Difficulties in simulating the Internet

Simulating how the global Internet behaves is an immensely challenging undertaking because of the network's great heterogeneity and rapid change. The heterogeneity ranges from the individual links that carry the network's traffic, to the protocols that interoperate over the links, the "mix" of different applications used at a site, and the levels of congestion seen on different links. We discuss two key strategies for developing meaningful simulations in the face of these difficulties: searching for invariants and judiciously exploring the simulation parameter space. We finish with a look at a collaborative effort within the research community to develop a common network simulator.

I. Introduction

Due TO the network's complexity, simulation plays a vital role in attempting to characterize both the behavior of the current Internet and the possible effects of proposed changes to its operation. Yet modeling and simulating the Internet is not an easy task. The goal of this paper is to discuss some of the issues and difficulties in modeling Internet traffic, topologies, and protocols. The discussion is not meant as a call to abandon Internet simulations as an impossible task; in fact, one of us (Sally) has continued to use simulations as a key component of her research for many years. Instead, the purpose is to share insights about some of the dangers and pitfalls in modeling and simulating the Internet, in order to strengthen the contribution of simulations in network research. A second purpose is to clearly and explicitly acknowledge the limitations as well as the potential of simulations and model-based research, so that we do not weaken our simulations by claiming too much for them.