Monitoring device, communication system, communication control method, and monitoring program

A communication system including a plurality of terminal devices that establish a connection based on TCP, and a monitoring device that monitors communications of the plurality of terminal devices, the communication system comprising:

• a request detection unit that detects a connection establishment request that a terminal device becoming a client sends to another terminal device becoming a server;
• an available bandwidth calculation unit that calculates an available bandwidth by subtracting a current amount of traffic from an allowable communication capacity, for each communication path identified by the connection establishment request detected by the request detection unit;
• and
• a setting command unit that, when the available bandwidth calculated by the available bandwidth calculation unit is smaller than a congestion window, transmits a command that sets the client to lower a slow start threshold to a predetermined value, or a command that sets the server to lower a window size to a predetermined value, regardless of whether or not a timeout occurs.

A communication control method for controlling communications of a plurality of terminal devices that establish a connection based on TCP, the communication control method comprising:

• a request detection step of detecting a connection establishment request that a terminal device becoming a client sends to another terminal device becoming a server;
• an available bandwidth calculation step of calculating an available bandwidth by subtracting a current amount of traffic from an allowable communication capacity, for each communication path identified by the detected connection establishment request;
• and
• a setting command step of, when the calculated available bandwidth is smaller than a congestion window, transmitting a command that sets the client to lower a slow start threshold to a predetermined value, or a command that sets the server to lower a window size to a predetermined value, regardless of whether or not a timeout occurs.