a proxy cache maintains a copy of multiple resources from various servers in a network . when the proxy cache must generate a validation request for at least one resource at one of the servers , the proxy cache piggybacks one or more additional cache validation requests related to documents presently stored in the cache but originating from or associated with the server in question . upon receipt of an indication of the freshness or validity of the cached copy of the document , the proxy cache can then make a determination as to whether to request an update of the document .