Analyzing a two-stage entry monitor for high-speed networks

Several researchers have suggested employing two-stage entry monitors for high-speed networks; the first stage enforcing a long term rate, the second stage enforcing a peak rate over a shorter time scale. In spite of this interest, little work on analyzing such systems has appeared. Not only is the analysis hard because of the the correlation between the stages, but for the most popular policing scheme, the leaky bucket, prior work is not easily generalized to two stages. This paper presents a performance analysis of a two-stage entry monitor, by analyzing the combined queue length of a two-stage system. Our analysis is for a two-stage buffered moving window. Analyzing such a conservative scheme enables us to bound the penalty of more optimistic schemes, in particular a two stage leaky bucket based system. We obtain a closed form analytical result, and show how to evaluate it for a wide range of parameters of interest. Our results show that the performance penalty of having the second stage is minimal under reasonable rate assumptions.