A method using a communication system including a first information processing device, a first communication device configured to relay packet communication between a network and the first information processing device, a second information processing device, a second communication device configured to relay packet communication between the network and the second information processing device, and a control device configured to control the first communication device and the second communication device, the method comprising: transmitting, from the first information processing device to the network via the first communication device, a first packet which belongs to a first flow, a destination node of the first packet being the second information processing device; receiving, by the second communication device, the first packet; discarding, by the second communication device, the first packet based on a first rule stipulating that a packet belonging to the first flow is to be discarded; sending, by the second communication device, first header information of the first packet to the control device; identifying, by the control device based on the first header information, the first information processing device which is a transmission source node of the first packet; transmitting, by the control device, to the network, a second packet of which a destination node is the identified first information processing device; receiving, by the first communication device, the second packet; based on the receiving of the second packet, setting (607, 608), by the control device, the first rule to the first communication device; after the setting of the first rule to the first communication device, transmitting, from the first information processing device, a third packet which belongs to the first flow; receiving, by the first communication device, the third packet; and discarding, by the first communication device, the third packet based on the first rule. transmitting, from the first information processing device to the network via the first communication device, a first packet which belongs to a first flow, a destination node of the first packet being the second information processing device; receiving, by the second communication device, the first packet; discarding, by the second communication device, the first packet based on a first rule stipulating that a packet belonging to the first flow is to be discarded; sending, by the second communication device, first header information of the first packet to the control device; identifying, by the control device based on the first header information, the first information processing device which is a transmission source node of the first packet; transmitting, by the control device, to the network, a second packet of which a destination node is the identified first information processing device; receiving, by the first communication device, the second packet; based on the receiving of the second packet, setting (607, 608), by the control device, the first rule to the first communication device; after the setting of the first rule to the first communication device, transmitting, from the first information processing device, a third packet which belongs to the first flow; receiving, by the first communication device, the third packet; and discarding, by the first communication device, the third packet based on the first rule.