A Near-Optimal Packet Scheduler for QoS Networks


Dallas E. Wrege
Jorg Liebeherr
Department of Computer Science
University of Virginia
Charlottesville, VA 22903

Abstract

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-Deadline-First (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-Priority-Queues$^+$ (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 derive admission control tests for RPQ$^+$ and show that it has the following desirable properties: its implementation requires operations independent of the number of queued packets, it can provide worst-case delay guarantees, and its efficiency is ``between'' that of EDF and Static-Priority (SP) schedulers. 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. %end italics mode }