Analysis and design of controllers for AQM routers supporting TCP flows

Abstract

In active queue management (AQM), core routers signal transmission control protocol (TCP) sources with the objective of managing queue utilization and delay. It is essentially a feedback control problem. Based on a recently developed dynamic model of TCPs congestion-avoidance mode, this paper does three things. First, it relates key network parameters such as the number of TCP sessions, link capacity and round-trip time to the underlying feedback control problem. Second, it analyzes the present de facto AQM standard: random early detection (RED) and determines that REDs queue-averaging is not beneficial. Finally, it recommends alternative AQM schemes which amount to classical proportional and proportional-integral control. We illustrate our results using ns simulations and demonstrate the practical impact of proportional-integral control on managing queue utilization and delay.

The use of a non-integer order PI controller with an active queue management mechanism

“…As has been discussed in many papers (e.g., Misra et al, 2000;Hollot et al, 2002), this feedback control system may be modeled and the model used to investigate the system stability with the use of control theory. The most common model is based on fluid flow approximation and is presented more formally in the next section.…”

“…Hollot et al (2002) focused on the design of a proportional-integral controller on low-frequency dynamics.…”

“…The H ∞ controller was designed for the original linear system proposed by Hollot et al (2002) but without neglecting high-frequency dynamics that ensure robust stability and good performance for a wider range of network parameters. The AQM scheme proposed by Quet and Ozbay (2004) performs better than RED by obtaining shorter transients and less oscillatory responses, which gives higher link utilization, a low packet loss rate and smaller queue fluctuations.…”

“…The PI AQM controller proposed by Hollot et al (2001b;2002) was designed following the small-gain theorem.…”

“…We also compare its performance with the RED mechanism, usually implemented in IP routers. The performance of TCP closed loop control is investigated with the use of fluid flow approximation, as proposed earlier (Misra et al, 2000;Hollot et al, 2002) and summarized in Section 2.1. The basic definition and properties of the theory of fractional calculus are summarized in Section 2.2.…”