A New View on What Limits TCP/IP Throughput in Local Area Networks


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Department of Computer Science, Virginia Polytechnic Institute & State University


This paper presents experimental results on what limits local area network throughput at the application program level for two popular transport protocols, TCP and UDP, using two application program interfaces, Berkeley sockets and System V transport layer interface. The sensitivity of application-level performance to the choice of host computer speed and background load on the host are also studied. Two sets of measurements are discussed. The first contains macroscopic measurement of throughput on 68020, 68030, 68040, 80386, SPARC, and MIPS R2000 and R3000 based computers over a single Ethernet subnet. The second presents a detailed timing analysis using a hardware monitor of a TCP/IP implementation for PC architecture computers. Previous studies implicate memory copying, checksumming, and the operating system interface as the major overheads in TCP/IP, rather than the time required to execute the protocol itself. This study indicates that these factors are secondary when the sender and receiver are closely matched in speed; rather the primary bottleneck is the TCP flow control mechanism. TCP flow control becomes closer to optimal as the degree of speed mismatch increases. We draw conclusions on why window mechanisms should be augmented by rate based flow control in the new generation of high data rate networks.