Scheduling TCP in the Nemesis Operating System

The Nemesis operating system is designed to provide Quality of Service to applications. Nemesis also allows applications to reserve CPU time and transmit bandwidth on network interfaces. We have implemented a TCP for Nemesis that makes use of these guarantees. We show that the Nemesis transmit scheduler rate-controls TCP traffic and thus leads to predictable traffic behavior when applications choose not to utilize non-allocated bandwidth. Applications that want to make use of the nonallocated transmit bandwidth receive the guaranteed bandwidth plus a share of the non-allocated bandwidth. We also study the impact of the guaranteed fraction of CPU time on the throughput that networked applications achieve. We measure the amount of CPU time applications have to reserve in order to run the TCP protocol stack and send data at a particular speed. We show that these values hold even when several applications strive for CPU time and transmit bandwidth.