Internet Topology: Discovery and Policy Impact

We cannot begin to understand the large-scale structure of the Internet without accurate representations of the Internet topology. Mercator is a program that uses hop-limited probes—the same primitive used in traceroute—to infer an Internet map. It uses informed random address probing to carefully explore the IP address space when determining router adjacencies, uses source-route capable routers wherever possible to enhance the fidelity of the resulting map, and employs novel mechanisms for resolving aliases (interfaces belonging to the same router). The first part of this chapter describes the design of these heuristics and highlights the limitations in collecting information about Internet topology. An accurate model of Internet routing paths also helps better understand the Internet’s large-scale structure. Internet routing is determined by policy. In theory, policy can inflate shortest-router-hop paths. Using a simplified model of routing policy in the Internet, we obtain approximate indications of the impact of policy routing on Internet paths. Our findings suggest that routing policy does impact the length of Internet paths significantly. For instance, in our model of routing policy, some 20% of Internet paths are inflated by more than five router-level hops. In this chapter, we present our initial findings of the impact of policy on Internet paths. In particular, our findings reveal answers to the following questions: How does policy based routing inflate Internet paths? For a source-destination pair, does there exist a detour path and how good is the best detour path comparing to the policy path? Does policy routing funnel Internet paths through larger ASs? Part A Internet Topology Discovery