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The differences between and appropriateness of branching versus linear time temporal logic for reasoning about concurrent programs are studied. These issues have been previously considered by Lamport. To facilitate a careful examination of these issues, a language, CTL*, in which a universal or existential path quantifier can prefix an arbitrary linear time assertion, is defined. The expressive power of a number of sublanguages is then compared. CTL* is also related to the logics MPL of Abrahamson and PL of Hare& Kozen, and Parikh. The paper concludes with a comparison of the utility of branching and linear time temporal logics.

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Distributed protocols are often composed of similar processes connected in a unidirectional ring network. Processes communicate by passing a token in a fixed direction; the process that holds the token is allowed to perform certain actions. Usually, correctness properties are expected to hold irrespective of the size of the ring. We show that the question of checking many useful correctness properties for rings of all sizes can be reduced to checking them on ring of sizes up to a small cutoff size. We apply our results to the verification of a mutual exclusion protocol and Milner's scheduler protocol.