A Near-Optimal Packet Scheduler for QoS Networks

A packet scheduler in a quality-of-service (QoS) network should be sophisticated enough to support stringent QoS constraints at high loads, but it must also have a simple implementation so that packets can be processed at the speed of the transmission link. The Earliest-DeadlineFirst (EDF) scheduler is the optimal scheduler for bounded-delay services in the sense that it provides the tightest delay guarantees of any scheduler, but an implementation of EDF requires the sorting of packets, a complex operation that is not practical for high-speed networks. In this study we present the design, implementation, and analysis of the novel Rotating-PriorityQueues (RPQ) scheduler that is near-optimal in the sense that it can approximate EDF with arbitrary precision. The RPQ scheduler uses a set of prioritized FIFO queues whose priorities are rearranged (rotated) periodically to increase the priority of waiting packets. We show that RPQ has the following desirable properties: its implementation requires operations independent of the number of queued packets, it can provide worst-case delay guarantees, and it is always superior to a Static-Priority (SP) scheduler. For shared-memory architectures, we show that RPQ can be implemented with little computational overhead. We derive expressions for the worst-case delays in an RPQ scheduler and demonstrate that the achievable network utilization increases with the frequency of queue rotations, approaching that of EDF in the limit. We use numerical examples, including examples based on MPEG video, to show that in realistic scenarios RPQ can closely approximate EDF even for infrequent queue rotations.