https://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-25#page-21
5.2. 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 Section 5.2.3 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.
5.2.1. Request Cache-Control Directives
5.2.1.1. max-age
Argument syntax:
delta-seconds (see Section 1.2.1)
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]
Internet-Draft HTTP/1.1 Caching November 2013 ‘max-age=5‘, not ‘max-age="5"‘. A sender SHOULD NOT generate the quoted-string form.