Fighting livelock in the GNU i-protocol: a case study in explicit-state model checking

The i-protocol, an optimized sliding-window protocol for GNU uucp, first came to our attention in 1995 when we used the Concurrency Factorys local model checker to detect, locate, and correct a non-trivial livelock in version 1.04 of the protocol. Since then, we have conducted a systematic case study on the protocol using four verification tools, viz. Cospan, Mur, Spin, and XMC, each of which supports some form of explicit-state model checking. Our results show that although the i-protocol is inherently complex – the size of its state space grows exponentially in the window size and it deploys several sophisticated optimizations aimed at minimizing control-message and retransmission overhead – it is nonetheless amenable to a number of general-purpose abstraction techniques whose application can significantly reduce the size of the protocols state space.