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Documents authored by Raynal, Michel

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Brief Announcement
DOI: 10.4230/LIPIcs.DISC.2024.41
Brief Announcement: Towards Optimal Communication Byzantine Reliable Broadcast Under a Message Adversary

Authors: Timothé Albouy, Davide Frey, Ran Gelles, Carmit Hazay, Michel Raynal, Elad Michael Schiller, François Taïani, and Vassilis Zikas

Published in: LIPIcs, Volume 319, 38th International Symposium on Distributed Computing (DISC 2024)

Abstract
We address the problem of Reliable Broadcast in asynchronous message-passing systems with n nodes, of which up to t are malicious (faulty), in addition to a message adversary that can drop some of the messages sent by correct (non-faulty) nodes. We present a Message-Adversary-Tolerant Byzantine Reliable Broadcast (MBRB) algorithm that communicates an almost optimal amount of O(|m|+n²κ) bits per node, where |m| represents the length of the application message and κ = Ω(log n) is a security parameter. This improves upon the state-of-the-art MBRB solution (Albouy, Frey, Raynal, and Taïani, TCS 2023), which incurs communication of O(n|m|+n²κ) bits per node. Our solution sends at most 4n² messages overall, which is asymptotically optimal. Reduced communication is achieved by employing coding techniques that replace the need for all nodes to (re-)broadcast the entire application message m. Instead, nodes forward authenticated fragments of the encoding of m using an erasure-correcting code. Under the cryptographic assumptions of PKI and collision-resistant hash, and assuming n > 3t+2d, where the adversary drops at most d messages per broadcast, our algorithm allows at least 𝓁 = n - t - (1 + ε)d (for any ε > 0) correct nodes to reconstruct m, despite missing fragments caused by the malicious nodes and the message adversary.
Cite as

Timothé Albouy, Davide Frey, Ran Gelles, Carmit Hazay, Michel Raynal, Elad Michael Schiller, François Taïani, and Vassilis Zikas. Brief Announcement: Towards Optimal Communication Byzantine Reliable Broadcast Under a Message Adversary. In 38th International Symposium on Distributed Computing (DISC 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 319, pp. 41:1-41:7, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{albouy_et_al:LIPIcs.DISC.2024.41,
  author =	{Albouy, Timoth\'{e} and Frey, Davide and Gelles, Ran and Hazay, Carmit and Raynal, Michel and Schiller, Elad Michael and Ta\"{i}ani, Fran\c{c}ois and Zikas, Vassilis},
  title =	{{Brief Announcement: Towards Optimal Communication Byzantine Reliable Broadcast Under a Message Adversary}},
  booktitle =	{38th International Symposium on Distributed Computing (DISC 2024)},
  pages =	{41:1--41:7},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-352-2},
  ISSN =	{1868-8969},
  year =	{2024},
  volume =	{319},
  editor =	{Alistarh, Dan},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2024.41},
  URN =		{urn:nbn:de:0030-drops-212697},
  doi =		{10.4230/LIPIcs.DISC.2024.41},
  annote =	{Keywords: Asynchronous message-passing, Byzantine fault-tolerance, Message adversary, Reliable Broadcast}
}
Document
DOI: 10.4230/LIPIcs.DISC.2023.16
Send/Receive Patterns Versus Read/Write Patterns in Crash-Prone Asynchronous Distributed Systems

Authors: Mathilde Déprés, Achour Mostéfaoui, Matthieu Perrin, and Michel Raynal

Published in: LIPIcs, Volume 281, 37th International Symposium on Distributed Computing (DISC 2023)

Abstract
This paper is on the power and computability limits of messages patterns in crash-prone asynchronous message-passing systems. It proposes and investigates three basic messages patterns (encountered in all these systems) each involving two processes, and compares them to their Read/Write counterparts. It is first shown that one of these patterns has no Read/Write counterpart. The paper proposes then a new one-to-all broadcast abstraction, denoted Mutual Broadcast (in short MBroadcast), whose implementation relies on two of the previous messages patterns. This abstraction provides each pair of processes with the following property (called mutual ordering): for any pair of processes p and p', if p broadcasts a message m and p' broadcasts a message m', it is not possible for p to deliver first (its message) m and then m' while p' delivers first (its message) m' and then m. It is shown that MBroadcast and atomic Read/Write registers have the same computability power (independently of the number of crashes). Finally, in addition to its theoretical contribution, the practical interest of MBroadcast is illustrated by its (very simple) use to solve basic upper level coordination problems such as mutual exclusion and consensus. Last but not least, looking for simplicity was also a target of this article.
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Mathilde Déprés, Achour Mostéfaoui, Matthieu Perrin, and Michel Raynal. Send/Receive Patterns Versus Read/Write Patterns in Crash-Prone Asynchronous Distributed Systems. In 37th International Symposium on Distributed Computing (DISC 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 281, pp. 16:1-16:24, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{depres_et_al:LIPIcs.DISC.2023.16,
  author =	{D\'{e}pr\'{e}s, Mathilde and Most\'{e}faoui, Achour and Perrin, Matthieu and Raynal, Michel},
  title =	{{Send/Receive Patterns Versus Read/Write Patterns in Crash-Prone Asynchronous Distributed Systems}},
  booktitle =	{37th International Symposium on Distributed Computing (DISC 2023)},
  pages =	{16:1--16:24},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-301-0},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{281},
  editor =	{Oshman, Rotem},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2023.16},
  URN =		{urn:nbn:de:0030-drops-191420},
  doi =		{10.4230/LIPIcs.DISC.2023.16},
  annote =	{Keywords: Asynchrony, Atomicity, Broadcast abstraction, Characterization, Consensus, Crash failure, Distributed Computability, Distributed software engineering, Computability, Lattice agreement, Message-passing, Message pattern, Mutual exclusion, Quorum, Read/write pattern, Read/Write register, Test\&Set, Simplicity, Two-process communication}
}
Document
DOI: 10.4230/LIPIcs.DISC.2023.21
The Synchronization Power (Consensus Number) of Access-Control Objects: the Case of AllowList and DenyList

Authors: Davide Frey, Mathieu Gestin, and Michel Raynal

Published in: LIPIcs, Volume 281, 37th International Symposium on Distributed Computing (DISC 2023)

Abstract
This article studies the synchronization power of AllowList and DenyList objects under the lens provided by Herlihy’s consensus hierarchy. It specifies AllowList and DenyList as distributed objects and shows that, while they can both be seen as specializations of a more general object type, they inherently have different synchronization power. While the AllowList object does not require synchronization between participating processes, a DenyList object requires processes to reach consensus on a specific set of processes. These results are then applied to a more global analysis of anonymity-preserving systems that use AllowList and DenyList objects. First, a blind-signature-based e-voting is presented. Second, DenyList and AllowList objects are used to determine the consensus number of a specific decentralized key management system. Third, an anonymous money transfer algorithm using the association of AllowList and DenyList objects is presented. Finally, this analysis is used to study the properties of these application, and to highlight efficiency gains that they can achieve in message passing environment.
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Davide Frey, Mathieu Gestin, and Michel Raynal. The Synchronization Power (Consensus Number) of Access-Control Objects: the Case of AllowList and DenyList. In 37th International Symposium on Distributed Computing (DISC 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 281, pp. 21:1-21:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{frey_et_al:LIPIcs.DISC.2023.21,
  author =	{Frey, Davide and Gestin, Mathieu and Raynal, Michel},
  title =	{{The Synchronization Power (Consensus Number) of Access-Control Objects: the Case of AllowList and DenyList}},
  booktitle =	{37th International Symposium on Distributed Computing (DISC 2023)},
  pages =	{21:1--21:23},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-301-0},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{281},
  editor =	{Oshman, Rotem},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2023.21},
  URN =		{urn:nbn:de:0030-drops-191473},
  doi =		{10.4230/LIPIcs.DISC.2023.21},
  annote =	{Keywords: Access control, AllowList/DenyList, Blockchain, Consensus number, Distributed objects, Modularity, Privacy, Synchronization power}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2022.26
A Modular Approach to Construct Signature-Free BRB Algorithms Under a Message Adversary

Authors: Timothé Albouy, Davide Frey, Michel Raynal, and François Taïani

Published in: LIPIcs, Volume 253, 26th International Conference on Principles of Distributed Systems (OPODIS 2022)

Abstract
This paper explores how reliable broadcast can be implemented without signatures when facing a dual adversary that can both corrupt processes and remove messages. More precisely, we consider an asynchronous n-process message-passing system in which up to t processes are Byzantine and where, at the network level, for each message broadcast by a correct process, an adversary can prevent up to d processes from receiving it (the integer d defines the power of the message adversary). So, unlike previous works, this work considers that not only can computing entities be faulty (Byzantine processes), but, in addition, that the network can also lose messages. To this end, the paper adopts a modular strategy and first introduces a new basic communication abstraction denoted k2𝓁-cast, which simplifies quorum engineering, and studies its properties in this new adversarial context. Then, the paper deconstructs existing signature-free Byzantine-tolerant asynchronous broadcast algorithms and, with the help of the k2𝓁-cast communication abstraction, reconstructs versions of them that tolerate both Byzantine processes and message adversaries. Interestingly, these reconstructed algorithms are also more efficient than the Byzantine-tolerant-only algorithms from which they originate.
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Timothé Albouy, Davide Frey, Michel Raynal, and François Taïani. A Modular Approach to Construct Signature-Free BRB Algorithms Under a Message Adversary. In 26th International Conference on Principles of Distributed Systems (OPODIS 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 253, pp. 26:1-26:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{albouy_et_al:LIPIcs.OPODIS.2022.26,
  author =	{Albouy, Timoth\'{e} and Frey, Davide and Raynal, Michel and Ta\"{i}ani, Fran\c{c}ois},
  title =	{{A Modular Approach to Construct Signature-Free BRB Algorithms Under a Message Adversary}},
  booktitle =	{26th International Conference on Principles of Distributed Systems (OPODIS 2022)},
  pages =	{26:1--26:23},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-265-5},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{253},
  editor =	{Hillel, Eshcar and Palmieri, Roberto and Rivi\`{e}re, Etienne},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2022.26},
  URN =		{urn:nbn:de:0030-drops-176464},
  doi =		{10.4230/LIPIcs.OPODIS.2022.26},
  annote =	{Keywords: Asynchronous system, Byzantine processes, Communication abstraction, Delivery predicate, Fault-Tolerance, Forwarding predicate, Message adversary, Message loss, Modularity, Quorum, Reliable broadcast, Signature-free algorithm, Two-phase commit}
}
Document
DOI: 10.4230/LIPIcs.DISC.2022.4
Good-Case Early-Stopping Latency of Synchronous Byzantine Reliable Broadcast: The Deterministic Case

Authors: Timothé Albouy, Davide Frey, Michel Raynal, and François Taïani

Published in: LIPIcs, Volume 246, 36th International Symposium on Distributed Computing (DISC 2022)

Abstract
This paper considers the good-case latency of Byzantine Reliable Broadcast (BRB), i.e., the time taken by correct processes to deliver a message when the initial sender is correct, and an essential property for practical distributed systems. Although significant strides have been made in recent years on this question, progress has mainly focused on either asynchronous or randomized algorithms. By contrast, the good-case latency of deterministic synchronous BRB under a majority of Byzantine faults has been little studied. In particular, it was not known whether a good-case latency below the worst-case bound of t+1 rounds could be obtained under a Byzantine majority. In this work, we answer this open question positively and propose a deterministic synchronous Byzantine reliable broadcast that achieves a good-case latency of max(2,t+3-c) rounds, where t is the upper bound on the number of Byzantine processes, and c the number of effectively correct processes.
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Timothé Albouy, Davide Frey, Michel Raynal, and François Taïani. Good-Case Early-Stopping Latency of Synchronous Byzantine Reliable Broadcast: The Deterministic Case. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 4:1-4:22, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{albouy_et_al:LIPIcs.DISC.2022.4,
  author =	{Albouy, Timoth\'{e} and Frey, Davide and Raynal, Michel and Ta\"{i}ani, Fran\c{c}ois},
  title =	{{Good-Case Early-Stopping Latency of Synchronous Byzantine Reliable Broadcast: The Deterministic Case}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{4:1--4:22},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.4},
  URN =		{urn:nbn:de:0030-drops-171953},
  doi =		{10.4230/LIPIcs.DISC.2022.4},
  annote =	{Keywords: Reliable Broadcast, Byzantine Faults, Synchronous Systems, Good-case latency, Deterministic Algorithms}
}
Document
DOI: 10.4230/OASIcs.FAB.2021.2
Byzantine-Tolerant Distributed Grow-Only Sets: Specification and Applications

Authors: Vicent Cholvi, Antonio Fernández Anta, Chryssis Georgiou, Nicolas Nicolaou, Michel Raynal, and Antonio Russo

Published in: OASIcs, Volume 92, 4th International Symposium on Foundations and Applications of Blockchain 2021 (FAB 2021)

Abstract
In order to formalize Distributed Ledger Technologies and their interconnections, a recent line of research work has formulated the notion of Distributed Ledger Object (DLO), which is a concurrent object that maintains a totally ordered sequence of records, abstracting blockchains and distributed ledgers. Through DLO, the Atomic Appends problem, intended as the need of a primitive able to append multiple records to distinct ledgers in an atomic way, is studied as a basic interconnection problem among ledgers. In this work, we propose the Distributed Grow-only Set object (DSO), which instead of maintaining a sequence of records, as in a DLO, maintains a set of records in an immutable way: only Add and Get operations are provided. This object is inspired by the Grow-only Set (G-Set) data type which is part of the Conflict-free Replicated Data Types. We formally specify the object and we provide a consensus-free Byzantine-tolerant implementation that guarantees eventual consistency. We then use our Byzantine-tolerant DSO (BDSO) implementation to provide consensus-free algorithmic solutions to the Atomic Appends and Atomic Adds (the analogous problem of atomic appends applied on G-Sets) problems, as well as to construct consensus-free Single-Writer BDLOs. We believe that the BDSO has applications beyond the above-mentioned problems.
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Vicent Cholvi, Antonio Fernández Anta, Chryssis Georgiou, Nicolas Nicolaou, Michel Raynal, and Antonio Russo. Byzantine-Tolerant Distributed Grow-Only Sets: Specification and Applications. In 4th International Symposium on Foundations and Applications of Blockchain 2021 (FAB 2021). Open Access Series in Informatics (OASIcs), Volume 92, pp. 2:1-2:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{cholvi_et_al:OASIcs.FAB.2021.2,
  author =	{Cholvi, Vicent and Fern\'{a}ndez Anta, Antonio and Georgiou, Chryssis and Nicolaou, Nicolas and Raynal, Michel and Russo, Antonio},
  title =	{{Byzantine-Tolerant Distributed Grow-Only Sets: Specification and Applications}},
  booktitle =	{4th International Symposium on Foundations and Applications of Blockchain 2021 (FAB 2021)},
  pages =	{2:1--2:19},
  series =	{Open Access Series in Informatics (OASIcs)},
  ISBN =	{978-3-95977-196-2},
  ISSN =	{2190-6807},
  year =	{2021},
  volume =	{92},
  editor =	{Gramoli, Vincent and Sadoghi, Mohammad},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.FAB.2021.2},
  URN =		{urn:nbn:de:0030-drops-139883},
  doi =		{10.4230/OASIcs.FAB.2021.2},
  annote =	{Keywords: Grow-only Sets, Distributed Ledgers, Blockchains, Atomic appends}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2020.13
Relaxed Queues and Stacks from Read/Write Operations

Authors: Armando Castañeda, Sergio Rajsbaum, and Michel Raynal

Published in: LIPIcs, Volume 184, 24th International Conference on Principles of Distributed Systems (OPODIS 2020)

Abstract
Considering asynchronous shared memory systems in which any number of processes may crash, this work identifies and formally defines relaxations of queues and stacks that can be non-blocking or wait-free while being implemented using only read/write operations. Set-linearizability and Interval-linearizability are used to specify the relaxations formally, and precisely identify the subset of executions which preserve the original sequential behavior. The relaxations allow for an item to be returned more than once by different operations, but only in case of concurrency; we call such a property multiplicity. The stack implementation is wait-free, while the queue implementation is non-blocking. Interval-linearizability is used to describe a queue with multiplicity, with the additional relaxation that a dequeue operation can return weak-empty, which means that the queue might be empty. We present a read/write wait-free interval-linearizable algorithm of a concurrent queue. As far as we know, this work is the first that provides formalizations of the notions of multiplicity and weak-emptiness, which can be implemented on top of read/write registers only.
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Armando Castañeda, Sergio Rajsbaum, and Michel Raynal. Relaxed Queues and Stacks from Read/Write Operations. In 24th International Conference on Principles of Distributed Systems (OPODIS 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 184, pp. 13:1-13:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{castaneda_et_al:LIPIcs.OPODIS.2020.13,
  author =	{Casta\~{n}eda, Armando and Rajsbaum, Sergio and Raynal, Michel},
  title =	{{Relaxed Queues and Stacks from Read/Write Operations}},
  booktitle =	{24th International Conference on Principles of Distributed Systems (OPODIS 2020)},
  pages =	{13:1--13:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-176-4},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{184},
  editor =	{Bramas, Quentin and Oshman, Rotem and Romano, Paolo},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2020.13},
  URN =		{urn:nbn:de:0030-drops-134983},
  doi =		{10.4230/LIPIcs.OPODIS.2020.13},
  annote =	{Keywords: Asynchrony, Correctness condition, Linearizability, Nonblocking, Process crash, Relaxed data type, Set-linearizability, Wait-freedom, Work-stealing}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2019.6
Byzantine-Tolerant Set-Constrained Delivery Broadcast

Authors: Alex Auvolat, Michel Raynal, and François Taïani

Published in: LIPIcs, Volume 153, 23rd International Conference on Principles of Distributed Systems (OPODIS 2019)

Abstract
Set-Constrained Delivery Broadcast (SCD-broadcast), recently introduced at ICDCN 2018, is a high-level communication abstraction that captures ordering properties not between individual messages but between sets of messages. More precisely, it allows processes to broadcast messages and deliver sets of messages, under the constraint that if a process delivers a set containing a message m before a set containing a message m', then no other process delivers first a set containing m' and later a set containing m. It has been shown that SCD-broadcast and read/write registers are computationally equivalent, and an algorithm implementing SCD-broadcast is known in the context of asynchronous message passing systems prone to crash failures. This paper introduces a Byzantine-tolerant SCD-broadcast algorithm, which we call BSCD-broadcast. Our proposed algorithm assumes an underlying basic Byzantine-tolerant reliable broadcast abstraction. We first introduce an intermediary communication primitive, Byzantine FIFO broadcast (BFIFO-broadcast), which we then use as a primitive in our final BSCD-broadcast algorithm. Unlike the original SCD-broadcast algorithm that is tolerant to up to t<n/2 crashing processes, and unlike the underlying Byzantine reliable broadcast primitive that is tolerant to up to t<n/3 Byzantine processes, our BSCD-broadcast algorithm is tolerant to up to t<n/4 Byzantine processes. As an illustration of the high abstraction power provided by the BSCD-broadcast primitive, we show that it can be used to implement a Byzantine-tolerant read/write snapshot object in an extremely simple way.
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Alex Auvolat, Michel Raynal, and François Taïani. Byzantine-Tolerant Set-Constrained Delivery Broadcast. In 23rd International Conference on Principles of Distributed Systems (OPODIS 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 153, pp. 6:1-6:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{auvolat_et_al:LIPIcs.OPODIS.2019.6,
  author =	{Auvolat, Alex and Raynal, Michel and Ta\"{i}ani, Fran\c{c}ois},
  title =	{{Byzantine-Tolerant Set-Constrained Delivery Broadcast}},
  booktitle =	{23rd International Conference on Principles of Distributed Systems (OPODIS 2019)},
  pages =	{6:1--6:23},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-133-7},
  ISSN =	{1868-8969},
  year =	{2020},
  volume =	{153},
  editor =	{Felber, Pascal and Friedman, Roy and Gilbert, Seth and Miller, Avery},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2019.6},
  URN =		{urn:nbn:de:0030-drops-117922},
  doi =		{10.4230/LIPIcs.OPODIS.2019.6},
  annote =	{Keywords: Algorithm, Asynchronous system, Byzantine process, Communication abstraction, Distributed computing, Distributed software engineering, Fault-tolerance, Message-passing, Modularity, Read/write snapshot object, Reliable broadcast, Set-constrained message delivery}
}
Document
DOI: 10.4230/LIPIcs.DISC.2017.27
Which Broadcast Abstraction Captures k-Set Agreement?

Authors: Damien Imbs, Achour Mostéfaoui, Matthieu Perrin, and Michel Raynal

Published in: LIPIcs, Volume 91, 31st International Symposium on Distributed Computing (DISC 2017)

Abstract
It is well-known that consensus (one-set agreement) and total order broadcast are equivalent in asynchronous systems prone to process crash failures. Considering wait-free systems, this article addresses and answers the following question: which is the communication abstraction that "captures" k-set agreement? To this end, it introduces a new broadcast communication abstraction, called k-BO-Broadcast, which restricts the disagreement on the local deliveries of the messages that have been broadcast (1-BO-Broadcast boils down to total order broadcast). Hence, in this context, k=1 is not a special number, but only the first integer in an increasing integer sequence. This establishes a new "correspondence" between distributed agreement problems and communication abstractions, which enriches our understanding of the relations linking fundamental issues of fault-tolerant distributed computing.
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Damien Imbs, Achour Mostéfaoui, Matthieu Perrin, and Michel Raynal. Which Broadcast Abstraction Captures k-Set Agreement?. In 31st International Symposium on Distributed Computing (DISC 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 91, pp. 27:1-27:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{imbs_et_al:LIPIcs.DISC.2017.27,
  author =	{Imbs, Damien and Most\'{e}faoui, Achour and Perrin, Matthieu and Raynal, Michel},
  title =	{{Which Broadcast Abstraction Captures k-Set Agreement?}},
  booktitle =	{31st International Symposium on Distributed Computing (DISC 2017)},
  pages =	{27:1--27:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-053-8},
  ISSN =	{1868-8969},
  year =	{2017},
  volume =	{91},
  editor =	{Richa, Andr\'{e}a},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2017.27},
  URN =		{urn:nbn:de:0030-drops-79943},
  doi =		{10.4230/LIPIcs.DISC.2017.27},
  annote =	{Keywords: Agreement problem, Antichain, Asynchronous system, Communication abstraction, Consensus, Message-passing system, Partially ordered set, Process crash}
}
Document
Tutorial
DOI: 10.4230/LIPIcs.OPODIS.2015.1
Signature-Free Communication and Agreement in the Presence of Byzantine Processes (Tutorial)

Authors: Michel Raynal

Published in: LIPIcs, Volume 46, 19th International Conference on Principles of Distributed Systems (OPODIS 2015)

Abstract
Communication and agreement are fundamental abstractions in any distributed system. (If the computing entities do not need to communicate or agree in one way or another, the system is not a distributed system!) This tutorial was devoted to the design of such abstractions built on top of signature-free asynchronous distributed systems prone to Byzantine process failures. It is made up of three parts, each devoted to an abstraction and algorithms that implement it.
Cite as

Michel Raynal. Signature-Free Communication and Agreement in the Presence of Byzantine Processes (Tutorial). In 19th International Conference on Principles of Distributed Systems (OPODIS 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 46, pp. 1:1-1:10, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2016)

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@InProceedings{raynal:LIPIcs.OPODIS.2015.1,
  author =	{Raynal, Michel},
  title =	{{Signature-Free Communication and Agreement in the Presence of Byzantine Processes}},
  booktitle =	{19th International Conference on Principles of Distributed Systems (OPODIS 2015)},
  pages =	{1:1--1:10},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-939897-98-9},
  ISSN =	{1868-8969},
  year =	{2016},
  volume =	{46},
  editor =	{Anceaume, Emmanuelle and Cachin, Christian and Potop-Butucaru, Maria},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2015.1},
  URN =		{urn:nbn:de:0030-drops-65929},
  doi =		{10.4230/LIPIcs.OPODIS.2015.1},
  annote =	{Keywords: Asynchronous system, Atomic read/write register, Byzantine process Consensus, Distributed algorithm, Distributed computability, Fault-tolerance, No-du}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2015.18
Anonymous Obstruction-Free (n,k)-Set Agreement with n-k+1 Atomic Read/Write Registers

Authors: Zohir Bouzid, Michel Raynal, and Pierre Sutra

Published in: LIPIcs, Volume 46, 19th International Conference on Principles of Distributed Systems (OPODIS 2015)

Abstract
The k-set agreement problem is a generalization of the consensus problem. Namely, assuming that each process proposes a value, every non-faulty process should decide one of the proposed values, and no more than k different values should be decided. This is a hard problem in the sense that we cannot solve it in an asynchronous system, as soon as k or more processes may crash. One way to sidestep this impossibility result consists in weakening the termination property, requiring that a process must decide a value only if it executes alone during a long enough period of time. This is the well-known obstruction-freedom progress condition. Consider a system of n anonymous asynchronous processes that communicate through atomic read/write registers, and such that any number of them may crash. In this paper, we address and solve the challenging open problem of designing an obstruction-free k-set agreement algorithm using only (n-k+1) atomic registers. From a shared memory cost point of view, our algorithm is the best algorithm known so far, thereby establishing a new upper bound on the number of registers needed to solve the problem, and in comparison to the previous upper bound, its gain is (n-k) registers. We then extend this algorithm into a space-optimal solution for the repeated version of k-set agreement, and an x-obstruction-free solution that employs 0(n-k+x) atomic registers (with 1 <= x <= k < n).
Cite as

Zohir Bouzid, Michel Raynal, and Pierre Sutra. Anonymous Obstruction-Free (n,k)-Set Agreement with n-k+1 Atomic Read/Write Registers. In 19th International Conference on Principles of Distributed Systems (OPODIS 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 46, pp. 18:1-18:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2016)

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@InProceedings{bouzid_et_al:LIPIcs.OPODIS.2015.18,
  author =	{Bouzid, Zohir and Raynal, Michel and Sutra, Pierre},
  title =	{{Anonymous Obstruction-Free (n,k)-Set Agreement with n-k+1 Atomic Read/Write Registers}},
  booktitle =	{19th International Conference on Principles of Distributed Systems (OPODIS 2015)},
  pages =	{18:1--18:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-939897-98-9},
  ISSN =	{1868-8969},
  year =	{2016},
  volume =	{46},
  editor =	{Anceaume, Emmanuelle and Cachin, Christian and Potop-Butucaru, Maria},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2015.18},
  URN =		{urn:nbn:de:0030-drops-66092},
  doi =		{10.4230/LIPIcs.OPODIS.2015.18},
  annote =	{Keywords: Anonymous processes, Asynchronous system, Atomic read/write register, Consensus, Fault-tolerance, \$k\$-Set agreement, Obstruction-freedom, Upper bound}
}
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