Byzantine Approximate Agreement on Graphs
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Consider a distributed system with n processors out of which f can be Byzantine faulty. In the approximate agreement task, each processor i receives an input value x_i and has to decide on an output value y_i such that - the output values are in the convex hull of the non-faulty processors' input values, - the output values are within distance d of each other. Classically, the values are assumed to be from an m-dimensional Euclidean space, where m > 1. In this work, we study the task in a discrete setting, where input values with some structure expressible as a graph. Namely, the input values are vertices of a finite graph G and the goal is to output vertices that are within distance d of each other in G, but still remain in the graph-induced convex hull of the input values. For d=0, the task reduces to consensus and cannot be solved with a deterministic algorithm in an asynchronous system even with a single crash fault. For any d > 1, we show that the task is solvable in asynchronous systems when G is chordal and n > (ω+1)f, where ω is the clique number of G. In addition, we give the first Byzantine-tolerant algorithm for a variant of lattice agreement. For synchronous systems, we show tight resilience bounds for the exact variants of these and related tasks over a large class of combinatorial structures.
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