Accuracy and scalability of ns-2's distributed emulation extension

摘要

Ns-2 is a well known network simulator, recently extended with improvements to its emulation facility. Real-time constraints and the boundary between real-world and simulated entities impose scalability and accuracy limitations, and distort the simulated network as perceived by the involved real-world applications. This paper presents results from a performance evaluation of the ns-2 emulation facility. By carrying out emulation experiments at different scales and in different scenarios, we identify scalability limitations and quantify the emulation accuracy. We find throughput limits using high- and low-end computers, and a significant throughput decrease when increasing the number of involved real-world applications. Furthermore, we show how the end-to-end delay increases with both the traffic load and the number of real-world applications. In experiments with many applications, we also uncover significant variations in throughput and end-to-end delay among flows. Finally, our jitter measurements indicate how kernel scheduling causing bursty traffic transmission is responsible for most jitter. This effect is most often relatively modest, yet somewhat variable for experiments including several applications. Based on analysis of our results, we propose a set of modifications of the system that improves the performance. Our implementation of one of these modifications are demonstrated to increase throughput significantly while reducing the variation among flows. Implementation and evaluation of the remaining modifications is reserved for future work.