Prose for HTTP 304

HTTP-304.txt

4.1.  304 Not Modified

   The 304 status code indicates that a conditional GET request has been
   received and would have resulted in a 200 (OK) response if it were
   not for the fact that the condition has evaluated to false.  In other
   words, there is no need for the server to transfer a representation
   of the target resource because the client's request indicates that it
   already has a valid representation, as indicated by the 304 response
   header fields, and is therefore redirecting the client to make use of
   that stored representation as if it were the payload of a 200
   response.  The 304 response MUST NOT contain a message-body, and thus
   is always terminated by the first empty line after the header fields.

   A 304 response MUST include a Date header field (Section 10.2 of
   [Part2]) unless the origin server does not have a clock that can
   provide a reasonable approximation of the current time.  If a 200
   response to the same request would have included any of the header
   fields Cache-Control, Content-Location, ETag, Expires, or Vary, then
   those same header fields MUST be sent in a 304 response.

   Since the goal of a 304 response is to minimize information transfer
   when the recipient already has one or more cached representations,
   the response SHOULD NOT include representation metadata other than
   the above listed fields unless said metadata exists for the purpose
   of guiding cache updates (e.g., future HTTP extensions).

   If the recipient of a 304 response does not have a cached
   representation corresponding to the entity-tag indicated by the 304
   response, then the recipient MUST NOT use the 304 to update its own
   cache.  If this conditional request originated with an outbound
   client, such as a user agent with its own cache sending a conditional
   GET to a shared proxy, then the 304 response MAY be forwarded to the
   outbound client.  Otherwise, the recipient MUST disregard the 304
   response and repeat the request without any preconditions.

   If a cache uses a received 304 response to update a cache entry, the
   cache MUST update the entry to reflect any new field values given in
   the response.