Consequently, many systems are fragile and vulnerable and only work well in a small part of the Internet. Simulation technology can help the development of fast, reliable, and secure network protocols and applications by presenting the system developer with far more heterogeneity than he or she could test alone. Simulation running time is usually exponential in the number of parameters simulated. New distributed computing services allow users to leverage the computing power of thousands of hosts on the Internet. These services could potentially speed up simulations or allow developers to simulate more parameters.
Most current simulators concentrate on testing for performance.
However, in many systems a larger problem is security, in particular intrusion and denial of service. A simulator may not be able to launch a sophisticated attack, but most exploited vulnerabilities result from incorrectly checked arguments and race conditions. Most current simulators require protocols to be re-implemented inside the simulator. This is a tedious and error-prone process. A few simulators implement emulation, where the simulator interfaces with real hosts running a live system, thus requiring minimal additional work by the developer.