Universal protocols for information dissemination using emergent signals

Abstract

We consider a population of n agents which communicate with each other in a decentralized manner, through random pairwise interactions. One or more agents in the population may act as authoritative sources of information, and the objective of the remaining agents is to obtain information from or about these source agents. We study two basic tasks: broadcasting, in which the agents are to learn the bit-state of an authoritative source which is present in the population, and source detection, in which the agents are required to decide if at least one source agent is present in the population or not. We focus on designing protocols which meet two natural conditions: (1) universality, i.e., independence of population size, and (2) rapid convergence to a correct global state after a reconfiguration, such as a change in the state of a source agent. Our main positive result is to show that both of these constraints can be met. For both the broadcasting problem and the source detection problem, we obtain solutions with an expected convergence time of O(logn), from any starting configuration. The solution to broadcasting is exact, which means that all agents reach the state broadcast by the source, while the solution to source detection admits one-sided error on a ε-fraction of the population (which is unavoidable for this problem). Both protocols are easy to implement in practice and are self-stabilizing, in the sense that the stated bounds on convergence time hold starting from any possible initial configuration of the system. Our protocols exploit the properties of self-organizing oscillatory dynamics. On the hardness side, our main structural insight is to prove that any protocol which meets the constraints of universality and of rapid convergence after reconfiguration must display a form of non-stationary behavior (of which oscillatory dynamics are an example). We also observe that the periodicity of the oscillatory behavior of the protocol, when present, must necessarily depend on the number #X of source agents present in the population. For instance, our protocols inherently rely on the emergence of a signal passing through the population, whose period is Θ(log(n/#X)) rounds for most starting configurations. The design of phase clocks with tunable frequency may be of independent interest, notably in modeling biological networks.