A study of the design requirements of the data link control protocol leads us to consider a throughput time delay criterion as a performance measure for optimal flow control of computer communication networks. A closed network model consisting of a

queuing system (the receiver node) controlled by an exponential queuing system with variable rate (the source node) is considered. The flow control maximizing the throughput of the M/M/1 queuing system in equilibrium under a bounded average time delay criterion is shown to be a window flow control mechanism. The window size

can be easily derived from the maximum tolerated average time delay

and the maximum offered load

. The dependence of the maximum throughput on the average time delay is also analyzed.