Two methods of dealing with the overflow problem in packet switches are compared: switch-retransmission and host-retransmission. The comparison is based on approximate analytic models of tandem queueing networks and is verified by simulation.

Consider a single packet switch with a finite number of packet buffers shared between several output queues. An arriving packet is lost if no free buffer is available, as in the CIGALE network. It has been observed by simulation that if load increases too much, congestion may occur, i.e., throughput declines; it appears that the busiest link's queue tends to hog the buffers. Therefore, we will limit the queue length and when the queue is full the packet will be dropped. We expect that this restricted buffer sharing policy will avoid congestion under conditions of heavy load. A queueing model of a packet switch is defined and solved by local balance. Loss probability is evaluated, and values of queue limit to minimize loss are found; they depend on load. A Square-Root rule is introduced to make the choice of queue limit independent of load. For a sample switch, with three output links, a comparison is made between performance under different buffer sharing policies; it is shown that restricted sharing prevents congestion by making throughput an increasing function of load.

A new technique for directing the flow of control in block structured programs is described. It allows a programmer to delay using the outcome of a test indefinitely, and to make the entry and exit of begin-end blocks conditional upon the last test performed. It was designed as a low-level technique in terms of which all the block structure control statements common in high-level programming languages can be implemented easily and efficiently. This technique is the main tool for directing the flow of control in the programming language NUCLEOL, and is considered to be particularly useful for program debugging.