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

Accept-Ranges: bytesbut are not required to do so. Clients MAY generate byte-rangerequests without having received this header for the resourceinvolved. Range units are defined in section 3.12.Servers that do not accept any kind of range request for aresource MAY sendAccept-Ranges: noneto advise the client not to attempt a range request.Fielding, et al. Standards Track [Page 105]RFC 2616 HTTP/1.1 June 199914.6 AgeThe Age response-header field conveys the sender's estimate of theamount of time since the response (or its revalidation) wasgenerated at the origin server. A cached response is "fresh" ifits age does not exceed its freshness lifetime. Age values arecalculated as specified in section 13.2.3.Age = "Age" ":" age-valueage-value = delta-secondsAge values are non-negative decimal integers, representing time inseconds.If a cache receives a value larger than the largest positiveinteger it can represent, or if any of its age calculationsoverflows, it MUST transmit an Age header with a value of2147483648 (2^31). An HTTP/1.1 server that includes a cache MUSTinclude an Age header field in every response generated from itsown cache. Caches SHOULD use an arithmetic type of at least 31bits of range.14.7 AllowThe Allow entity-header field lists the set of methods supportedby the resource identified by the Request-URI. The purpose of thisfield is strictly to inform the recipient of valid methodsassociated with the resource. An Allow header field MUST bepresent in a 405 (Method Not Allowed) response.