Network Working Group R. Fielding Request for Comments: 2616 ...

Fielding, et al. Standards Track [Page 82]RFC 2616 HTTP/1.1 June 1999response is first-hand unless all caches along the request path arecompliant with HTTP/1.1 (i.e., older HTTP caches did not implementthe Age header field).13.2.4 Expiration CalculationsIn order to decide whether a response is fresh or stale, we need tocompare its freshness lifetime to its age. The age is calculated asdescribed in section 13.2.3; this section describes how to calculatethe freshness lifetime, and to determine if a response has expired.In the discussion below, the values can be represented in any formappropriate for arithmetic operations.We use the term "expires_value" to denote the value of the Expiresheader. We use the term "max_age_value" to denote an appropriatevalue of the number of seconds carried by the "max-age" directive ofthe Cache-Control header in a response (see section 14.9.3).The max-age directive takes priority over Expires, so if max-age ispresent in a response, the calculation is simply:freshness_lifetime = max_age_valueOtherwise, if Expires is present in the response, the calculation is:freshness_lifetime = expires_value - date_valueNote that neither of these calculations is vulnerable to clock skew,since all of the information comes from the origin server.If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage (see section 14.9.3) appears in the response, and the responsedoes not include other restrictions on caching, the cache MAY computea freshness lifetime using a heuristic. The cache MUST attach Warning113 to any response whose age is more than 24 hours if such warninghas not already been added.Also, if the response does have a Last-Modified time, the heuristicexpiration value SHOULD be no more than some fraction of the intervalsince that time. A typical setting of this fraction might be 10%.The calculation to determine if a response has expired is quite