On the Borowsky-Gafni Simulation Algorithm

The first precise description of a version of the BorowskyGafni fault-tolerant simulation algorithm is given, along with a careful description of what it accomplishes and a proof of correctness. The algorithm implements a notion of fault-tolerant reducibility between decision problems. This notion of reducibility is defined, and examples of its use are provided. The algorithm is presented and verified in terms of I/O automata. The presentation has a great deal of interesting modularity, expressed by I/O automaton composition and both forward and backward simulation relations. Composition is used to include a safe agreement module as a subroutine. Forward and backward simulation relations are used to view the algorithm as implementing a multi-try snapshot strategy.