The problem of optimizing the efficiency of large capacity communication networks is considered. An important objective is to reduce the effects of traffic uncertainties on network efficiency, especially in cases where statistical models of the traffic uncertainties are not available. The main result is the development of algorithms for adjusting the data flow of each network customer so as to optimize network efficiency. It is shown that, in some common situations, the use of feedback control makes it possible to eliminate completely the effects of traffic uncertainties on network efficiency, despite the lack of statistical models.