Abstract: The asynchronous transfer mode （ATM） forum has chosen rate based control as the flow control scheme for available bit rate（ABR） service. Through the study of rate based and credit based traffic control, A cell discarding traffic control principle is proposed, which is the dynamic rate adjusting scheme based on counting mechanism of credit control. The principle controls the source rate by setting up high/low threshold of buffer and modulating the increase/decrease factor of rate, and at the same time, it uses the cell discarding mechanism to protect the switch from congestion efficiently. Simulation results show that this kind of scheme not only can give a guarantee of buffer utilization, but also can enhance the performance in cell loss, etc.