An application of the control theoretic modeling for a scalable TCP ACK pacer

We present the design of a TCP ACK pacer which regulates the downstream buffer occupancy at an edge router in the Internet to avoid quality of service (QoS) damaging congestion while maximizing the link utilization. This technique has the advantage of requiring no changes in existing TCP implementations. Based on a feedback system representation of the network, a PI-type controller is proposed to determine the aggregate ACK releasing rate according to the buffer occupancy. This approach is scalable in that it requires no per-flow state information. The robustness of the controller is addressed with respect to the system uncertainty such as the traffic dynamics and the system delay. Generalizations of the control objective are discussed as well. A special case of the controller, corresponding to a proportional plus lag compensator is analyzed and used to provide a systematic approach for satisfying a given delay requirement. Numerical examples to illustrate this approach are also given.