The basic architecture of our testbed is shown in Figure 3. A set of P client machines are connected to the server machine being tested. Each client machine runs a number of S-Client (short for Scalable Client) processes. The structure of a S-Client, and the number of S-Clients that run on a single machine are critical to our method and are described in detail below. If WAN effects are to be evaluated, the client machines should be connected to the server through a router that has sufficient capacity to carry the maximum traffic anticipated. The purpose of the router is to simulate WAN delays by introducing an artificial delay in the router's forwarding mechanism.