Inferring and Characterizing Internet Routing Policies

Border Gateway Protocol allows Autonomous Systems (ASs) to apply diverse routing policies for selecting routes and for propagating reachability information to other ASs. Although a significant number of studies have been focused on the Internet topology, little is known about what routing policies network operators employ to configure their networks. In this paper, we infer and characterize routing policies employed in the Internet. We find that routes learned from customers are preferred over those from peers and providers, and those from peers are typically preferred over those from providers. We present an algorithm for inferring and characterizing export policies. We show that ASs announce their prefixes to a selected subset of providers. The main reasons for the selective announcement are due to traffic engineering strategy for controlling incoming traffic. The impact of these routing policies might be significant. For example, many Tier-1 ASs reach their (direct or indirect) customers via their peers instead of customers. Furthermore, the selective announcement routing policies imply that there are much less available paths in the Internet than shown in the AS connectivity graph. We hope that our findings will caution network operators in choosing the selective announcement routing policy for traffic engineering. Finally, we study export policies to peers and find that ASs tend to announce all of their prefixes to other peers. To the best of our knowledge, this is the first study on systematically understanding routing policies applied in the Internet. Keywords— Routing Policies, BGP, Selective Announcement, Traffic Engineering