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

for the associated entity, its entity-tag SHOULD NOT be included inthe If-None-Match header field unless the request is for a range thatwould be fully satisfied by that entry.If a cache receives a successful response whose Content-Locationfield matches that of an existing cache entry for the same Request-]URI, whose entity-tag differs from that of the existing entry, andwhose Date is more recent than that of the existing entry, theexisting entry SHOULD NOT be returned in response to future requestsand SHOULD be deleted from the cache.13.7 Shared and Non-Shared CachesFor reasons of security and privacy, it is necessary to make adistinction between "shared" and "non-shared" caches. A non-sharedcache is one that is accessible only to a single user. Accessibilityin this case SHOULD be enforced by appropriate security mechanisms.All other caches are considered to be "shared." Other sections ofFielding, et al. Standards Track [Page 96]RFC 2616 HTTP/1.1 June 1999this specification place certain constraints on the operation ofshared caches in order to prevent loss of privacy or failure ofaccess controls.13.8 Errors or Incomplete Response Cache BehaviorA cache that receives an incomplete response (for example, with fewerbytes of data than specified in a Content-Length header) MAY storethe response. However, the cache MUST treat this as a partialresponse. Partial responses MAY be combined as described in section13.5.4; the result might be a full response or might still bepartial. A cache MUST NOT return a partial response to a clientwithout explicitly marking it as such, using the 206 (PartialContent) status code. A cache MUST NOT return a partial responseusing a status code of 200 (OK).If a cache receives a 5xx response while attempting to revalidate anentry, it MAY either forward this response to the requesting client,or act as if the server failed to respond. In the latter case, it MAYreturn a previously received response unless the cached entryincludes the "must-revalidate" cache-control directive (see section14.9).