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

- Restrictions on what may be stored by a cache; these may beimposed by either the origin server or the user agent.- Modifications of the basic expiration mechanism; these may beimposed by either the origin server or the user agent.- Controls over cache revalidation and reload; these may only beimposed by a user agent.- Control over transformation of entities.- Extensions to the caching system.14.9.1 What is CacheableBy default, a response is cacheable if the requirements of therequest method, request header fields, and the response statusindicate that it is cacheable. Section 13.4 summarizes these defaultsfor cacheability. The following Cache-Control response directivesallow an origin server to override the default cacheability of aresponse:publicIndicates that the response MAY be cached by any cache, even if itwould normally be non-cacheable or cacheable only within a nonsharedcache. (See also Authorization, section 14.8, foradditional details.)privateIndicates that all or part of the response message is intended fora single user and MUST NOT be cached by a shared cache. Thisallows an origin server to state that the specified parts of theFielding, et al. Standards Track [Page 109]RFC 2616 HTTP/1.1 June 1999response are intended for only one user and are not a validresponse for requests by other users. A private (non-shared) cacheMAY cache the response.Note: This usage of the word private only controls where theresponse may be cached, and cannot ensure the privacy of themessage content.no-cache