Conclusions

In this paper, we have presented geography as a means for analyzing various aspects of Internet routing. First, our analysis based on extensive traceroute data shows the existence of many circuitous routes in the Internet. From the end-to-end perspective, we observe that the circuitousness of routes depends on the geographic and network locations of the end-hosts. We also find that the minimum delay along a path is more strongly correlated with the linearized distance the path than it is with the geographic distance between the end-points. This suggests that the circuitousness of a path does impact its minimum delay characteristics, which is an important end-to-end performance metric. In ongoing work, we are studying the correlation between geography and network performance. Second, a more careful examination shows that many circuitous paths tend to traverse multiple major ISPs. Although many of these major ISPs have points of presence in common locations, the peering between them is restricted to specific geographic locations, which causes the paths traversing multiple ISPs to be more circuitous. We also found that intra-ISP paths are far less circuitous than inter-ISP paths. An important requirement to reduce the circuitousness of paths is for ISPs to have peering relationships at many geographic locations. Third, the fraction of the end-to-end path that lies within an ISP's network varies widely from one ISP to another. Furthermore, when we consider paths that traverse two or more major ISPs, we find that the path generally traverses a significantly shorter distance in the first ISP's network than in the second. This finding is consistent with the hot-potato routing policy. Using geographic information, we are able to quantify the degree to which an ISP's routing policy resembles hot-potato routing. Finally, our analysis of geographic fault tolerance of ISPs indicates that the (IP-level) network topologies of many tier-1 ISPs exhibit skewed degree distributions which may induce a low tolerance to the failure of a single, critical geographic node. The combined topology of multiple ISPs exhibits better fault tolerance characteristics, assuming that the ISPs peer at all geographic locations that are in common.