Real-time systems must accomplish executive and application tasks within specified timing constraints. In distributed real-time systems, the mechanisms that ensure fair access to shared resources, achieve consistent deadlines, meet timing or precedence constraints, and avoid deadlocks all utilize the notion of a common system-wide time base. A synchronization primitive is essential in meeting the demands of real-time critical computing. This paper provides a tutorial on the terminology, issues, and techniques essential to synchronization in real-time systems. >

Synchronization issues in real-time systems

The authors consider a system consisting of a sender that wants to send a value to certain receivers. Byzantine agreement protocols have previously been proposed to achieve this in the presence of arbitrary failures. The imposed requirement typically is that the fault-free receivers must all agree on the same value. An agreement protocol is proposed that achieves Lamport's Byzantine agreement (L. Lamport et al., 1982) up to a certain number of faults and a degraded form of agreement with a higher number of faults. The degraded form of agreement allows the fault-free receivers to agree on at most two different values, one of which is necessarily the default value. The proposed approach is named degradable agreement. An algorithm for degradable agreement is presented along with bounds on the number of nodes and network connectivity necessary to achieve degradable agreement. >

Degradable agreement in the presence of Byzantine faults

Traditional Byzantine agreement protocols require all fault-free receivers to agree on an identical value. The proposed degradable agreement approach achieves traditional agreement up to m faults and a degraded form of agreement up to u faults (u/spl ges/m), which allows fault-free receivers to agree on at most two different values (one of which is necessarily the default value). A degradable agreement algorithm and lower bounds are presented. >

Degradable Byzantine agreement

Fault-tolerant synchronization using phase-locked clocks

The use of phase-locked clocks to limit clock skews to fractions of the clock period while keeping the algorithm overhead very small is investigated. The number of clocks in the system required to ensure that up to m arbitrary failures can be tolerated with all the good clocks still in synchrony has been shown to be N>or=3m+1. It is shown here that, if N >

I.S. Bhandari

Papers

On the graceful degradation of phase-locked clocks