 HTTP response delay

In the HTTP current age calculation, HTTP response delay is the interval between the cache's request time and the time it receives the corresponding HTTP response:The interval corrects the age information carried in the response for time spent obtaining it. It is one input to cache-age estimation.^[ref]

For an own example, a cache initiates a request at local time $1000.0$ seconds and receives the response at $1003.5$. Its response delay is $3.5$ seconds. If the response carries Age: 12, the RFC's delay-corrected value isAfter another four seconds of local residence, using that corrected initial age would give $19.5$ seconds of current age.^[ref]

This interval does not isolate server computation. Transport, waiting and response handling can all contribute; a single elapsed interval does not reveal their separate amounts. Likewise, the received Date value comes from response metadata rather than defining either local endpoint of this subtraction.

The RFC permits the corrected age value as the corrected initial age. It also gives a conservative alternative that takes the maximum of that value and HTTP apparent age, particularly for paths involving old caches that might not supply appropriate Age information. The example uses the delay-corrected route explicitly instead of silently assuming both routes always agree.^[ref]

A correct elapsed-time calculation matters because HTTP freshness compares the resulting age with an HTTP freshness lifetime. Treating a received response as age zero simply because it has just reached the local cache would discard both the received age estimate and the acquisition interval.