1. A cache manager for controlling distribution of content stored in caches in a cache domain in a packet data network, the cache manager comprising: an object monitor configured to receive notifications from caches in the cache domain about data objects requested at a respective cache; an object database configured to collate and store object information about the requested data objects, the object information including information about the caches at which the data objects have been requested and frequency with which the requests have been made; an object distributor configured to: classify each requested data object into one of at least three classes based on the object information, the classes including: a high popularity class comprising objects which should be distributed to all caches in the cache domain; a medium popularity class comprising objects which should be distributed to one or more subsets of the caches in the cache domain; and a low popularity class comprising objects which should not be distributed; identify data objects for distribution in accordance with the classification; and instruct the caches in the cache domain to distribute the identified data objects stored in those caches between themselves.