Throughput in the ARPANET--Protocols and Measurement

The speed at which large files can travel across a computer network is an important performance measure of that network. In this paper we examine the achievable sustained throughput in the ARPANET. Our point of departure is to describe the procedures used for controlling the flow of long messages (multipacket messages) and to identify the limitations that these procedures place on the throughput. We then present the quantitative results of experiments which measured the maximum throughput as a function of topological distance in the ARPANET. We observed a throughput of approximately 38 kbit/s at short distances. This throughput falls off at longer distances in a fashion which depends upon which particular version of the flow control procedure is in use; for example, at a distance of 9 hops, an October 1974 measurement gave 30 kbit/s, whereas a May 1975 experiment gave 27 kbit/s. The two different flow control procedures for these experiments are described, and the sources of throughput degradation at longer distances are identified, a major cause being due to a poor movement of critical limiting resources around in the network (this we call "phasing"). We conclude that flow control is a tricky business, but in spite of this, the ARPANET throughput is respectably high.