Unreliable Channels Are Easier to Verify Than Perfect Channels

Abstract

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 of interest in verification of FIFO channels that can lose messages have been considered by Finkel and by Abdulla and Jonsson. 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 are a model of computation that forms the backbone of the ISO standard protocol specification languages Estelle and SDL. While the 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.

Domain-specific regular acceleration

“…In order to model more accurately some systems, it is also useful to consider lossy FIFO channels, in which symbols introduced by send operations can either be successfully delivered or be dropped by the channel [AJ96,ACABJ04]. It is known that safety properties of systems modeled by finite-state machines extended with lossy FIFO channels are decidable [AJ96], but the set of reachable configurations of such systems is generally uncomputable [CFI96].…”

“…In addition to lossy FIFO EHA, we can also consider systems in which the source of unreliability is the insertion of arbitrary symbols in the contents of the communication queues [CFI96]. Such systems are defined as follows.…”

Bisimulation and Other Undecidable Equivalences for Lossy Channel Systems

“…The relative merits of these several semantics have never been discussed in the literature (even in [CFP95] where both Front Lossiness and Classic Lossiness are considered). It is not clear which proposal better fits the real world.…”

“…Therefore it is really ironic, and somewhat paradoxical, that lossy channels are "easier to analyze than perfect ones! ", quoting [CFP95].…”

Lossy Counter Machines Decidability Cheat Sheet

“…In general, the results come from [11,42,39] when they are specific to lossy counter machines. Some results have been first shown for lossy channel systems [15,7,6] or even well-structured systems [25,26,5,28,29].…”