1. A method for simulating network traffic from a user comprising: collecting data representing actual user behavior when interacting with a network; generating a non-deterministic behavior model of a behavior of a network user based on the collected data, the non-deterministic behavior model comprising a state machine having a plurality of nodes interconnected by a set of directed edges, each of the nodes representing an action that the user can perform on the network system to produce a corresponding network communication, and each of the directed edges representing a transition from one of the node in the state machine to a different one of the nodes in the state machine, wherein each of the nodes specifies an average time that the user waits before or after generating the network communication and a distribution for user deviation from the average time; and generating and outputting network communications to a network system based on the generated non-deterministic behavior model to simulate the behavior of the network user and test the network system.