RFC 2616                        HTTP/1.1                       June 1999


   origin server
      The server on which a given resource resides or is to be created.

   proxy
      An intermediary program which acts as both a server and a client
      for the purpose of making requests on behalf of other clients.
      Requests are serviced internally or by passing them on, with
      possible translation, to other servers. A proxy MUST implement
      both the client and server requirements of this specification. A
      "transparent proxy" is a proxy that does not modify the request or
      response beyond what is required for proxy authentication and
      identification. A "non-transparent proxy" is a proxy that modifies
      the request or response in order to provide some added service to
      the user agent, such as group annotation services, media type
      transformation, protocol reduction, or anonymity filtering. Except
      where either transparent or non-transparent behavior is explicitly
      stated, the HTTP proxy requirements apply to both types of
      proxies.

   gateway
      A server which acts as an intermediary for some other server.
      Unlike a proxy, a gateway receives requests as if it were the
      origin server for the requested resource; the requesting client
      may not be aware that it is communicating with a gateway.

   tunnel
      An intermediary program which is acting as a blind relay between
      two connections. Once active, a tunnel is not considered a party
      to the HTTP communication, though the tunnel may have been
      initiated by an HTTP request. The tunnel ceases to exist when both
      ends of the relayed connections are closed.

   cache
      A program's local store of response messages and the subsystem
      that controls its message storage, retrieval, and deletion. A
      cache stores cacheable responses in order to reduce the response
      time and network bandwidth consumption on future, equivalent
      requests. Any client or server may include a cache, though a cache
      cannot be used by a server that is acting as a tunnel.

   cacheable
      A response is cacheable if a cache is allowed to store a copy of
      the response message for use in answering subsequent requests. The
      rules for determining the cacheability of HTTP responses are
      defined in section 13. Even if a resource is cacheable, there may
      be additional constraints on whether a cache can use the cached
      copy for a particular request.





 
RFC 2616                        HTTP/1.1                       June 1999


   first-hand
      A response is first-hand if it comes directly and without
      unnecessary delay from the origin server, perhaps via one or more
      proxies. A response is also first-hand if its validity has just
      been checked directly with the origin server.

   explicit expiration time
      The time at which the origin server intends that an entity should
      no longer be returned by a cache without further validation.

   heuristic expiration time
      An expiration time assigned by a cache when no explicit expiration
      time is available.

   age
      The age of a response is the time since it was sent by, or
      successfully validated with, the origin server.

   freshness lifetime
      The length of time between the generation of a response and its
      expiration time.

   fresh
      A response is fresh if its age has not yet exceeded its freshness
      lifetime.

   stale
      A response is stale if its age has passed its freshness lifetime.

   semantically transparent
      A cache behaves in a "semantically transparent" manner, with
      respect to a particular response, when its use affects neither the
      requesting client nor the origin server, except to improve
      performance. When a cache is semantically transparent, the client
      receives exactly the same response (except for hop-by-hop headers)