5.2. Age and Expiration of Cached Objects
HTTP also provides support for caches to decide when to check the validity of the cached objects. This support is based on a time to live(TTL) concept, which is similar to the concept with the same name that we already encountered in DNS caching (Section 4.2). The time to live of an object is the time during which the object is considered valid in a cache. An HTTP object that has been cached longer than its time to live is said to expire. When an HTTP request for an expired object arrives, the cache must validate the object by sending a conditional request to the server before serving the object from the cache. A related concept that plays an important role in HTTP 1.1 is the age of a cached object, which ...
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