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Network Working Group R. Fielding Request for Comments: 2616 ...

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<strong>Fielding</strong>, et al. Standards Track [Page 10]RFC <strong>2616</strong> HTTP/1.1 June 1999first-handA response is first-hand if it comes directly and withoutunnecessary delay from the origin server, perhaps via one or moreproxies. A response is also first-hand if its validity has justbeen checked directly with the origin server.explicit expiration timeThe time at which the origin server intends that an entity shouldno longer be returned by a cache without further validation.heuristic expiration timeAn expiration time assigned by a cache when no explicit expirationtime is available.ageThe age of a response is the time since it was sent by, orsuccessfully validated with, the origin server.freshness lifetimeThe length of time between the generation of a response and itsexpiration time.freshA response is fresh if its age has not yet exceeded its freshnesslifetime.staleA response is stale if its age has passed its freshness lifetime.semantically transparentA cache behaves in a "semantically transparent" manner, withrespect to a particular response, when its use affects neither therequesting client nor the origin server, except to improveper<strong>for</strong>mance. When a cache is semantically transparent, the clientreceives exactly the same response (except <strong>for</strong> hop-by-hop headers)that it would have received had its request been handled directlyby the origin server.validatorA protocol element (e.g., an entity tag or a Last-Modified time)that is used to find out whether a cache entry is an equivalentcopy of an entity.upstream/downstreamUpstream and downstream describe the flow of a message: all

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