The interrelation of TCP responsiveness and smoothness in heterogeneous networks

TCP(α,β) protocols trade the congestion window increase value α for the decrease ratio β, to generate smoother traffic patterns and to maintain a friendly behavior. We study the design assumptions of TCP(α,β) protocols and discuss the impact of equation-based modulation of α and β 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 or 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, 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 α dictates a conservative behavior that degrades the protocol performance with both delay-tolerant and delay-sensitive applications; and in the context of high contention of heterogeneous flows, a low α does not contribute to efficiency and friendliness.