Design of robust AQM controllers for improved TCP Westwood congestion control

This paper is concerned with the application of a novel robust AQM controller recently presented in the literature to TCP Westwood congestion control. The aim is twofold. Firstly we assess the performance of the scheme when applied to networks where the traffic is generated by sources controlled by Westwood TCP. Secondly, we validate the controller through numerical simulations showing that it guarantees better performance in the presence of unwanted variations of the load, the round trip time and the link capacity when compared to other existing AQM control schemes. I. I NTRODUCTION Traditional TCP implements an end-to-end congestion control mechanism based on the fundamental assumption that routers implements a drop tail policy so that packet losses are due to congestion [15]. When a packet loss is detected by timeout or 3 duplicate acknowledgements the TCP congestion controller reacts by multiplicatively shrinking the congestion window, i. e. the sending rate. On the other hand, each time a packet is successfully acknowledged the TCP linearly increments the congestion window. Interesting mathematical models of the closed loop behaviour of the TCP have been proposed in literature [16]-[19]. These models provide differential stochastic equations that relate the packet drop probability with the congestion window dynamics. The basic idea of Active Queue Management is to actively throttle the packet drop rate in order to affect the TCP input rate and provide a nice behaviour of TCP flows, such as low queuing delays, or traffic differentiation, or control of misbehaving flows and so on. Since the first time that AQM was proposed in [20] as a simple Random Early Discard policy (RED), it was clear how difficult was to tune the AQM controller, which, as a matter of fact, delayed the employment in real routers. As a consequence, a consistent amount of literature has been devoted to the issue of designing controllers for actively throttling the packet drop rate [19],[21]. Beside the usual AQM controls, i.e. RED, PI, perform poorly in the presence of network parameter variations such as round-trip time, load variations and link capacity which are bound to occur in more realistic networks. Indeed in more realistic networks, the round-trip time can also vary with respect to its nominal value because of congestion phenomena and other effects such as the presence of responsive sources variations at different points in the network (causing the variation of equivalent round trip time propagation delay), rerouting, additional unresponsive source traffic. Also the assumption of fixed long-lived TCP workload, often taken in the derivation of these models, is also violated in realistic network scenarios. Moreover variations in the link capacity advertised from long-lived TCP flows can also occur because of unresponsive traffic generated by UDP and short-lived TCP flows. Thus, the ultimate aim of AQM control is to design a strategy taking explicitly into account the presence of round trip time delays in the network model and to make the system robust against network parameter uncertainties while guaranteeing acceptable performances in terms of margin stability, reduced number of packet losses, a more efficient queue utilization and better regulation, and shorter queuing delays. Recent robust control technics are applied to AQM with comfortable results in Simulink and NS-2 simulations [9], [10]. In this paper we consider TCP flows controlled by Westwood+ TCP, which is a new end-to-end sender based modification of standard TCP that employs the stream of returning acknowledgment packets to estimate the available bandwidth and set the congestion window after congestion [2],[3]. After developing a mathematical model of Westwood+ TCP, we propose a robust AQM control approach on this model able to cope with unwanted variations of characteristic parameters such as the average round-trip time, load and link capacity, just proposed in [9], [10] for Hollot’model [22]. In particular: (1) an appropriate robust control method for time-delay systems is proposed to cope with round-trip time, load variations and link capacity and hence used to design a suitable AQM control scheme; (2) A robust observer based on the mathematical model of the Westwood+ TCP source behaviour is used in the control loop to avoid direct measurement of the congestion window, as this would be unpractical in applications. Finally, the resulting control law is validated and tested through numerical simulations. II. TCP-WESTWOOD: AN OVERVIEW The basic idea of TCP Westwood is to exploit the stream of returning ACK packets in order to obtain an estimate of the connection available bandwidth (BWE). The bandwidth estimate is used when a congestion episode is detected to properly set thecwndandssthtresh. In the absence of congestion, the dynamics of these variables is conform to RFC2581 [1]. A pseudo-code of the Westwood TCP algorithm can be given as follows: When 3 DUPACKs are received: ssthresh = (BWE * RTTmin) / MSS; if (ssthresh<2) ssthresh=2; cwnd = ssthresh; When coarse timeout expires: ssthresh = (BWE * RTTmin) / MSS; if (ssthresh<2) ssthresh=2; cwnd = 1; When ACKs are successfully received: cwnd increases as stated in RFC2581. It is worth noting that the adaptive decrease mechanism employed by TCP Westwood improves the stability of the standard TCP multiplicative decrease. In fact, the adaptive window shrinking provides a congestion window that is decreased enough in the presence of heavy congestion and not too much in the presence of light congestion or losses not due to a real congestion episode, such as in the case of unreliable radio links. Moreover the setting cwnd = BWE ∗RTTmin injects a transmission rate equal to BWE ∗ RTTmin/RTT , which is less than bandwidth used at the time of congestion. As a consequence, a Westwood TCP flow clears out its path backlog after the setting thus leaving room in the routers’ buffers for coexisting flows. This obviously improves network stability, statistical multiplexing and fairness. TCP Westwood congestion control is heavily based on bandwidth estimation algorithm. Bandwidth estimation algorithm proposed in [2] overestimates the available bandwidth in the presence of ACK compression [3]. In order to avoid this undesirable behavior, Westwood+ TCP proposes a new bandwidth estimation algorithm that works properly also in the presence of ACK compression [3]. The new bandwidth estimation algorithm proposed along with Westwood+ TCP relies on bandwidth samples which are collected every RTT instead that every time an ACK is received by the sender. Basically, the Westwood+ TCP sender counts the amount of data Dk, which is acknowledged during the last RTT = Tk and then computes a bandwidth samples as Bk = Dk/Tk. Details concerning the ACK counting can be found in [2]. Briefly, a duplicate ACK counts for one delivered segments, a delayed ACK for two segments, whereas a cumulative ACK counts for 1 segment or for the number of segments exceeding those already accounted for by previous duplicate acknowledgments. Since congestion depends on low frequency components of available bandwidth [4], Bk samples are filtered using a discrete time low-pass filter, details of the filtering algorithm can be found in [3]. A. A Fluid flow Model for Westwood TCP This sub-section derives a fluid flow model of Westwood TCP by following the approach proposed in [5], [6]. We consider a packet switching network made of a single bottleneck with capacity C, shared byN Westwood TCP connections, all having the same round trip propadation delay Tp. In particular, under the assumption of neglecting the TCP timeout and the effect of the low-pass filter used to obtain the bandwidth estimation, the following set of nonlinear coupled ODEs is obtained: