Deficit Round Robin with Limited Deficit Savings (DRR-LDS) for Fairness Among TCP Users

Abstract

Deficit Round Robin (DRR) is a simple and computationally efficient approximation of the Weighted Fair Queueing (WFQ) scheduling discipline. Its intention is to share resources among several queues, e.g., flows or users, according to given weights. However, when users hold different numbers of TCP connections with saturated sources, the throughput among these users may differ significantly. In this work, we quantify the difference in throughput for heavy and light users with saturated TCP flows for equal weights and for two different buffer management strategies. The difference is large if low queueing delay for packets is enforced through shallow buffers on the bottleneck link. To address this shortcoming, we propose limited deficit savings (LDS), a modification of the DRR scheduler, which can be combined with different buffer management schemes. We show that LDS reduces unequal throughput for heavy and light users with saturated TCP flows. Moreover, we illustrate that LDS clearly decreases download times for data chunks of moderate size in the presence of high background load.