There is also a provision to extend this list. Access Authentication Provides simple challenge-response authentication mechanism. Heuristic expiration time Expiration time assigned by cache when no explicit expiration time is available. Age Age of response is time since it was sent by, or successfully validated with, origin server. Client receives exactly the same response from cache as from origin server.
Warning is sent to the user if relaxed.
Not Modified, Proxy Redirect, or error response returned. Expiration Model Recall that expiration is used to avoid sending requests . Client may also specify that it will accept stale responses, up to some maximum amount of staleness. This may be necessary to support disconnected operation, or high availability in the face of poor connectivity. Server could also assign expiration time in past to force caches to revalidate every request.