On the Sensitivity of Network Simulation to Topology

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

cing: Measuring Network-Internal Delays using only Existing Infrastructure

Several techniques have been proposed for measuring network-internal delays. However, those that rely on router responses have questionable performance, and all proposed alternatives require either new functionality in routers or the existence of a measurement infrastructure. In this paper we revisit the feasibility of measuring network-internal delays using only existing infrastructure, focusing on the use of ICMP Timestamp probes to routers. We present network measurements showing that ICMP Timestamp is widely supported and that TTL-responses often perform poorly, and we analyze the effect of path instability and routing irregularities on the performance and applicability of using ICMP Timestamp. We also confirm that router responses rarely introduce errors in our measurements. Finally, we present a practical algorithm for clock artifact removal that addresses problems with previous methods and has been found to perform well in our setting