An investigation of the clique problem in FlexRay

Distributed, time-triggered communication based on FlexRay is likely to become an enabler for future safety related applications in the automotive domain. Prior to series deployment, however, confidence has to be established that all mechanisms work as expected. One particular issue in this regard is the clique problem. Cliques are groups of nodes connected to the same bus which are only able to communicate with the members of the same clique, but not with members from another one. In time-triggered systems cliques might be caused, for instance, by Byzantine faults leading to a different view of the global time. In constrast to other hard real-time systems, FlexRay lacks explicit protocol support for detecting and resolving cliques. Thus, in this paper we investigate the clique problem in a real FlexRay system. We explain the different types of cliques and give examples how they might emerge and look like. One focus is the experimental study of the behavior of a real FlexRay cluster with existing cliques. In particular, we investigate the integration behavior of nodes into a system with present cliques as well as the behavior of the system when it is in a clique state. After a presentation of the results, we conclude by giving some directions about some protocol features which might be used for implementing clique avoidance or clique detection mechanisms.