Gathering in Dynamic Rings

The gathering problem requires a set of mobile agents, arbitrarily positioned at different nodes of the network to group within finite time at the same location, not fixed in advanced. The extensive existing literature on this problem shares the same fundamental assumption that the topological structure does not change during the rendezvous or the gathering. In this paper we start the investigation of gathering in dynamic graphs, that is networks where the topology changes continuously and at unpredictable locations. We study the feasibility of gathering mobile agents, identical and without explicit communication capabilities, in a dynamic ring of anonymous nodes; the class of dynamics we consider is the classic 1-interval-connectivity. In particular, we focus on the impact that factors such as chirality (i.e., common sense of orientation) and cross detection (i.e., the ability to detect, when traversing an edge, whether some agent is traversing it in the other direction), have on the solvability of the problem. We establish several results. We provide a complete characterization of the classes of initial configurations from which gathering problem is solvable in presence and in absence of cross detection. We provide distributed algorithms that allow the agents to gather within low polynomial time. In particular, the protocol for gathering with cross detection is time optimal. We show that cross detection is a powerful computational element; furthermore, we prove that, with cross detection, knowledge of the ring size is strictly more powerful than knowledge of the number of agents. From our investigation it follows that, for the gathering problem, the computational obstacles created by the dynamic nature of the ring can be overcome by the presence of chirality or of cross-detection.