On the Impact of Filters on Analyzing Prefix Reachability in the Internet

The reachability of IP address prefixes exhibits significant fluctuations due to changes in both physical connectivity and ISP routing policies. In the late 1990s, Labovitz et al. performed an extensive study of inter-domain path stability by analyzing BGP routing data. To reduce the noise in the BGP data, e.g., transient updates during route convergence, they applied several filters to preprocess the raw BGP data. In this work, we investigate prefix reachability as advertised by BGP, while revisiting the preprocessing filter design problem. We show that the reachability analysis results are highly sensitive to the specific filters applied and the parameters that control the strength of the filters. In particular, we compute the mean time to failure and recovery (MTTF and MTTR) as well as the up- to-downtime ratios of prefixes, and find that these can fluctuate by a factor of 10 by varying the filter parameters. We analyze the impact of recent fiber cuts in the Mediterranean sea and the Middle East, and study prefix reachability during a nine-month period in 2007 to evaluate the general health of the Internet.