Enhanced weighted round robin schedulers for accurate bandwidth distribution in packet networks

Abstract

Weighted round robin (WRR) schedulers constitute a popular solution for differentiating the bandwidth guarantees of heterogeneous IP flows, mostly because of their minimal implementation cost. However, the existing WRR schedulers are not sufficient to satisfy all the requirements of emerging quality-of-service frameworks. Flexible bandwidth management at the network nodes requires the deployment of hierarchical scheduling structures, where bandwidth can be allocated not only to individual flows, but also to aggregations of those flows. With currently available WRR schedulers, the superimposition of a hierarchical structure compromises the simplicity of the basic scheduler. WRR schedulers are also known for their burstiness in distributing service, which exposes the scheduled flows to higher packet-loss probability at downstream nodes. By construction, WRR schedulers distribute bandwidth proportionally to the service shares allocated to the individual flows. For best-effort (BE) flows, having no specified bandwidth requirements, existing WRR schedulers typically allocate arbitrary service shares. This approach conflicts with the intrinsic nature of BE flows and reduces the availability of bandwidth for the allocation of guaranteed-bandwidth (GB) flows. We present three enhancements for WRR schedulers that solve these problems. In the first enhancement, we superimpose a “soft” scheduling layer on the basic WRR scheduler by simply redefining the computation of the flow timestamps. The second enhancement substantially reduces the service burstiness of the WRR scheduler with only marginal impact on its implementation cost. Finally, the third enhancement allows the smooth integration of GB and BE flows, with efficient management of the available bandwidth and total compliance with the nature of BE flows.