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##### GANG - 2019

Application Domains
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## Section: New Results

### Distributed Computing

#### Distributed Interactive Proofs

In a distributed locally-checkable proof, we are interested in checking the legality of a given network configuration with respect to some Boolean predicate. To do so, the network enlists the help of a prover — a computationally-unbounded oracle that aims at convincing the network that its state is legal, by providing the nodes with certificates that form a distributed proof of legality. The nodes then verify the proof by examining their certificate, their local neighborhood and the certificates of their neighbors.

In [24], we examine the power of a randomized form of locally-checkable proof, called distributed Merlin-Arthur protocols, or $dMA$ for short. In a $dMA$ protocol, the prover assigns each node a short certificate, and the nodes then exchange random messages with their neighbors. We show that while there exist problems for which $dMA$ protocols are more efficient than protocols that do not use randomness, for several natural problems, including Leader Election, Diameter, Symmetry, and Counting Distinct Elements, $dMA$ protocols are no more efficient than standard nondeterministic protocols. This is in contrast with Arthur-Merlin ($dAM$) protocols and Randomized Proof Labeling Schemes (RPLS), which are known to provide improvements in certificate size, at least for some of the aforementioned properties.

The study of interactive proofs in the context of distributed network computing is a novel topic, recently introduced by Kol, Oshman, and Saxena [PODC 2018]. In the spirit of sequential interactive proofs theory, we study in [20] the power of distributed interactive proofs. This is achieved via a series of results establishing trade-offs between various parameters impacting the power of interactive proofs, including the number of interactions, the certificate size, the communication complexity, and the form of randomness used. Our results also connect distributed interactive proofs with the established field of distributed verification. In general, our results contribute to providing structure to the landscape of distributed interactive proofs.

#### Topological Approach of Network Computing

More than two decades ago, combinatorial topology was shown to be useful for analyzing distributed fault-tolerant algorithms in shared memory systems and in message passing systems. In [18], we show that combinatorial topology can also be useful for analyzing distributed algorithms in networks of arbitrary structure. To illustrate this, we analyze consensus, set-agreement, and approximate agreement in networks, and derive lower bounds for these problems under classical computational settings, such as the LOCAL model and dynamic networks.

In [19], we study the number of rounds needed to solve consensus in a synchronous network $G$ where at most $t$ nodes may fail by crashing. This problem has been thoroughly studied when $G$ is a complete graph, but very little is known when $G$ is arbitrary. We define a notion of radius that considers all ways in which $t$ nodes may crash, and present an algorithm that solves consensus in radius rounds. Then we derive a lower bound showing that our algorithm is optimal for vertex-transitive graphs, among oblivious algorithms.

#### Making Local Algorithms Wait-Free

When considering distributed computing, reliable message-passing synchronous systems on the one side, and asynchronous failure-prone shared-memory systems on the other side, remain two quite independently studied ends of the reliability/asynchrony spectrum. The concept of locality of a computation is central to the first one, while the concept of wait-freedom is central to the second one. In [8], we propose a new DECOUPLED model in an attempt to reconcile these two worlds. It consists of a synchronous and reliable communication graph of nodes, and on top a set of asynchronous crash-prone processes, each attached to a communication node. To illustrate the DECOUPLED model, the paper presents an asynchronous 3-coloring algorithm for the processes of a ring. From the processes point of view, the algorithm is wait-free. From a locality point of view, each process uses information only from processes at distance $O\left(lo{g}^{*}n\right)$ from it. This local wait-free algorithm is based on an extension of the classical Cole and Vishkin’s vertex coloring algorithm in which the processes are not required to start simultaneously.

In [31], we show that, for any task $T$ associated to a locally checkable labeling (lcl), if T is solvable in t rounds by a deterministic algorithm in the local model, then T remains solvable by a deterministic algorithm in $O\left(t\right)$ rounds in an asynchronous variant of the local model whenever $t=O\left(polylogn\right)$.

#### Towards Synthesis of Distributed Algorithms with SMT Solvers

In [32], we consider the problem of synthesizing distributed algorithms working on a specific execution context. We show it is possible to use the linear time temporal logic in order to both specify the correctness of algorithms and their execution contexts. We then provide a method allowing to reduce the synthesis problem of finite state algorithms to some model-checking problems. We finally apply our technique to automatically generate algorithms for consensus and epsilon-agreement in the case of two processes using the SMT solver Z3.

#### On Weakest Failure Detector

Failure detectors are devices (objects) that provide the processes with information on failures. They were introduced to enrich asynchronous systems so that it becomes possible to solve problems (or implement concurrent objects) that are otherwise impossible to solve in pure asynchronous systems where processes are prone to crash failures. The most famous failure detector (which is called “eventual leader” and denoted $\Omega$ ) is the weakest failure detector which allows consensus to be solved in n-process asynchronous systems where up to $t=n-1$ processes may crash in the read/write communication model, and up to $t processes may crash in the message-passing communication model.

When looking at the mutual exclusion problem (or equivalently the construction of a lock object), while the weakest failure detectors are known for both asynchronous message-passing systems and read/write systems in which up to $t processes may crash, for the starvation-freedom progress condition, it is not yet known for weaker deadlock-freedom progress condition in read/write systems. In [34], we extend the previous results, namely, it presents the weakest failure detector that allows mutual exclusion to be solved in asynchronous n-process read/write systems where any number of processes may crash, whatever the progress condition (deadlock-freedom or starvation-freedom).

In read/read/write communication model, and in the message-passing communication model, all correct processes are supposed to participate in a consensus instance and in particular the eventual leader.

In [33], we considers the case where some subset of processes that do not crash (not predefined in advance) are allowed not to participate in a consensus instance. In this context $\Omega$ cannot be used to solve consensus as it could elect as eventual leader a non-participating process. This paper presents the weakest failure detector that allows correct processes not to participate in a consensus instance.This failure detector, denoted ${\Omega }^{*}$, is a variant of $\Omega$. The paper presents also an ${\Omega }^{*}$-based consensus algorithm for the asynchronous read/write model, in which any number of processes may crash, and not all the correct processes are required to participate.

#### Multi-Round Cooperative Search Games with Multiple Players

We study search in the context of competing agents. The setting we consider combines game-theoretic concepts with notions related to parallel computing. Assume that a treasure is placed in one of $M$ boxes according to a known distribution and that $k$ searchers are searching for it in parallel during $T$ rounds. In [27], we study the question of how to incentivize selfish players so that group performance would be maximized. Here, this is measured by the success probability, namely, the probability that at least one player finds the treasure. We focus on congestion policies $C\left(\ell \right)$ that specify the reward that a player receives if it is one of $\ell$ players that (simultaneously) find the treasure for the first time. Our main technical contribution is proving that the exclusive policy, in which $C\left(1\right)=1$ and $C\left(\ell \right)=0$ for $\ell >1$, yields a price of anarchy of ${\left(1-{\left(1-1/k\right)}^{k}\right)}^{-1}$, and that this is the best possible price among all symmetric reward mechanisms. For this policy we also have an explicit description of a symmetric equilibrium, which is in some sense unique, and moreover enjoys the best success probability among all symmetric profiles. For general congestion policies, we show how to polynomially find, for any $\theta >0$, a symmetric multiplicative $\left(1+\theta \right)\left(1+C\left(k\right)\right)$-equilibrium.

Together with an appropriate reward policy, a central entity can suggest players to play a particular profile at equilibrium. As our main conceptual contribution, we advocate the use of symmetric equilibria for such purposes. Besides being fair, we argue that symmetric equilibria can also become highly robust to crashes of players. Indeed, in many cases, despite the fact that some small fraction of players crash (or refuse to participate), symmetric equilibria remain efficient in terms of their group performances and, at the same time, serve as approximate equilibria. We show that this principle holds for a class of games, which we call monotonously scalable games. This applies in particular to our search game, assuming the natural sharing policy, in which $C\left(\ell \right)=1/\ell$. For the exclusive policy, this general result does not hold, but we show that the symmetric equilibrium is nevertheless robust under mild assumptions.