A covert channel is established between a network service and one or more service monitors in a service group. The covert channel minimizes overhead by providing an indication of the status of the service through use of operating system utilities rather than conventional remote procedure calls (RPCs)...http://www.google.de/patents/US6412001?utm_source=gb-gplus-sharePatent US6412001 - Method to monitor and control server applications using low cost covert channels
