In order to know if a cached entry is fresh, a cache needs to know if its age exceeds its freshness lifetime. We discuss how to calculate the latter in section 13.2.4; this section describes how to calculate the age of a response or cache entry.
In this discussion, we use the term "now" to mean "the current value of the clock at the host performing the calculation." Hosts that use HTTP, but especially hosts running origin servers and caches, should use NTP  or some similar protocol to synchronize their clocks to a globally accurate time standard.
Also note that HTTP/1.1 requires origin servers to send a Date header with every response, giving the time at which the response was generated. We use the term "date_value" to denote the value of the Date header, in a form appropriate for arithmetic operations.
HTTP/1.1 uses the Age response-header to help convey age information between caches. The Age header value is the sender's estimate of the amount of time since the response was generated at the origin server. In the case of a cached response that has been revalidated with the origin server, the Age value is based on the time of revalidation, not of the original response.
In essence, the Age value is the sum of the time that the response has been resident in each of the caches along the path from the origin server, plus the amount of time it has been in transit along network paths.
We use the term "age_value" to denote the value of the Age header, in a form appropriate for arithmetic operations.
A response's age can be calculated in two entirely independent ways:
Given that we have two independent ways to compute the age of a response when it is received, we can combine these as
corrected_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.
Note that this correction is applied at each HTTP/1.1 cache along the path, so that if there is an HTTP/1.0 cache in the path, the correct received age is computed as long as the receiving cache's clock is nearly in sync. We don't need end-to-end clock synchronization (although it is good to have), and there is no explicit clock synchronization step.
Because of network-imposed delays, some significant interval may pass from the time that a server generates a response and the time it 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 have been initiated prior to that Age value's generation, we can correct for delays imposed by the network by recording the time at which the request was initiated. Then, when an Age value is received, it MUST be interpreted relative to the time the request was initiated, not the time that the response was received. This algorithm results in conservative behavior no matter how much delay is experienced. So, we compute:
corrected_initial_age = corrected_received_age + (now - request_time)
where "request_time" is the time (according to the local clock) when the request that elicited this response was sent.
Summary of age calculation algorithm, when a cache receives a response:
/* * age_value * is the value of Age: header received by the cache with * this response. * date_value * is the value of the origin server's Date: header * request_time * is the (local) time when the cache made the request * that resulted in this cached response * response_time * is the (local) time when the cache received the * response * now * is the current (local) time */ apparent_age = max(0, response_time - date_value); corrected_received_age = max(apparent_age, age_value); response_delay = response_time - request_time; corrected_initial_age = corrected_received_age + response_delay; resident_time = now - response_time; current_age = corrected_initial_age + resident_time;
When a cache sends a response, it must add to the corrected_initial_age the amount of time that the response was resident locally. It must then transmit this total age, using the Age header, to the next recipient cache.