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Cache Control Extensions

The Cache-Control header field can be extended through the use of one or more cache-extension tokens, each with an optional value. A cache MUST ignore unrecognized cache directives.

Informational extensions (those that do not require a change in cache behavior) can be added without changing the semantics of other directives.

Behavioral extensions are designed to work by acting as modifiers to the existing base of cache directives. Both the new directive and the standard directive are supplied, such that applications that do not understand the new directive will default to the behavior specified by the standard directive, and those that understand the new directive will recognize it as modifying the requirements associated with the standard directive. In this way, extensions to the Cache-Control directives can be made without breaking deployed caches.

For example, consider a hypothetical new response directive called "community" that acts as a modifier to the private directive: in addition to private caches, any cache that is shared only by members of the named community is allowed to cache the response. An origin server wishing to allow the UCI community to use an otherwise private response in their shared cache(s) could do so by including

Cache-Control: private, community="UCI"

A cache that recognizes such a community cache-extension could broaden its behavior in accordance with that extension. A cache that does not recognize the community cache-extension would ignore it and adhere to the private directive.

Cache-Control: private, community="UCI"
Cache-Control: private, community="UCI"