Performance of a Linux implementation of class based queueing

Class based queueing (CBQ) is a link-sharing and resource management mechanism for packet networks. We have ported the CBQ implementation on FreeBSD, also known as alternate queueing (ALTQ), to the Linux platform. This paper investigates the performance of CBQ in terms of latency, bandwidth guarantee and delay. CBQ is found to be able to preserve bandwidth allocated to the classes. However it does not scale well with the number of data flows as it incurs additional delay with each new data flow admitted. Due to this additional delay, the admission control cannot predict the delay experienced by the existing flows after admitting a new flow. Although CBQ is able to provide lower delay to higher priority flows, it also introduces jitter to the traffic owing to its packet scheduling artifact. As such, CBQ is not suitable for fine-grained scheduling of real-time traffic. It is more suited for providing bandwidth guarantee to aggregated data flows.