Fractional Exponent Coupling of RIO

The IETF Differentiated Service (DiffServ) architecture can allow for establishing a modern large scale network which guarantees the quality of service. In order to realize the multiple levels of packet drop precedence required for the Assured Forwarding (AF) framework of DiffServ, a multi-level RED algorithm is needed. RIO (RED with In/Out) is suitable for the AF scheme, and two major RIO variants such as RIO-D and RIO-C have been proposed and widely used. Both use the average lengths of virtual queues to determine the multiple levels of drop precedence and have different coupling strengths between virtual queues. The key difference between RIO-C and RIO-D lies with the fact that RIO-C takes full coupling and RIO-D has zero coupling. In this paper, a novel algorithm called RIO-FEC (RIO based on Fractional Exponent Coupling for determining the coupling strength) is proposed, and can achieve partial coupling with a controllable coupling power for calculating each average virtual queue length. The effects of the fractional exponent based coupling on the drop rates and throughputs of color-labeled virtual queues are analyzed, and a fractional power 1/3 is found to be optimal. The queue weight factor for calculating the EWMAs (Exponential Weighted Moving Averages) of virtual queue lengths, thus affecting the drop probability, is also presented. The results show that the proposed algorithm can achieve a total drop rate and a total throughput as good as RIO-D. Furthermore, RIO-FEC outperforms RIO-C and RIO-D in terms of preventing the lowest-priority virtual queue from bandwidth starvation, and can effectively adjudge the coupling strengths to gain the desired control on the allocation of priority and fairness.