A communication system includes: a server configured to transmit a series of data packets through a relay device and a transmitting device to receiving device; the receiving device configured to transmit a plurality of acknowledgement packets for the data packets; the transmitting device configured to provide, when executing link aggregation on a downlink flow including the data packets, a notification that represents that the downlink flow is subjected to the link aggregation; and the relay device configured to: execute a process of concealing a specific acknowledgement packet that is among the plurality of acknowledgement packets and included in an uplink flow corresponding to the downlink flow subjected to the link aggregation and represents that an order in which the data packets arrive at the receiving device is reversed, when receiving the notification from the transmitting device, and transmit the other acknowledgement packets remaining after the concealment process to the server.