Hybrid Simulation of Packet-Level Networks and Functional-Level Routers

We discuss our approach to federating dissimilar discrete event simulations, leveraging the strengths and design goals of both, to produce a packet-level detailed network model federated with a component-level detailed input-queuing router model. All existing network simulation tools that we are aware of incorporate a very simplistic model for the flow of packets through a router. The simplistic model simply responds to a packet receipt event by performing a route look-up and adding the packet to the output queue of the next-hop output interface. This is often simulated to take place in zero time, or with rudimentary probabilistic models of delay within a router. However, modern high-end routers are designed using a complex input-queuing methodology and a sophisticated scheduling approach to move packets through a crossbar switch from the input queue to the output queue. We used the popular ns -- 3 network simulator to create realistic packet-level models of network load, and the Manifold computer architecture simulator to create a realistic model of data movement through an input-queued router. We federated the two by means of two alternative approaches: First, two POSIX threads run within a single simulation process and utilize the shared memory for both time synchronization and packet exchange. Second, we used the well-known MPI message passing library for the federation. Our results show that the detailed router models can in fact produce somewhat different packet delay and loss characteristics than the simplistic router models at the expense of considerable computational complexity.