# What can be decided locally without identifiers?

## Pierre Fraigniaud, Mika Göös, Amos Korman, and Jukka Suomela

Abstract

Do unique node identifiers help in deciding whether a network G has a prescribed property P? We study this question in the context of distributed local decision, where the objective is to decide whether G \in P by having each node run a constant-time distributed decision algorithm. If G \in P, all the nodes should output yes; if G \notin P, at least one node should output no. A recent work (Fraigniaud et al., OPODIS 2012) studied the role of identifiers in local decision and gave several conditions under which identifiers are not needed. In this  article, we answer their original question. More than that, we do so under all combinations of the following two critical variations on the underlying model of distributed computing: (B): the size of the identifiers is bounded by a function of the size of the input network; as  opposed to (\neg B): the identifiers are unbounded. (C): the nodes run a computable algorithm; as opposed to (\neg C): the nodes can compute any, possibly uncomputable function. While it is easy to see that under (\neg B, \neg C) identifiers are not needed, we show that under all other combinations there are properties that can be decided locally if and only if identifiers are present. Our constructions use ideas from classical computability theory.

Guest: Mika Göös
Host: Stefan Schmid

# Fast Byzantine Agreement

## Nicolas Braud-Santoni, Rachid Guerraoui, Florian Huc

Abstract: This paper presents the ﬁrst probabilistic Byzantine Agreement algorithm whose communication and time complexities are poly-logarithmic. So far, the most effective probabilistic Byzantine Agreement algorithm had communication complexity O*(sqrt n), and time complexity O*(1). Our algorithm is based on a novel, unbalanced, almost everywhere to everywhere Agreement protocol which is interesting in its own right.

Guest: Florian Huc
Host: Stefan Schmid