Efficiency of TCP-friendly window adjustment strategies in wired/wireless networks

TCP(a,b) protocols parameterize the congestion window increase value and decrease ratio , motivated by the QoS requirements of multimedia applications for smooth rate adjustments. Based on a projection of the expected throughput of standard TCP(1,.5), modified versions of the protocol have been designed to generate smoother traffic patterns and to maintain a friendly behavior. In this paper, we study the design assumptions of TCP(a,b) and we discuss the impact of equation-based modulation of a and b on application efficiency. We confirm experimentally that, in general, smoothness and responsiveness constitute a tradeoff; however, we uncover undesirable dynamics of the protocols when the network is heterogeneous (wired/wireless) or the flow characteristics do not follow a prescribed and static behavior. For example, we show that smooth backward adjustments confine the protocol's capability to exploit resources that become available rapidly after a handoff in mobile network, and embarrass the fair and efficient growth of incoming flows. Furthermore, we show that in the context of wireless networks with high error rate, a low a dictates a conservative behavior that degrades the protocol performance with both best-effort and real-time applications; and in the context of high contention of heterogeneous flows, a low a does not contribute to efficiency and friendliness.