Reasoning About Internet Connectivity

Abstract: Innovation in the Internet requires a global Internet core to enable communication between users in ISPs and services in the cloud. Today, this Internet core is challenged by partial reachability: political pressure threatens fragmentation by nationality, architectural changes such as carrier-grade NAT make connectivity conditional, and operational problems and commercial disputes make reachability incomplete for months. We assert that partial reachability is a fundamental part of the Internet core. While other studies address partial reachability, this paper is the first to provide a conceptual definition of the Internet core so we can reason about reachability from principles first. Following the Internet design, our definition is guided by reachability, not authority. Its corollaries are peninsulas: persistent regions of partial connectivity; and islands: when networks are partitioned from the Internet core. We show that the concept of peninsulas and islands can improve existing measurement systems. In one example, they show that RIPE's DNSmon suffers misconfiguration and persistent network problems that are important, but risk obscuring operationally important connectivity changes because they are $5\times$ to $9.7\times$ larger. Our evaluation also informs policy questions, showing no single country or organization can unilaterally control the Internet core.