Maximum delivery time and hot spots in ServerNet TM topologies

Abstract

This paper will center on analysis of the performance characteristics of ServerNet topologies, concentrating on the prediction, through simulation and statistical analysis, of the maximum two-way delivery time, the identification of congested links (hot spots) and tree saturation. ServerNet TM, developed by Tandem Computers Inc., is a wormhole-routed, packet-switched, point-to-point network, with special attention paid to reducing latency and to assuring reliability. ServerNet uses multiple high-speed, low-cost routers to rapidly switch data directly between data sources and destinations. The maximum two-way delivery time is necessary for determining the values of timeout counters, which are relevant to the fault-tolerant aspect of the system, and the QoS guarantees. Hot spots may cause the occurrence of tree saturation in the network i.e., where an individual tree will become congested (tree saturation) while all other trees are mostly idle, which leads to significant performance degradation. Our study is based on data generated by a simulation tool. Statistical analysis and inference methods were used to process the samples generated by the simulator and to obtain estimates for the maximum two-way packet delivery time. These methods were also used to determine the number of the samples that were required to produce estimates of the desired accuracy. Both the degree of confidence and the percentage of packets are determined by the number of samples generated by the simulator. This allows us to control the quality and accuracy of the estimate to suit our needs. Link usage statistics were recorded by the simulator for the purpose of performing a detailed investigation of congestion effects and hot spots. The study of the static and dynamic link usage statistics revealed that the extreme values of the two-way maximum delivery time were caused by links operating under heavy traffic loads and that under certain traffic patterns a tree saturation effect occurs.