The ever increasing complexity of networks dramatically increases the challenges faced by service providers to analyze network behavior and (re)provision resources to support multiple complex distributed applications. Robust and accurate simulation tools are pivotal to this cause. The current state of the art in network modeling and simulation tools, namely, packet-based discrete event simulations and fluid models, are limited in that they are either applicable only to particular phases of the

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... etwork's dynamics or do not scale well to capture the behavior of immense or complex networks. The recently proposed hybrid system model for data communication networks shows promise in addressing these challenges. Based on the hybrid system modeling paradigm, this paper provides novel contributions to TCP network modeling and analysis/simulation of large scale networks. The salient distinguishing features of our approach include a new finite state machine model for a drop-tail queue, a new model for fast recovery/fast retransmit mode, a revised sending rate model, and an embedded time-out mode transition mechanism all of which employ a time-stepped solution method to solve the hybrid system network models. Our simulation results are consistent with well-known simulators, such as ns-2.