Cache-Control官方文档

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]

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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.