REPORT DOCUMENTATION PAGEForm Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Researchers from the University of Massachusetts developed fluid-based methodologies for characterizing the behavior of large IP networks handling large numbers of TCP and UDP flows. These methodologies provide for rapid and efficient rates of individual and aggregate flows. These fluid models were also applied to the problems of characterizing the spread of worms and viruses and to the cascade of failures within the BGP routing infrastructure. The resulting fluid models were used to develop novel active queue management mechanisms resulting in more stable TCP performance and novel rate controllers for the purpose of providing minimum rate guarantees to TCP flow aggregates. Last, methodologies and tools were developed to integrate fluid network simulation with packet-level simulation.