Cache Control
A Cache-Control header with cache directives from a server or client. These directives set policy on what responses can be stored, and which requests can be satisfied by those stored responses.
See RFC 7234, 5.2.
Types
Properties
The duration past the response's served date that it can be served without validation.
This field's name "only-if-cached" is misleading. It actually means "do not use the network". It is set by a client who only wants to make a request if it can be fully satisfied by the cache. Cached responses that would require validation (ie. conditional gets) are not permitted if this header is set.
The "s-maxage" directive is the max age for shared caches. Not to be confused with "max-age" for non-shared caches, As in Firefox and Chrome, this directive is not honored by this cache.