Concurrency Theory: Lecture 21, 17 April 2018 --------------------------------------------- Lossy channel systems --------------------- We allow channels to lose messages: one additional global move is one where the control state does not change but some channel (i,j) changes from w.m.w' to w.w', dropping the message m. Forward transition relation allows message loss at any point. More restrictive backward transition relation assumes lost messages are dropped as soon as they are dropped. Order configurations by equality on control state and subsequence ordering on channel contents. Higman's Lemma says that any set of incomparable configurations is finite. Define a backward search procedure starting from a finite set of target states that computes if the initial state is reachable via the backward transition relation. Correctness is guaranteed by an invariant property of the search. Termination follows from Higman's Lemma. See Verifying Programs with Unreliable Channels by Abdulla and Jonsson. ----------------------------------------------------------------------