This research investigates various aspects of user-perceived network performance over a local area network for two transport layer protocols: TCP and UDP. The sensitivity of user-level performance to the choice of different speed hosts, host loads, and application program interfaces are studied. Our measurements serve as a guide in designing performance critical applications. Moreover, we present a detailed timing analysis of the dynamic behavior of the TCP/IP implementation in the MD-DOS/IP package. The analysis shows that the TCP flow control mechanism has a severely negative impact on the performance. The TCP data copy and checksum are the major overheads of TCP segment processing. Finally, the bottleneck of data communication using TCP/IP is identified based on queueing theory and empirical measurements. / Master of Science