Information protocol of network devices for an open network system, such as a UPnP-based home network) is provided. When a device receives a state information setting action for an internal service, the device extracts an input argument from the received state information setting action, and determines whether to set state variables of the internal service to values carried in state information included in the input argument, based on source information of the state information (device identification information (UDN), service identification information, and service type information) included in the input argument. This prevents malfunction of a device or service, which may be caused by setting state information whose values are invalid for the device or service.