On the Sensitivity of Network Simulation to Topology

Abstract

While network simulations for congestion control studies have often varied traffic loads and protocol parameters, they have typically investigated only a few topologies. The most common is by far the so-called ``barbell'' topology. In this paper we argue, first, that the barbell topology is not representative of the Internet. In particular, we report that a measurable fraction of packets pass through multiple congestion points. Second, we argue that the distinction between the ``barbell'' topology and more complex topologies is relevant by presenting a scenario with multiple congestion points that exhibits behavior that seems unexpected based on intuition derived from the barbell topology (in particular, a TCP-only system that exhibits behavior technically considered ``congestion collapse''). We make the larger argument that the typical methodology currently accepted for evaluating network protocols is flawed. Finally, we briefly comment on some issues that arise in designing a simulation methodology that will be better suited to comparison of network protocol performance