Normalized Queueing Delay: Congestion Control Jointly Utilizing Delay and Marking

Depending upon the type of feedback that is primarily used as a congestion measure, congestion control methods can be generally classified into two categories: marking/loss-based or delay-based. While both marking and queueing delay provide information about the congestion state of a network, they have been largely treated with separate control strategies. In this paper, we propose the notion of the normalized queueing delay, which serves as a congestion measure by combining both delay and marking information. Utilizing normalized queueing delay (NQD), we propose an approach to congestion control that allows a source to scale its rate dynamically to prevailing network conditions through the use of a time-variant set-point. In ns-2 simulation studies, an NQD-enabled FAST TCP demonstrates a significant link utilization improvement over FAST TCP under certain conditions. In addition, we propose another NQD-based controller ${\rm D}+{\rm M}$ TCP (Delay+Marking TCP) that achieves quick convergence to fair and stable rates with nearly full link utilization. Therefore, NQD is a suitable candidate as a congestion measure for practical congestion control.