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392 IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 9, NO. 4, AUGUST 2001 Difficulties in Simulating the Internet Sally Floyd , Senior Member, IEEE, and Vern Paxson Abstract— Simulating how the global Internet behaves is an im- mensely 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 simula- tions in the face of these difficulties: searching for invariants and judiciously exploring the simulation parameter space. We finish with a brief look at a collaborative effort within the research com- munity to develop a common network simulator. Index Terms— Internet, modeling, simulation. I. INTRODUCTION D UE 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 pro- tocols. 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 simula- tions and model-based research, so that we do not weaken our simulations by claiming too much for them. We begin with the fundamental role of simulation in Internet research (Section II), and next explore the underlying difficul- ties (Sections III–V) rooted in the network’s immense hetero- geneity and the great degree to which it changes over time. We then discuss some strategies for accommodating these difficul- ties (Section VI). We finish with a brief look at a collaborative effort within the research community to develop a common net- work simulator (Section VII). II. THE ROLE OF SIMULATION While measurement and experimentation provide a means for exploring the “real world,” simulation and analysis are restricted Manuscript received October 14, 1999; revised June 29, 2000; approved by IEEE/ACM TRANSACTIONS ON NETWORKING Editor K. Calvert. This work was supported by the Director of the Office of Energy Research, Office of Compu- tational and Technology Research (Mathematical, Information, and Computa- tional Sciences Division) of the United States Department of Energy under Con- tract DE-AC03-76SF00098, and by ACIRI. This paper was presented in part at
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