The "only-if-cached" request directive indicates that the client only
wishes to obtain a stored response. If it receives this directive, a
cache SHOULD either respond using a stored response that is
consistent with the other constraints of the request, or respond with
a 504 (Gateway Timeout) status code. If a group of caches is being
operated as a unified system with good internal connectivity, a
member cache MAY forward such a request within that group of caches.