Structuring communication software for quality-of-service guarantees

Abstract

A growing number of real-time applications require quality-of-service (QoS) guarantees from the underlying communication subsystem. The communication subsystem (host and network) must support real-time communication services to provide the required QoS of these applications. In this paper, we propose architectural mechanisms for structuring host communication software to provide QoS guarantees. In particular, we present and evaluate a QoS-sensitive communication subsystem architecture for end hosts that provides real-time communication support for generic network hardware. This architecture provides services for managing communication resources for guaranteed-QoS (real-time) connections, such as admission control, traffic enforcement, buffer management, and CPU and link scheduling. The design of the architecture is based on three key goals: maintenance of QoS-guarantees on a par-connection basis, overload protection between established connections, and fairness in delivered performance to best-effort traffic. Using this architecture we implement real-time channels, a paradigm for real-time communication services in packet-switched networks. The proposed architecture features a process-per-channel model that associates a channel handler with each established channel. The model employed for handler execution is one of "cooperative" preemption, where an executing handler yields the CPU to a waiting higher-priority handler at well-defined preemption points. The architecture provides several configurable policies for protocol processing and overload protection. We present extensions to the admission control procedure for real-time channels to account for cooperative preemption and overlap between protocol processing and link transmission at a sending host. We evaluate the implementation to demonstrate the efficacy with which the architecture maintains QoS guarantees on outgoing traffic while adhering to the stated design goals. The evaluation also demonstrates the need for specific features and policies provided in the architecture. In subsequent work, we have refined this architecture and used it to realize a full-fledged guaranteed-QoS communication service that performs QoS-sensitive resource management for outgoing as well as incoming traffic.