Parallel discrete event simulation techniques have enabled the realization of large-scale models of communication networks containing millions of end hosts and routers. However, the performance of these parallel simulators could be severely degraded if proper synchronization algorithms are not utilized. In this paper, we compare the performance and scalability of synchronous and asynchronous algorithms for conservative parallel network simulation. We develop an analytical model to evaluate the efficiency and scalability of certain variations of the well-known null message algorithm, and present experimental data to verify the accuracy of this model. This analysis and initial performance measurements on parallel machines containing hundreds of processors suggest that for scenarios simulating scaled network models with constant number of input and output channels per logical process, an optimized null message algorithm offers better scalability than efficient global reduction based synchronous protocols.