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

with every response, giving the time at which the response wasgenerated (see section 14.18). We use the term "date_value" to denotethe value of the Date header, in a form appropriate for arithmeticoperations.HTTP/1.1 uses the Age response-header to convey the estimated age ofthe response message when obtained from a cache. The Age field valueis the cache's estimate of the amount of time since the response wasgenerated or revalidated by the origin server.In essence, the Age value is the sum of the time that the responsehas been resident in each of the caches along the path from theorigin server, plus the amount of time it has been in transit alongnetwork paths.We use the term "age_value" to denote the value of the Age header, ina form appropriate for arithmetic operations.A response's age can be calculated in two entirely independent ways:1. now minus date_value, if the local clock is reasonably wellsynchronized to the origin server's clock. If the result isnegative, the result is replaced by zero.2. age_value, if all of the caches along the response pathimplement HTTP/1.1.Given that we have two independent ways to compute the age of aresponse when it is received, we can combine these ascorrected_received_age = max(now - date_value, age_value)and as long as we have either nearly synchronized clocks or all-HTTP/1.1 paths, one gets a reliable (conservative) result.Because of network-imposed delays, some significant interval mightpass between the time that a server generates a response and the timeit is received at the next outbound cache or client. If uncorrected,this delay could result in improperly low ages.Because the request that resulted in the returned Age value must havebeen initiated prior to that Age value's generation, we can correctfor delays imposed by the network by recording the time at which therequest was initiated. Then, when an Age value is received, it MUSTbe interpreted relative to the time the request was initiated, notFielding, et al. Standards Track [Page 81]