Earticle

Open Shortest Path First (OSPF) is a popular link state routing protocol widely used in Internet infrastructure. OSPF implements several timers to limit the protocol overhead. With these timers, it usually takes several tens of seconds for OSPF network to recover from a failure. The convergence time is delayed mainly by the timers of failure detection and routing calculation scheduling. In this paper we analyze OSPF convergence behavior in presence of multiple failures, where the interactions between failure detection and routing calculation scheduling could generate complicated dynamics during convergence process. We also present experimental study to understand the impact of multiple failures on convergence. The results demonstrate that multiple failures have a greater chance to delay the convergence. This suggests that operators should take it into account while configuring OSPF network.

A fault tolerant control system design technique has been proposed and analyzed for managing performance degradation in the presence of multiple faults in actuators. The method is based on a control structure with a model reference reconfigurable control design in an inner loop and command input adjustment in an outer loop. The reduced dynamic performance requirements in the presence of different actuator faults are accounted for through different performance reduced (degraded) reference models. The degraded steady-state performances are governed by the reduced levels of command input. The reconfigurable controller is designed on-line automatically in an explicit model reference control framework so that the dynamics of the closed-loop system follow that of the performance reduced reference model under each fault condition. The reduced command input level is determined to prevent potential actuator saturation. The proposed method has been evaluated and analyzed using an aircraft example against actuator faults subject to constraints on the magnitude and slew-rate of actuators.