Duplication, insertion and lossiness errors in unreliable communication channels

We consider the problem of verifying correctness of finite state machines that communicate with each other over unbounded FIFO channels that are unreliable. Various problems regarding verification of FIFO channels that can lose messages have been considered by Finkel [10], and by Abdulla and Johnson [1, 2]. We consider, in this paper, other possible unreliable behaviors of communication channels, viz. (a) duplication and (b) insertion errors. Furthermore, we also consider various combinations of duplication, insertion and lossiness errors.Finite state machines that communicate over unbounded FIFO buffers is a model of computation that forms the backbone of ISO standard protocol specification languages Estelle and SDL. While an assumption of a perfect communication medium is reasonable at the higher levels of the OSI protocol stack, the lower levels have to deal with an unreliable communication medium; hence our motivation for the present work.The verification problems that are of interest are reachability, unboundedness, deadlock, and model-checking against CTL. All of these problems are undecidable for machines communicating over reliable unbounded FIFO channels. So, it is perhaps surprising that some of these problems become decidable when unreliable channels are modeled. The contributions of this paper are: (a) An investigation of solutions to these problems for machines with insertion errors, duplication errors, or a combination of duplication, insertion and lossiness errors, and (b) A comparison of the relative expressive power of the various errors.