that copy
may be delivered in the response. Otherwise the cache should treat
the pragma exactly as "no-cache" is presently defined.