Experiences with the ns-2 network simulator - explicitly setting seeds considered harmful

The ns-2 network simulator is one of the most widely used packet network simulators. In version 2.1b9 its old random number generator was replaced by an implementation of MRG32k3a to fix sensitivity to seeds. Due to bad documentation and re-use of old scripts many people still use the old API functions to explicitly set seeds. Unfortunately, this corrupts the correct function of the new generator and can lead to correlated simulation results. This might affect the majority of ns-2 simulation results currently published. We show why this is the case, illustrate possible effects, and how to avoid the problem.