In this paper we present a scalable model of a network of Active Queue Management (AQM) routers serving a large population of TCP flows. We present efficient solution techniques that allow one to obtain the transient behavior of the average queue lengths, packet loss probabilities, and average end-to-end latencies. We model different versions of TCP as well as different versions of RED, the most popular AQM scheme currently in use. Comparisons between our models and ns simulation show our models to be quite accurate while at the same time requiring substantially less time to solve, especially when workloads and bandwidths are high. Categories and Subject Descriptors C.2 [Computer-Communication Networks]: Miscellaneous ; C.4 [Performance of Systems]: Modeling techniques General Terms Performance Keywords Fluid model, Simulation, Large-scale IP networks