Knowledge, Level of Symmetry, and Time of Leader Election

We study the time needed for deterministic leader election in the $\mathcal{LOCAL}$ model, where in every round a node can exchange any messages with its neighbors and perform any local computations. The topology of the network is unknown and nodes are unlabeled, but ports at each node have arbitrary fixed labelings which, together with the topology of the network, can create asymmetries to be exploited in leader election. We consider two versions of the leader election problem: strong LE in which exactly one leader has to be elected, if this is possible, while all nodes must terminate declaring that leader election is impossible otherwise, and weak LE, which differs from strong LE in that no requirement on the behavior of nodes is imposed, if leader election is impossible. We show that the time of leader election depends on three parameters of the network: its diameter D, its size n, and its level of symmetryλ, which, when leader election is feasible, is the smallest depth at which some node has a unique view of the network. It also depends on the knowledge by the nodes, or lack of it, of parameters D and n. Optimal time of weak LE is shown to be Θ(D+λ) if either D or n is known to the nodes. (If none of these parameters is known, even weak LE is impossible.) For strong LE, knowing only D is insufficient to perform it. If only n is known then optimal time is Θ(n), and if both n and D are known, then optimal time is Θ(D+λ).