Use of server-driven content negotiation (section 12), as indicated by the presence of a Vary header field in a response, alters the conditions and procedure by which a cache can use the response for subsequent requests.
A server MUST use the Vary header field (section 14.43) to inform a cache of what header field dimensions are used to select among multiple representations of a cachable response. A cache may use the selected representation (the entity included with that particular response) for replying to subsequent requests on that resource only when the subsequent requests have the same or equivalent values for all header fields specified in the Vary response-header. Requests with a different value for one or more of those header fields would be forwarded toward the origin server.
If an entity tag was assigned to the representation, the forwarded request SHOULD be conditional and include the entity tags in an If- None-Match header field from all its cache entries for the Request- URI. This conveys to the server the set of entities currently held by the cache, so that if any one of these entities matches the requested entity, the server can use the ETag header in its 304 (Not Modified) response to tell the cache which entry is appropriate. If the entity-tag of the new response matches that of an existing entry, the new response SHOULD be used to update the header fields of the existing entry, and the result MUST be returned to the client.
The Vary header field may also inform the cache that the representation was selected using criteria not limited to the request-headers; in this case, a cache MUST NOT use the response in a reply to a subsequent request unless the cache relays the new request to the origin server in a conditional request and the server responds with 304 (Not Modified), including an entity tag or Content-Location that indicates which entity should be used.
If any of the existing cache entries contains only partial content for the associated entity, its entity-tag SHOULD NOT be included in the If-None-Match header unless the request is for a range that would be fully satisfied by that entry.
If a cache receives a successful response whose Content-Location field matches that of an existing cache entry for the same Request- URI, whose entity-tag differs from that of the existing entry, and whose Date is more recent than that of the existing entry, the existing entry SHOULD NOT be returned in response to future requests, and should be deleted from the cache.