Abstract: Characterizing the set of routes used between domains is an important and difficult problem. The size and complexity of the millions of BGP paths in use at any time can hide important phenomena and hinder attempts to understand the path selection behavior of ASes. In this paper we introduce a new approach to analysis of the interdomain routing system designed to shed light on collective routing policies. Our approach starts by defining a new metric for
distance between prefixes, which we call routing state distance (RSD). We show that RSD has a number of properties that make it attractive for use in visualizing and analyzing the state of the BGP system. Further, since RSD is a metric, it lends itself naturally to use in clustering prefixes or ASes. In fact, the properties of RSD allow us to define a natural clustering criterion, and we show that this criterion admits to a simple clustering algorithm with provable approximation guarantees. We then show that by clustering ASes using RSD, one can uncover macroscopic behavior in BGP that was previously hidden. For example, we show how to identify groups of ASes having similar routing policies with respect to certain destinations, which apparently reflects shared sensitivity to economic or performance considerations. These routing patterns represent a considerable
generalization and extension of the notion of BGP atoms to the case where routing policies are only locally and approximately similar across a set of prefixes.