Response HeaderExpires
Beispiel:
Expires: Thu, 01 Dec 1994 16:00:00 GMT
Original Beschreibung (engl.):
The Expires entity-header field gives the date/time after which the
response is considered stale. A stale cache entry may not normally be
returned by a cache (either a proxy cache or a user agent cache)
unless it is first validated with the origin server (or with an
intermediate cache that has a fresh copy of the entity). See section
13.2 for further discussion of the expiration model.
The presence of an Expires field does not imply that the original
resource will change or cease to exist at, before, or after that
time.
The format is an absolute date and time as defined by HTTP-date in
section 3.3.1; it MUST be in RFC 1123 date format:
Expires = "Expires" ":" HTTP-date
An example of its use is
Expires: Thu, 01 Dec 1994 16:00:00 GMT
Note: if a response includes a Cache-Control field with the max-
age directive (see section 14.9.3), that directive overrides the
Expires field.
HTTP/1.1 clients and caches MUST treat other invalid date formats,
especially including the value "0", as in the past (i.e., "already
expired").
To mark a response as "already expired," an origin server sends an
Expires date that is equal to the Date header value. (See the rules
for expiration calculations in section 13.2.4.)
To mark a response as "never expires," an origin server sends an
Expires date approximately one year from the time the response is
sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one
year in the future.
The presence of an Expires header field with a date value of some
time in the future on a response that otherwise would by default be
non-cacheable indicates that the response is cacheable, unless
indicated otherwise by a Cache-Control header field (section 14.9).
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