In-vehicle communication system and control method of in-vehicle communication system

A control method of an in-vehicle communication system including a plurality of control devices, wherein

• the plurality of control devices includes a upper-level control device that transmits a communication frame including a control message, a middle-level control device that relays the communication frame transmitted by the upper-level control device, and a lower-level control device that receives the communication frame relayed by the middle-level control device and performs a control process based on the control message included in the received communication frame, and
• one or more communication lines are connected between the upper-level control device, the middle-level control device and the lower-level control device so that at least two or more communication paths are present for the communication frame including the control message transmitted from the upper-level control device to reach the lower-level control device,
• the control method comprising:
• the upper-level control device repeatedly transmitting the communication frame including the control message to the at least two or more communication paths over time, wherein each communication frame includes identification information used to identify an ordinal of the communication frame in a transmitting order;
• the lower-level control device determining based on the identification information of the received communication frame whether or not the ordinal of the received communication frame in the transmitting order is the same as or older than that of the previously received communication frame;
• and
• the lower-level control device performing processes such that:
• upon determining that the ordinal of the received communication frame in the transmitting order is not the same as nor older than that of the previously received communication frame but is newer than that of the previously received communication frame, the lower-level control device executes the control process based on the control message included in the received communication frame;
• and
• upon determining that the ordinal of the received communication frame in the transmitting order is the same as or older than that of the previously received communication frame, the lower-level control device does not execute the control process based on the control message included in the received communication frame.