Simulation is an important tool in network protocol development, providing an effective way to perform controlled experiments, consider alternative designs, understand protocol interactions, and examine scales and topologies that are difficult to create in the laboratory. However, the scale and heterogeneity of today's networks create challenges for network simulation. In particular, configuring simulation inputs, properly modelling the myriad of interacting protocols, and processing and visualizing simulation output is becoming increasing difficult. The VINT project is developing a set of tools centered around ns simulator and nam network animator to meet these challenges. This paper describes the current status and future directions of this ongoing work.