Race Conditions in Coexisting Overlay Networks

By allowing end hosts to make independent routing decisions at the application level, different overlay networks may unintentionally interfere with each other. This paper describes how multiple similar or dissimilar overlay networks could experience race conditions, resulting in oscillations (in both route selection and network load) and cascading reactions. We pinpoint the causes for synchronization and derive an analytic formulation for the synchronization probability of two overlays. Our model indicates that the probability of synchronization is non-negligible across a wide range of parameter settings, thus implying that the ill effects of synchronization should not be ignored. Using the analytical model, we find an upper bound on the duration of traffic oscillations. We also show that the model can be easily extended to include a large number of co-existing overlays. We validate our model through simulations that are designed to capture the transient routing behavior of both the IP- and overlay-layers. We use our model to study the effects of factors such as path diversity (measured in round trip times) and probing aggressiveness on these race conditions. Finally, we discuss the implications of our study on the design of path probing process in overlay networks and examine strategies to reduce the impact of race conditions.