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

s-maxageIf a response includes an s-maxage directive, then for a sharedcache (but not for a private cache), the maximum age specified bythis directive overrides the maximum age specified by either themax-age directive or the Expires header. The s-maxage directivealso implies the semantics of the proxy-revalidate directive (seesection 14.9.4), i.e., that the shared cache must not use theentry after it becomes stale to respond to a subsequent requestwithout first revalidating it with the origin server. The s-maxage directive is always ignored by a private cache.Note that most older caches, not compliant with this specification,do not implement any cache-control directives. An origin serverwishing to use a cache-control directive that restricts, but does notprevent, caching by an HTTP/1.1-compliant cache MAY exploit therequirement that the max-age directive overrides the Expires header,and the fact that pre-HTTP/1.1-compliant caches do not observe themax-age directive.Other directives allow a user agent to modify the basic expirationmechanism. These directives MAY be specified on a request:max-ageIndicates that the client is willing to accept a response whoseage is no greater than the specified time in seconds. Unless maxstaledirective is also included, the client is not willing toaccept a stale response.min-freshIndicates that the client is willing to accept a response whosefreshness lifetime is no less than its current age plus thespecified time in seconds. That is, the client wants a responsethat will still be fresh for at least the specified number ofseconds.max-staleIndicates that the client is willing to accept a response that hasexceeded its expiration time. If max-stale is assigned a value,then the client is willing to accept a response that has exceededits expiration time by no more than the specified number ofseconds. If no value is assigned to max-stale, then the client iswilling to accept a stale response of any age.If a cache returns a stale response, either because of a max-staledirective on a request, or because the cache is configured tooverride the expiration time of a response, the cache MUST attach aWarning header to the stale response, using Warning 110 (Response isstale).