論文 - 大下　福仁

Crash-tolerant perpetual exploration with myopic luminous robots on rings 査読

Fukuhito Ooshita, Naoki Kitamura, Ryota Eguchi, Michiko Inoue, Hirotsugu Kakugawa, Sayaka Kamei, Masahiro Shibata, and Yuichi Sudo

28th International Conference on Principles of Distributed Systems (OPODIS)   2024年12月

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We investigate crash-tolerant perpetual exploration algorithms by myopic luminous robots on ring networks. Myopic robots mean that they can observe nodes only within a certain fixed distance \phi, and luminous robots mean that they have light devices that can emit a color from a set of colors. The goal of perpetual exploration is to ensure that robots, starting from specific initial positions and colors, move in such a way that every node is visited by at least one robot infinitely often. As a main contribution, we clarify the tight necessary and sufficient number of robots to realize perpetual exploration when at most f robots crash. In the fully synchronous model, we prove that f+2 robots are necessary and sufficient for any \phi>=1. In the semi-synchronous and asynchronous models, we prove that 3f+3 (resp., 2f+2) robots are necessary and sufficient if \phi=1 (resp., \phi>=2).

A near time-optimal population protocol for self-stabilizing leader election on rings with a poly-logarithmic number of states 査読 国際誌

Daisuke Yokota, Yuichi Sudo, Fukuhito Ooshita, and Toshimitsu Masuzawa

42nd ACM Symposium on Principles of Distributed Computing (PODC)   2023年7月

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We propose a self-stabilizing leader election (SS-LE) protocol on ring networks in the population protocolmodel. Given a rough knowledge ψ = ⌈log n⌉ + O(1) on the population size n, the proposed protocol lets the population reach a safe configuration within O(n2 log n) steps with high probability starting from any configuration. Thereafter, the population keeps the unique leader forever. Since no protocol solves SS-LE in o(n2) steps with high probability, the convergence time is near-optimal: the gap is only an O(log n) multiplicative factor. This protocol uses only polylog(n) states. There exist two state-of-the-art algorithms in current literature that solve SS-LE on ring networks. The first algorithm uses a polynomial number of states and solves SS-LE in O(n2) steps, whereas the second algorithm requires exponential time but it uses only a constant number of states. Our proposed algorithm provides an excellent middle ground between these two.

Ring exploration of myopic luminous robots with visibility more than one 査読

Shota Nagahama, Fukuhito Ooshita, and Michiko Inoue

Information and Computation   292   2023年6月

Uniform Bipartition in the Population Protocol Model with Arbitrary Graphs 査読 国際共著

Hiroto Yasumi, Fukuhito Ooshita, Michiko Inoue, and Sebastien Tixeuil

Theoretical Computer Science   892   187 - 207   2021年9月

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In this paper, we focus on the uniform bipartition problem in the population protocol model. This problem aims to divide a population into two groups of equal size. In particular, we consider the problem in the context of arbitrary communication graphs. As a result, we investigate the solvability of the uniform bipartition problem with arbitrary communication graphs when agents in the population have designated initial states, under various assumptions such as the existence of a base station, symmetry of the protocol, and fairness of the execution. When the problem is solvable, we present protocols for uniform bipartition. When global fairness is assumed, the space complexity of our solutions is tight.

Ring exploration with myopic luminous robots 査読 国際共著

Fukuhito Ooshita and Sebastien Tixeuil

Information and Computation   2021年1月

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We investigate exploration algorithms for autonomous mobile robots evolving in uniform ring-shaped networks. Unlike previous works, we consider myopia (each robot can only observe its neighboring nodes) and luminosity (each robot can use only two colors for its light) properties for the robots.

As a main contribution, we prove that (i) in the FSYNC model, two (resp. three) myopic robots are necessary and sufficient to achieve perpetual (resp. terminating) exploration, and (ii) in the SSYNC and ASYNC models, three (resp. four) myopic robots are necessary and sufficient to achieve perpetual (resp. terminating) exploration.

We also show that, in all execution models, our perpetual exploration algorithm is universal. That is, the algorithm solves perpetual exploration from any solvable initial configuration with two (resp. three) myopic robots and two colors in the FSYNC model (resp. the SSYNC and ASYNC models). On the other hand, we provide impossibility results for universal terminating exploration.

Exploration of dynamic tori by multiple agents 査読

Tsuyoshi Gotoh, Yuichi Sudo, Fukuhito Ooshita, Hirotsugu Kakugawa, and Toshimitsu Masuzawa

Theoretical Computer Science   850 ( 5 )   202 - 220   2021年1月

Time-optimal leader election in population protocols 査読

Yuichi Sudo, Fukuhito Ooshita, Taisuke Izumi, Hirotsugu Kakugawa, and Toshimitsu Masuzawa

IEEE Transactions on Parallel and Distributed Systems   31 ( 11 )   2620 - 2632   2020年11月

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In this article, we present the first leader election protocol in the population protocol model that stabilizes within O(logn) parallel time in expectation with O(logn) states per agent, where n is the number of agents. Given a rough knowledge m of lg n such that m ≥ lg n and m = O(logn), the proposed protocol guarantees that exactly one leader is elected and the unique leader is kept forever thereafter. This protocol is time-optimal because it was recently proven that any leader election protocol requires Ω(logn) parallel time.

Brief announcement: Self-stabilizing graph exploration by a single agent 査読

Yuichi Sudo, Fukuhito Ooshita, and Sayaka Kamei

38th International Symposium on Distributed Computing (DISC)   2024年10月

A self-stabilizing distributed algorithm for the 1-MIS problem under the distance-3 model 査読

Hirotsugu kakugawa, Sayaka Kamei, Masahiro Shibata, and Fukuhito Ooshita

Concurrency and Computation: Practice and Experience   36 ( 26 )   2024年9月

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Fault-tolerance and self-organization are critical properties in modern distributed systems. Self-stabilization is a class of fault-tolerant distributed algorithms which has the ability to recover from any kind and any finite number of transient faults and topology changes. In this article, we propose a self-stabilizing distributed algorithm for the 1-MIS problem under the unfair central daemon assuming the distance-3 model. Here, in the distance-3 model, each process can refer to the values of local variables of processes within three hops. Intuitively speaking, the 1-MIS problem is a variant of the maximal independent set (MIS) problem with improved local optimizations. The time complexity (convergence time) of our algorithm is O(n) steps and the space complexity is O(log n) bits, where n is the number of processes. Finally, we extend the notion of 1-MIS to p-MIS for each nonnegative integer p, and compare the set sizes of p-MIS (p=0,1,2...) and the maximum independent set.

Gathering in carrier graphs: Meeting via public transportation system 査読

Haozhi Zheng, Ryota Eguchi, Fukuhito Ooshita, and Michiko Inoue

3rd Symposium on Algorithmic Foundation of Dynamic Networks (SAND)   2024年6月

Fast gathering despite a linear number of weakly Byzantine agents 査読

Jion Hirose, Junya Nakamura, Fukuhito Ooshita, and Michiko Inoue

Concurrency and Computation: Practice and Experience   36 ( 14 )   2024年4月

Neighborhood Mutual Remainder: Self-Stabilizing Distributed Implementation and Applications 査読 国際共著

Shlomi Dolev, Sayaka Kamei, Yoshiaki Katayama, Fukuhito Ooshita, and Koichi Wada

Acta Informatica   2023年12月

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Motivated by the need to convert move-atomic assumption in LOOK-COMPUTE-MOVE (LCM) robot algorithms to be implemented in existing distributed systems, we define a new distributed fundamental task, the neighborhood mutual remainder (NMR). Consider a situation where each process has a set of operations Op and executes each operation in Op infinitely often in distributed systems. Then, let Oe⊂Op be a subset of operations, which a process cannot execute, while its closed neighborhood executes operations in Op-Oe. The NMR is defined for such a situation. A distributed algorithm that satisfies the NMR requirement should satisfy the following two properties: (1) Liveness is satisfied if a process executes each operation in Op infinitely often and (2) safety is satisfied if, when each process executes operations in Oe, no process in its closed neighborhood executes operations in Op-Oe. We formalize the concept of NMR and give a simple self-stabilizing algorithm using the pigeon-hole principle to demonstrate the design paradigm to achieve NMR. A self-stabilizing algorithm tolerates transient faults (e.g., message loss, memory corruption, etc.) by its ability to converge from an arbitrary configuration to the legitimate one. In addition, we present an application of NMR to an LCM robot system for implementing a move-atomic property, where robots possess an independent clock that is advanced at the same speed. It is the first self-stabilizing implementation of the LCM synchronization for environments where each robot can have limited visibility and lights.

A self-stabilizing distributed algorithm for 1-MIS problem under the distance-3 model 査読

Hirotsugu kakugawa, Sayaka Kamei, Masahiro Shibata, and Fukuhito Ooshita

15th International Workshop on Parallel and Distributed Algorithms and Applications (PDAA)   2023年11月

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Fault-tolerance and self-organization are critical
properties in modern distributed systems. Self-stabilization is a class of fault-tolerant distributed algorithms which has the ability to recover from any kind and any finite number of transient faults and topology changes. In this paper, we propose a selfstabilizing distributed algorithm for the 1-MIS problem under the unfair central daemon assuming the distance-3 model. Here, in the distance-3 model, each process can refer to the values of local variables of processes within three hops. Intuitively speaking, the
1-MIS problem is a variant of the maximal independent set (MIS) problem with improved local optimizations. The time complexity (convergence time) of our algorithm is O(n) steps and the space complexity is O(log n) bits, where n is the number of processes.

Meeting times of non-atomic random walks 査読 国際共著 国際誌

Ryota Eguchi, Fukuhito Ooshita, Michiko Inoue, and Sebastien Tixeuil

25th International Symposium on Stabilization, Safety, and Security of Distributed Systems (SSS)   2023年10月

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In this paper, we revisit the problem of classical meeting times of random walks in graphs. In the process that two tokens (called agents) perform random walks on an undirected graph, the meeting times are defined as the expected times until they meet when the two agents are initially located at different vertices. A key feature of the problem is that, in each discrete time-clock (called round) of the process, the scheduler selects only one of the two agents, and the agent performs one move of the random walk. In the adversarial setting, the scheduler utilizes the strategy that intends to maximizing the expected time to meet. In the seminal papers [5, 11, 18], for the random walks of two agents, the notion of atomicity is implicitly considered. That is, each move of agents should complete while the other agent waits. In this paper, we consider and formalize the meeting time of non-atomic random walks. In the non-atomic random walks, we assume that in each round, only one agent can move but the move does not necessarily complete in the next round. In other words, we assume that an agent can move at a round while the other agent is still moving on an edge. For the non-atomic random walks with the adversarial schedulers, we give a polynomial upper bound on the meeting times.

Location Functions for Self-Stabilizing Byzantine Tolerant Swarms 査読 国際共著

Yotam Ashkenazi, Shlomi Dolev, Sayaka Kamei, Yoshiaki Katayama, Fukuhito Ooshita, and Koichi Wada

Theoretical Computer Science   954   2023年4月

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This paper proposes a novel framework to realize self-stabilizing Byzantine tolerant swarms. In this framework, non-Byzantine robots execute tasks while satisfying location functions. That is, the robots use a policy for their location choice, which restricts their location to satisfy the functions. We give a general Byzantine-resilient self-stabilizing algorithm based on the location function. Then we provide an efficient implementation of the self-stabilizing algorithms for special classes of tasks, called polynomial-based tasks and shape-based tasks. We also demonstrate the usefulness of the proposed framework by implementing typical tasks of robots.

Weakly Byzantine Gathering with a Strong Team 査読

Jion Hirose, Junya Nakamura, Fukuhito Ooshita, and Michiko Inoue

IEICE Transactions on Information and Systems   E105-D ( 3 )   2022年3月

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We study the gathering problem requiring a team of mobile agents to gather at a single node in arbitrary networks. The team consists of k agents with unique identifiers (IDs), and f of them are weakly Byzantine agents, which behave arbitrarily except falsifying their identifiers. The agents move in synchronous rounds and cannot leave any information on nodes. If the number of nodes n is given to agents, the existing fastest algorithm tolerates any number of weakly Byzantine agents and achieves gathering with simultaneous termination in O(n^4·|Λgood|·X(n)) rounds, where |Λgood| is the length of the maximum ID of non-Byzantine agents and X(n) is the number of rounds required to explore any network composed of n nodes. In this paper, we ask the question of whether we can reduce the time complexity if we have a strong team, i.e., a team with a few Byzantine agents, because not so many agents are subject to faults in practice. We give a positive answer to this question by proposing two algorithms in the case where at least 4f^2+9f+4 agents exist. Both the algorithms assume that the upper bound N of n is given to agents. The first algorithm achieves gathering with non-simultaneous termination in O((f+|Λgood|)·X(N)) rounds. The second algorithm achieves gathering with simultaneous termination in O((f+|Λall|)·X(N)) rounds, where |Λall| is the length of the maximum ID of all agents. The second algorithm significantly reduces the time complexity compared to the existing one if n is given to agents and |Λall|=O(|Λgood|) holds.

Terminating grid exploration with myopic luminous robots 査読

Shota Nagahama, Fukuhito Ooshita, and Michiko Inoue

International Journal of Networking and Computing   2022年1月

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We investigate the terminating grid exploration for autonomous myopic luminous robots. Myopic robots mean that they can observe nodes only within a certain fixed distance, and luminous robots mean that they have light devices that can emit colors. First, we prove that, in the semi-synchronous and asynchronous models, three myopic robots are necessary to achieve the terminating grid exploration if the visible distance is one. Next, we give fourteen algorithms for the terminating grid exploration in various assumptions of synchrony (fully-synchronous, semi-synchronous, and asynchronous models), visible distance, the number of colors, and a chirality. Six of them are optimal in terms of the number of robots.

Eventually consistent distributed ledger despite degraded atomic broadcast 査読 国際共著

Gregory Benassy, Fukuhito Ooshita, and Michiko Inoue

Concurrency and Computation: Practice and Experience   2021年1月

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The distributed ledger or blockchain technologies originated from the Bitcoin have been rapidly widespread in recent years. However, it also gives incentive to malicious users who would like to break the system or take advantage of it (steal money, hide some information stored in the ledger, isolate a particular node from the rest of the network, and so forth). Thus, research focusing on overcoming potential attacks to distributed ledgers is required. In this article, we focus on attacks that damage underlying networks of distributed ledgers. Underlying networks offer useful communication primitives such as an atomic broadcast, however, such attacks may degrade the property of the primitives and make distributed ledgers relying on the primitives no longer work. Hence we should design algorithms to make the distributed ledgers still work even when some attacks degrade the primitives. As the first study for such situations, we consider a problem to implement distributed ledgers tolerating the degradation of an underlying atomic broadcast service that distributed ledgers are relying on. We consider the case where the uniform agreement property of the atomic broadcast is degraded, and propose new algorithms that could ensure to reach eventual consistency despite degraded atomic broadcast.

Forgive & Forget: Self-stabilizing swarms in spite of Byzantine robots 査読 国際共著

Yotam Ashkenazi, Shlomi Dolev, Sayaka Kamei, Fukuhito Ooshita, and Koichi Wada

Concurrency and Computation: Practice and Experience   2020年11月

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In this article, we consider the case in which a swarm of robots collaborates in a mission, where a few of the robots behave maliciously. These malicious Byzantine robots may be temporally or constantly controlled by an adversary. The scope is synchronized full information robot operations, where a robot that does not follow the program/policy of the swarm is immediately identified and can be remembered as Byzantine. As robots may be suspected of being Byzantine due to benign temporal malfunctions, it is imperative to forgive and forget, otherwise, a robot cannot assume collaborative actions with any other robot in the swarm. Still, remembering for a while may facilitate a policy of surrounding, isolating and freezing the movement of the misbehaving robots, by several robots, allowing the rest to perform the swarm task with no intervention. We demonstrate the need to periodically forgive and forget to realize swarm several tasks including patrolling/cleaning in the presence of possible Byzantine robots. The policy for achieving the task consists of blocking the movement of the Byzantine robot(s) by some of the robots, while the rest patrol/clean the plane.

Byzantine-tolerant gathering of mobile agents in asynchronous arbitrary networks with authenticated whiteboards 査読

Masashi Tsuchida, Fukuhito Ooshita, and Michiko Inoue

IEICE Transactions on Information and Systems   E103-D ( 7 )   1672 - 1682   2020年7月