The Cache-Control general-header field is used to specify directives
that MUST be obeyed by all caching mechanisms along the
request/response chain. The directives specify behavior intended to
prevent caches from adversely interfering with the request or
response. These directives typically override the default caching
algorithms. Cache directives are unidirectional in that the presence
of a directive in a request does not imply that the same directive
should be given in the response.
Note that HTTP/1.0 caches may not implement Cache-Control and may
only implement Pragma: no-cache (see section 14.32).
Cache directives must be passed through by a proxy or gateway
application, regardless of their significance to that application,
since the directives may be applicable to all recipients along the
request/response chain. It is not possible to specify a cache-
directive for a specific cache.
When a directive appears without any 1#field-name parameter, the
directive applies to the entire request or response. When such a
directive appears with a 1#field-name parameter, it applies only to
the named field or fields, and not to the rest of the request or
response. This mechanism supports extensibility; implementations of
future versions of the HTTP protocol may apply these directives to
header fields not defined in HTTP/1.1.
The cache-control directives can be broken down into these general
Restrictions on what is cachable; these may only be imposed by the
Restrictions on what may be stored by a cache; these may be imposed
by either the origin server or the user agent.
Modifications of the basic expiration mechanism; these may be
imposed by either the origin server or the user agent.
Controls over cache revalidation and reload; these may only be
imposed by a user agent.