A method is described for controlling the conformance of a data flow in a communication system to a traffic definition. The communication system comprises a transmitter and a receiver and data packets transporting the data flow are sent from the transmitter to the receiver. The receiver sends responses identifying received data packets to the transmitter, and the transmitter comprises a control function which controls the number of sent data packets for which no response is received according to a limit for the number of outstanding data packets. A conditioning criterion is obtained for the data flow, the conditioning criterion being associated with the traffic definition. The receiver determines a conformance parameter for the received data packets transporting the data flow. The limit and/or the sending of the responses is modified according to the conditioning criterion and the determined conformance parameter for controlling the number of transmitted data packets.