Performance evaluation of the random replacement policy for networks of caches

The overall performance of content distribution networks as well as recently proposed information-centric networks rely on both memory and bandwidth capacities. The hit ratio is the key performance indicator which captures the bandwidth/memory tradeoff for a given global performance. This paper focuses on the estimation of the hit ratio in a network of caches that employ the Random replacement policy (RND). Assuming that requests are independent and identically distributed, general expressions of miss probabilities for a single RND cache are provided as well as exact results for specific popularity distributions (such results also hold for the FIFO replacement policy). Moreover, for any Zipf popularity distribution with exponent @a>1, we obtain asymptotic equivalents for the miss probability in the case of large cache size. We extend the analysis to networks of RND caches, when the topology is either a line or a homogeneous tree. In that case, approximations for miss probabilities across the network are derived by neglecting time correlations between miss events at any node; the obtained results are compared to the same network using the Least-Recently-Used discipline, already addressed in the literature. We further analyze the case of a mixed tandem cache network where the two nodes employ either Random or Least-Recently-Used policies. In all scenarios, asymptotic formulas and approximations are extensively compared to simulation results and shown to be very accurate. Finally, our results enable us to propose recommendations for cache replacement disciplines in a network dedicated to content distribution.