if-modified-since (ims)

A request sent by a cache to the origin server, used to check the freshness of a cached web object that appears stale, for example, if the object has an explicit expiration date. The cache sends the request to the origin server, then the server examines the time and date stamp of the object, then compares that to information provided by the cache. If the time and date stamps are the same, then the object is considered fresh and can be served from the cache. If the time and date stamps differ, then the object is considered stale and the server sends a new version of the content to the cache, which then forwards it to the client.