https://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-25#page-21

 

. Cache-Control

The "Cache-Control" header field is used to specify directives for   caches along the request/response chain.  Such cache directives are   unidirectional in that the presence of a directive in a request does   not imply that the same directive is to be given in the response.   A cache MUST obey the requirements of the Cache-Control directives   defined in this section.  See  for information about how   Cache-Control directives defined elsewhere are handled.      Note: Some HTTP/1.0 caches might not implement Cache-Control.   A proxy, whether or not it implements a cache, MUST pass cache   directives through in forwarded messages, regardless of their   significance to that application, since the directives might be   applicable to all recipients along the request/response chain.  It is   not possible to target a directive to a specific cache.   Cache directives are identified by a token, to be compared case-   insensitively, and have an optional argument, that can use both token   and quoted-string syntax.  For the directives defined below that   define arguments, recipients ought to accept both forms, even if one   is documented to be preferred.  For any directive not defined by this   specification, a recipient MUST accept both forms.     Cache-Control   = 1#cache-directive     cache-directive = token [ "=" ( token / quoted-string ) ]   For the cache directives defined below, no argument is defined (nor   allowed) unless stated otherwise.

. Request Cache-Control Directives

. max-age

Argument syntax:      delta-seconds (see )   The "max-age" request directive indicates that the client is   unwilling to accept a response whose age is greater than the   specified number of seconds.  Unless the max-stale request directive   is also present, the client is not willing to accept a stale   response.   This directive uses the token form of the argument syntax; e.g.,Fielding, et al.          Expires May 21, 2014                 [Page 21]

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Internet-Draft              HTTP/1.1 Caching               November 2013   'max-age=5', not 'max-age="5"'.  A sender SHOULD NOT generate the   quoted-string form.