In such a situation, in a set time period (may be called a “unit time period” or a “measurement time period”), the FW #4 collects information of packets discarded in accordance with a flow entry (may be abbreviated to “discarded-packet information”) for each of the flows A and B.

The discarded-packet information may exemplarily include header information of the discarded packets. The header information may include a transmission source address, a service identifier, and a destination address. The transmission source address, the service identifier, and the destination address are examples of parameters able to be set in a packet header.

The service identifier is an example of a parameter able to identify the service type of communication. The service identifier may be exemplarily expressed by a combination of the protocol type of packet and a port number thereof. In this regard, however, the service identifier may be replaced with another parameter.

While the transmission source address and the destination address may be exemplarily IP addresses, the transmission source address and the destination address may be addresses based on another communication protocol.

The discarded-packet information may be exemplarily totalized for each of packets each having an identical transmission source address, an identical destination address, and an identical service identifier (exemplarily includes a protocol type and a port number). FIG. 9 illustrates an example of a totalization result of the discarded-packet information.