Abstract: A system-on-chip comprising a sender unit and a receiver unit and a method of operating thereof are described. The sender unit comprises a send counter for providing a send counter value. The receiver unit comprises a receive counter for providing a receive counter value. The sender unit is arranged to increment the send counter value by an increment and send a data packet to the receiver unit. The receiver unit is arranged to receive the data packet and increment the receive counter value by the increment. In a challenge operation, the sender unit sends the send counter value to the receiver unit. In a challenge response operation, the receiver unit receives the send counter value, compares the values, and generates a comparison result. Loss of data packets from the sender unit as well as reception of unexpected data packets from a third functional unit may thus be detected.