An approach to verifying control flow in distributed computer systems (DCS) is presented. The approach is based on control flow checking among software components distributed over processors and cooperating among them. In this approach, control-flow behavior of DCS software is modeled and contained in special software components called verifiers. The verifiers are distributed over the processors and consulted to check the correctness of the control flow in DCS software during its execution. Algorithms for deriving the verifiers are presented. This technique can detect global errors including synchronization errors as well as local errors. It can be used for sequential or concurrent software at various levels of details. Experiments show that using this technique requires no significant overhead.>

Notable among local computer network protocols is the carrier sense multiple access (CSMA) network protocol. Many versions of CSMA have been proposed. The purpose of this paper is to derive an optimized performance of one version of CSMA, slotted nonpersistent CSMA with truncation (CSMA-SNP/TR), which is known to have better throughput performance than all other versions of CSMA. In this paper, the optimum average retransmission delay which provides the maximum throughput is presented in terms of message propagation delay, average retransmission delay, and number of terminals. Since the optimum average retransmission delay varies depending upon the number of terminals, an optimal average retransmission delay which nearly maximizes the throughput for a range of terminals is derived. A queueing model for representing the CSMA-SNP/TR protocol to analyze the performance of a computer network is also established. This work can be useful for design and performance analysis of distributed computer systems, including the computer systems and the communication channel connecting the computer systems.