LIPIcs, Volume 209

35th International Symposium on Distributed Computing (DISC 2021)



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Event

DISC 2021, October 4-8, 2021, Freiburg, Germany (Virtual Conference)

Editor

Seth Gilbert
  • National University of Singapore

Publication Details

  • published at: 2021-10-04
  • Publisher: Schloss Dagstuhl – Leibniz-Zentrum für Informatik
  • ISBN: 978-3-95977-210-5
  • DBLP: db/conf/wdag/disc2021

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Document
Complete Volume
LIPIcs, Volume 209, DISC 2021, Complete Volume

Authors: Seth Gilbert


Abstract
LIPIcs, Volume 209, DISC 2021, Complete Volume

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35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 1-860, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@Proceedings{gilbert:LIPIcs.DISC.2021,
  title =	{{LIPIcs, Volume 209, DISC 2021, Complete Volume}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{1--860},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021},
  URN =		{urn:nbn:de:0030-drops-148015},
  doi =		{10.4230/LIPIcs.DISC.2021},
  annote =	{Keywords: LIPIcs, Volume 209, DISC 2021, Complete Volume}
}
Document
Front Matter
Front Matter, Table of Contents, Preface, Conference Organization

Authors: Seth Gilbert


Abstract
Front Matter, Table of Contents, Preface, Conference Organization

Cite as

35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 0:i-0:xxii, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{gilbert:LIPIcs.DISC.2021.0,
  author =	{Gilbert, Seth},
  title =	{{Front Matter, Table of Contents, Preface, Conference Organization}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{0:i--0:xxii},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.0},
  URN =		{urn:nbn:de:0030-drops-148028},
  doi =		{10.4230/LIPIcs.DISC.2021.0},
  annote =	{Keywords: Front Matter, Table of Contents, Preface, Conference Organization}
}
Document
Invited Talk
The Quest for Universally-Optimal Distributed Algorithms (Invited Talk)

Authors: Bernhard Haeupler


Abstract
Many distributed optimization algorithms achieve an existentially-optimal round complexity (of (Õ(√n + D)), i.e., there exists some pathological worst-case topology on which no algorithm can be faster. However, most networks of interest allow for exponentially faster algorithms. This motivates two questions: - What network topology parameters determine the complexity of distributed optimization? - Are there universally-optimal algorithms that are as fast as possible on every single topology? This talk provides an overview over the freshly-completed 6-year program that resolves these 25-year-old open problems for a wide class of global network optimization problems including MST, (1+ε)-min cut, various approximate shortest path problems, sub-graph connectivity, etc. We provide several equivalent graph parameters that are tight universal lower bounds for the above problems, fully characterizing their inherent complexity. We also give the first universally-optimal algorithms approximately achieving this complexity on every topology. The quest for universally-optimal distributed algorithms required novel techniques that also answer fundamental (open) questions in seemingly unrelated fields, such as, network information theory, approximation algorithms, (oblivious) packet routing, (algorithmic & topological) graph theory, and metric embeddings. Generally, the problems addressed in these fields explicitly or implicitly ask to jointly optimize 𝓁_∞ & 𝓁₁ parameters such as congestion & dilation, communication rate & delay, capacities & diameters of subnetworks, or the makespan of packet routings. In particular, results obtained on the way include the following firsts: (Congestion+Dilation)-Competitive Oblivious Routing, Network Coding Gaps for Completion-Times, Hop-Constrained Expanders & Expander Decompositions, Bi-Criteria (Online / Demand-Robust) Approximation Algorithms for many Diameter-Constrained Network Design Problems (e.g., (Group) Steiner Tree/Forest), Makespan-Competitive (Compact and Distributed) Routing Tables, and (Probabilistic) Tree Embeddings for Hop-Constrained Distances. (Joint work with M. Ghaffari, G. Zuzic, D.E. Hershkowitz, D. Wajc, J. Li, H. Raecke, T. Izumi)

Cite as

Bernhard Haeupler. The Quest for Universally-Optimal Distributed Algorithms (Invited Talk). In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, p. 1:1, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{haeupler:LIPIcs.DISC.2021.1,
  author =	{Haeupler, Bernhard},
  title =	{{The Quest for Universally-Optimal Distributed Algorithms}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{1:1--1:1},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.1},
  URN =		{urn:nbn:de:0030-drops-148030},
  doi =		{10.4230/LIPIcs.DISC.2021.1},
  annote =	{Keywords: Distributed algorithms}
}
Document
Invited Talk
Tech Transfer Stories and Takeaways (Invited Talk)

Authors: Dahlia Malkhi


Abstract
In this talk, I will share impressions from several industrial research project experiences that reached production and became part of successful products. I will go through four stories of how these systems transpired and their journey to impact. All of the stories are in the distributed computing arena, and more specifically, they revolve around the state-machine-replication paradigm. Yet, I hope that the take-aways from the experience of building foundations for these systems may be of interest and value to everyone, no matter the discipline.

Cite as

Dahlia Malkhi. Tech Transfer Stories and Takeaways (Invited Talk). In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, p. 2:1, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{malkhi:LIPIcs.DISC.2021.2,
  author =	{Malkhi, Dahlia},
  title =	{{Tech Transfer Stories and Takeaways}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{2:1--2:1},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.2},
  URN =		{urn:nbn:de:0030-drops-148045},
  doi =		{10.4230/LIPIcs.DISC.2021.2},
  annote =	{Keywords: Tech Transfer, Distributed Systems}
}
Document
Frugal Byzantine Computing

Authors: Marcos K. Aguilera, Naama Ben-David, Rachid Guerraoui, Dalia Papuc, Athanasios Xygkis, and Igor Zablotchi


Abstract
Traditional techniques for handling Byzantine failures are expensive: digital signatures are too costly, while using 3f+1 replicas is uneconomical (f denotes the maximum number of Byzantine processes). We seek algorithms that reduce the number of replicas to 2f+1 and minimize the number of signatures. While the first goal can be achieved in the message-and-memory model, accomplishing the second goal simultaneously is challenging. We first address this challenge for the problem of broadcasting messages reliably. We study two variants of this problem, Consistent Broadcast and Reliable Broadcast, typically considered very close. Perhaps surprisingly, we establish a separation between them in terms of signatures required. In particular, we show that Consistent Broadcast requires at least 1 signature in some execution, while Reliable Broadcast requires O(n) signatures in some execution. We present matching upper bounds for both primitives within constant factors. We then turn to the problem of consensus and argue that this separation matters for solving consensus with Byzantine failures: we present a practical consensus algorithm that uses Consistent Broadcast as its main communication primitive. This algorithm works for n = 2f+1 and avoids signatures in the common case - properties that have not been simultaneously achieved previously. Overall, our work approaches Byzantine computing in a frugal manner and motivates the use of Consistent Broadcast - rather than Reliable Broadcast - as a key primitive for reaching agreement.

Cite as

Marcos K. Aguilera, Naama Ben-David, Rachid Guerraoui, Dalia Papuc, Athanasios Xygkis, and Igor Zablotchi. Frugal Byzantine Computing. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 3:1-3:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{aguilera_et_al:LIPIcs.DISC.2021.3,
  author =	{Aguilera, Marcos K. and Ben-David, Naama and Guerraoui, Rachid and Papuc, Dalia and Xygkis, Athanasios and Zablotchi, Igor},
  title =	{{Frugal Byzantine Computing}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{3:1--3:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.3},
  URN =		{urn:nbn:de:0030-drops-148051},
  doi =		{10.4230/LIPIcs.DISC.2021.3},
  annote =	{Keywords: Reliable Broadcast, Consistent Broadcast, Consensus, Byzantine Failure, Message-and-memory}
}
Document
Lower Bounds for Shared-Memory Leader Election Under Bounded Write Contention

Authors: Dan Alistarh, Rati Gelashvili, and Giorgi Nadiradze


Abstract
This paper gives tight logarithmic lower bounds on the solo step complexity of leader election in an asynchronous shared-memory model with single-writer multi-reader (SWMR) registers, for both deterministic and randomized obstruction-free algorithms. The approach extends to lower bounds for deterministic and randomized obstruction-free algorithms using multi-writer registers under bounded write concurrency, showing a trade-off between the solo step complexity of a leader election algorithm, and the worst-case number of stalls incurred by a processor in an execution.

Cite as

Dan Alistarh, Rati Gelashvili, and Giorgi Nadiradze. Lower Bounds for Shared-Memory Leader Election Under Bounded Write Contention. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 4:1-4:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{alistarh_et_al:LIPIcs.DISC.2021.4,
  author =	{Alistarh, Dan and Gelashvili, Rati and Nadiradze, Giorgi},
  title =	{{Lower Bounds for Shared-Memory Leader Election Under Bounded Write Contention}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{4:1--4:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.4},
  URN =		{urn:nbn:de:0030-drops-148063},
  doi =		{10.4230/LIPIcs.DISC.2021.4},
  annote =	{Keywords: Lower Bounds, Leader Election, Shared-Memory}
}
Document
Deterministic Distributed Algorithms and Lower Bounds in the Hybrid Model

Authors: Ioannis Anagnostides and Themis Gouleakis


Abstract
The HYBRID model was recently introduced by Augustine et al. [John Augustine et al., 2020] in order to characterize from an algorithmic standpoint the capabilities of networks which combine multiple communication modes. Concretely, it is assumed that the standard LOCAL model of distributed computing is enhanced with the feature of all-to-all communication, but with very limited bandwidth, captured by the node-capacitated clique (NCC). In this work we provide several new insights on the power of hybrid networks for fundamental problems in distributed algorithms. First, we present a deterministic algorithm which solves any problem on a sparse n-node graph in 𝒪̃(√n) rounds of HYBRID, where the notation 𝒪̃(⋅) suppresses polylogarithmic factors of n. We combine this primitive with several sparsification techniques to obtain efficient distributed algorithms for general graphs. Most notably, for the all-pairs shortest paths problem we give deterministic (1 + ε)- and log n/log log n-approximate algorithms for unweighted and weighted graphs respectively with round complexity 𝒪̃(√n) in HYBRID, closely matching the performance of the state of the art randomized algorithm of Kuhn and Schneider [Kuhn and Schneider, 2020]. Moreover, we make a connection with the Ghaffari-Haeupler framework of low-congestion shortcuts [Mohsen Ghaffari and Bernhard Haeupler, 2016], leading - among others - to a (1 + ε)-approximate algorithm for Min-Cut after 𝒪(polylog (n)) rounds, with high probability, even if we restrict local edges to transfer 𝒪(log n) bits per round. Finally, we prove via a reduction from the set disjointness problem that Ω̃(n^{1/3}) rounds are required to determine the radius of an unweighted graph, as well as a (3/2 - ε)-approximation for weighted graphs. As a byproduct, we show an Ω̃(n) round-complexity lower bound for computing a (4/3 - ε)-approximation of the radius in the broadcast variant of the congested clique, even for unweighted graphs.

Cite as

Ioannis Anagnostides and Themis Gouleakis. Deterministic Distributed Algorithms and Lower Bounds in the Hybrid Model. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 5:1-5:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{anagnostides_et_al:LIPIcs.DISC.2021.5,
  author =	{Anagnostides, Ioannis and Gouleakis, Themis},
  title =	{{Deterministic Distributed Algorithms and Lower Bounds in the Hybrid Model}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{5:1--5:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.5},
  URN =		{urn:nbn:de:0030-drops-148077},
  doi =		{10.4230/LIPIcs.DISC.2021.5},
  annote =	{Keywords: Distributed Computing, Hybrid Model, Sparse Graphs, Deterministic Algorithms, All-Pairs Shortest Paths, Minimum Cut, Radius}
}
Document
Ruling Sets in Random Order and Adversarial Streams

Authors: Sepehr Assadi and Aditi Dudeja


Abstract
The goal of this paper is to understand the complexity of a key symmetry breaking problem, namely the (α,β)-ruling set problem in the graph streaming model. Given a graph G = (V,E), an (α, β)-ruling set is a subset I ⊆ V such that the distance between any two vertices in I is at least α and the distance between a vertex in V and the closest vertex in I is at most β. This is a fundamental problem in distributed computing where it finds applications as a useful subroutine for other problems such as maximal matching, distributed colouring, or shortest paths. Additionally, it is a generalization of MIS, which is a (2,1)-ruling set. Our main results are two algorithms for (2,2)-ruling sets: 1) In adversarial streams, where the order in which edges arrive is arbitrary, we give an algorithm with Õ(n^{4/3}) space, improving upon the best known algorithm due to Konrad et al. [DISC 2019], with space Õ(n^{3/2}). 2) In random-order streams, where the edges arrive in a random order, we give a semi-streaming algorithm, that is an algorithm that takes Õ(n) space. Finally, we present new algorithms and lower bounds for (α,β)-ruling sets for other values of α and β. Our algorithms improve and generalize the previous work of Konrad et al. [DISC 2019] for (2,β)-ruling sets, while our lower bound establishes the impossibility of obtaining any non-trivial streaming algorithm for (α,α-1)-ruling sets for all even α > 2.

Cite as

Sepehr Assadi and Aditi Dudeja. Ruling Sets in Random Order and Adversarial Streams. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 6:1-6:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{assadi_et_al:LIPIcs.DISC.2021.6,
  author =	{Assadi, Sepehr and Dudeja, Aditi},
  title =	{{Ruling Sets in Random Order and Adversarial Streams}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{6:1--6:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.6},
  URN =		{urn:nbn:de:0030-drops-148086},
  doi =		{10.4230/LIPIcs.DISC.2021.6},
  annote =	{Keywords: Symmetry breaking, Ruling sets, Lower bounds, Communication Complexity}
}
Document
Impossibility of Strongly-Linearizable Message-Passing Objects via Simulation by Single-Writer Registers

Authors: Hagit Attiya, Constantin Enea, and Jennifer L. Welch


Abstract
A key way to construct complex distributed systems is through modular composition of linearizable concurrent objects. A prominent example is shared registers, which have crash-tolerant implementations on top of message-passing systems, allowing the advantages of shared memory to carry over to message-passing. Yet linearizable registers do not always behave properly when used inside randomized programs. A strengthening of linearizability, called strong linearizability, has been shown to preserve probabilistic behavior, as well as other "hypersafety" properties. In order to exploit composition and abstraction in message-passing systems, it is crucial to know whether there exist strongly-linearizable implementations of registers in message-passing. This paper answers the question in the negative: there are no strongly-linearizable fault-tolerant message-passing implementations of multi-writer registers, max-registers, snapshots or counters. This result is proved by reduction from the corresponding result by Helmi et al. The reduction is a novel extension of the BG simulation that connects shared-memory and message-passing, supports long-lived objects, and preserves strong linearizability. The main technical challenge arises from the discrepancy between the potentially minuscule fraction of failures to be tolerated in the simulated message-passing algorithm and the large fraction of failures that can afflict the simulating shared-memory system. The reduction is general and can be viewed as the inverse of the ABD simulation of shared memory in message-passing.

Cite as

Hagit Attiya, Constantin Enea, and Jennifer L. Welch. Impossibility of Strongly-Linearizable Message-Passing Objects via Simulation by Single-Writer Registers. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 7:1-7:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{attiya_et_al:LIPIcs.DISC.2021.7,
  author =	{Attiya, Hagit and Enea, Constantin and Welch, Jennifer L.},
  title =	{{Impossibility of Strongly-Linearizable Message-Passing Objects via Simulation by Single-Writer Registers}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{7:1--7:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.7},
  URN =		{urn:nbn:de:0030-drops-148096},
  doi =		{10.4230/LIPIcs.DISC.2021.7},
  annote =	{Keywords: Concurrent Objects, Message-passing systems, Strong linearizability, Impossibility proofs, BG simulation, Shared registers}
}
Document
Locally Checkable Labelings with Small Messages

Authors: Alkida Balliu, Keren Censor-Hillel, Yannic Maus, Dennis Olivetti, and Jukka Suomela


Abstract
A rich line of work has been addressing the computational complexity of locally checkable labelings (LCLs), illustrating the landscape of possible complexities. In this paper, we study the landscape of LCL complexities under bandwidth restrictions. Our main results are twofold. First, we show that on trees, the CONGEST complexity of an LCL problem is asymptotically equal to its complexity in the LOCAL model. An analog statement for non-LCL problems is known to be false. Second, we show that for general graphs this equivalence does not hold, by providing an LCL problem for which we show that it can be solved in O(log n) rounds in the LOCAL model, but requires Ω̃(n^{1/2}) rounds in the CONGEST model.

Cite as

Alkida Balliu, Keren Censor-Hillel, Yannic Maus, Dennis Olivetti, and Jukka Suomela. Locally Checkable Labelings with Small Messages. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 8:1-8:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{balliu_et_al:LIPIcs.DISC.2021.8,
  author =	{Balliu, Alkida and Censor-Hillel, Keren and Maus, Yannic and Olivetti, Dennis and Suomela, Jukka},
  title =	{{Locally Checkable Labelings with Small Messages}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{8:1--8:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.8},
  URN =		{urn:nbn:de:0030-drops-148109},
  doi =		{10.4230/LIPIcs.DISC.2021.8},
  annote =	{Keywords: distributed graph algorithms, CONGEST, locally checkable labelings}
}
Document
Randomized Local Fast Rerouting for Datacenter Networks with Almost Optimal Congestion

Authors: Gregor Bankhamer, Robert Elsässer, and Stefan Schmid


Abstract
To ensure high availability, datacenter networks must rely on local fast rerouting mechanisms that allow routers to quickly react to link failures, in a fully decentralized manner. However, configuring these mechanisms to provide a high resilience against multiple failures while avoiding congestion along failover routes is algorithmically challenging, as the rerouting rules can only depend on local failure information and must be defined ahead of time. This paper presents a randomized local fast rerouting algorithm for Clos networks, the predominant datacenter topologies. Given a graph G = (V,E) describing a Clos topology, our algorithm defines local routing rules for each node v ∈ V, which only depend on the packet’s destination and are conditioned on the incident link failures. We prove that as long as number of failures at each node does not exceed a certain bound, our algorithm achieves an asymptotically minimal congestion up to polyloglog factors along failover paths. Our lower bounds are developed under some natural routing assumptions.

Cite as

Gregor Bankhamer, Robert Elsässer, and Stefan Schmid. Randomized Local Fast Rerouting for Datacenter Networks with Almost Optimal Congestion. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 9:1-9:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{bankhamer_et_al:LIPIcs.DISC.2021.9,
  author =	{Bankhamer, Gregor and Els\"{a}sser, Robert and Schmid, Stefan},
  title =	{{Randomized Local Fast Rerouting for Datacenter Networks with Almost Optimal Congestion}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{9:1--9:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.9},
  URN =		{urn:nbn:de:0030-drops-148117},
  doi =		{10.4230/LIPIcs.DISC.2021.9},
  annote =	{Keywords: local failover routing, congestion, randomized algorithms, datacenter networks}
}
Document
Deterministic Logarithmic Completeness in the Distributed Sleeping Model

Authors: Leonid Barenboim and Tzalik Maimon


Abstract
In this paper we provide a deterministic scheme for solving any decidable problem in the distributed sleeping model. The sleeping model [Valerie King et al., 2011; Soumyottam Chatterjee et al., 2020] is a generalization of the standard message-passing model, with an additional capability of network nodes to enter a sleeping state occasionally. As long as a vertex is in the awake state, it is similar to the standard message-passing setting. However, when a vertex is asleep it cannot receive or send messages in the network nor can it perform internal computations. On the other hand, sleeping rounds do not count towards awake complexity. Awake complexity is the main complexity measurement in this setting, which is the number of awake rounds a vertex spends during an execution. In this paper we devise algorithms with worst-case guarantees on the awake complexity. We devise a deterministic scheme with awake complexity of O(log n) for solving any decidable problem in this model by constructing a structure we call Distributed Layered Tree. This structure turns out to be very powerful in the sleeping model, since it allows one to collect the entire graph information within a constant number of awake rounds. Moreover, we prove that our general technique cannot be improved in this model, by showing that the construction of distributed layered trees itself requires Ω(log n) awake rounds. This is obtained by a reduction from message-complexity lower bounds, which is of independent interest. Furthermore, our scheme also works in the CONGEST setting where we are limited to messages of size at most O(log n) bits. This result is shown for a certain class of problems, which contains problems of great interest in the research of the distributed setting. Examples for problems we can solve under this limitation are leader election, computing exact number of edges and average degree. Another result we obtain in this work is a deterministic scheme for solving any problem from a class of problems, denoted O-LOCAL, in O(log Δ + log^*n) awake rounds. This class contains various well-studied problems, such as MIS and (Δ+1)-vertex-coloring. Our main structure in this case is a tree as well, but is sharply different from a distributed layered tree. In particular, it is constructed in the local memory of each processor, rather than distributively. Nevertheless, it provides an efficient synchronization scheme for problems of the O-LOCAL class.

Cite as

Leonid Barenboim and Tzalik Maimon. Deterministic Logarithmic Completeness in the Distributed Sleeping Model. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 10:1-10:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{barenboim_et_al:LIPIcs.DISC.2021.10,
  author =	{Barenboim, Leonid and Maimon, Tzalik},
  title =	{{Deterministic Logarithmic Completeness in the Distributed Sleeping Model}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{10:1--10:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.10},
  URN =		{urn:nbn:de:0030-drops-148123},
  doi =		{10.4230/LIPIcs.DISC.2021.10},
  annote =	{Keywords: Distributed Computing, Sleeping Model, Complexity Class}
}
Document
Wait-Free CAS-Based Algorithms: The Burden of the Past

Authors: Denis Bédin, François Lépine, Achour Mostéfaoui, Damien Perez, and Matthieu Perrin


Abstract
Herlihy proved that CAS is universal in the classical computing system model composed of an a priori known number of processes. This means that CAS can implement, together with reads and writes, any object with a sequential specification. For this, he proposed the first universal construction capable of emulating any data structure. It has recently been proved that CAS is still universal in the infinite arrival computing model, a model where any number of processes can be created on the fly (e.g. multi-threaded systems). In this paper, we prove that CAS does not allow to implement wait-free and linearizable visible objects in the infinite model with a space complexity bounded by the number of active processes (i.e. ones that have operations in progress on this object). This paper also shows that this lower bound is tight, in the sense that this dependency can be made as low as desired (e.g. logarithmic) by proposing a wait-free and linearizable universal construction, using the compare-and-swap operation, whose space complexity in the number of ever issued operations is defined by a parameter that can be linked to any unbounded function.

Cite as

Denis Bédin, François Lépine, Achour Mostéfaoui, Damien Perez, and Matthieu Perrin. Wait-Free CAS-Based Algorithms: The Burden of the Past. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 11:1-11:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{bedin_et_al:LIPIcs.DISC.2021.11,
  author =	{B\'{e}din, Denis and L\'{e}pine, Fran\c{c}ois and Most\'{e}faoui, Achour and Perez, Damien and Perrin, Matthieu},
  title =	{{Wait-Free CAS-Based Algorithms: The Burden of the Past}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{11:1--11:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.11},
  URN =		{urn:nbn:de:0030-drops-148131},
  doi =		{10.4230/LIPIcs.DISC.2021.11},
  annote =	{Keywords: Compare-And-Swap, Concurrent Object, Infinite arrival model, Linearizability, Memory complexity, Multi-Threaded Systems, Shared-Memory, Universality, Wait-freedom}
}
Document
Space and Time Bounded Multiversion Garbage Collection

Authors: Naama Ben-David, Guy E. Blelloch, Panagiota Fatourou, Eric Ruppert, Yihan Sun, and Yuanhao Wei


Abstract
We present a general technique for garbage collecting old versions for multiversion concurrency control that simultaneously achieves good time and space complexity. Our technique takes only O(1) time on average to reclaim each version and maintains only a constant factor more versions than needed (plus an additive term). It is designed for multiversion schemes using version lists, which are the most common. Our approach uses two components that are of independent interest. First, we define a novel range-tracking data structure which stores a set of old versions and efficiently finds those that are no longer needed. We provide a wait-free implementation in which all operations take amortized constant time. Second, we represent version lists using a new lock-free doubly-linked list algorithm that supports efficient (amortized constant time) removals given a pointer to any node in the list. These two components naturally fit together to solve the multiversion garbage collection problem - the range-tracker identifies which versions to remove and our list algorithm can then be used to remove them from their version lists. We apply our garbage collection technique to generate end-to-end time and space bounds for the multiversioning system of Wei et al. (PPoPP 2021).

Cite as

Naama Ben-David, Guy E. Blelloch, Panagiota Fatourou, Eric Ruppert, Yihan Sun, and Yuanhao Wei. Space and Time Bounded Multiversion Garbage Collection. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 12:1-12:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{bendavid_et_al:LIPIcs.DISC.2021.12,
  author =	{Ben-David, Naama and Blelloch, Guy E. and Fatourou, Panagiota and Ruppert, Eric and Sun, Yihan and Wei, Yuanhao},
  title =	{{Space and Time Bounded Multiversion Garbage Collection}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{12:1--12:20},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.12},
  URN =		{urn:nbn:de:0030-drops-148143},
  doi =		{10.4230/LIPIcs.DISC.2021.12},
  annote =	{Keywords: Lock-free, data structures, memory management, snapshot, version lists}
}
Document
A Tight Local Algorithm for the Minimum Dominating Set Problem in Outerplanar Graphs

Authors: Marthe Bonamy, Linda Cook, Carla Groenland, and Alexandra Wesolek


Abstract
We show that there is a deterministic local algorithm (constant-time distributed graph algorithm) that finds a 5-approximation of a minimum dominating set on outerplanar graphs. We show there is no such algorithm that finds a (5-ε)-approximation, for any ε > 0. Our algorithm only requires knowledge of the degree of a vertex and of its neighbors, so that large messages and unique identifiers are not needed.

Cite as

Marthe Bonamy, Linda Cook, Carla Groenland, and Alexandra Wesolek. A Tight Local Algorithm for the Minimum Dominating Set Problem in Outerplanar Graphs. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 13:1-13:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{bonamy_et_al:LIPIcs.DISC.2021.13,
  author =	{Bonamy, Marthe and Cook, Linda and Groenland, Carla and Wesolek, Alexandra},
  title =	{{A Tight Local Algorithm for the Minimum Dominating Set Problem in Outerplanar Graphs}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{13:1--13:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.13},
  URN =		{urn:nbn:de:0030-drops-148159},
  doi =		{10.4230/LIPIcs.DISC.2021.13},
  annote =	{Keywords: Outerplanar graphs, dominating set, LOCAL model, constant-factor approximation algorithm}
}
Document
Fast Nonblocking Persistence for Concurrent Data Structures

Authors: Wentao Cai, Haosen Wen, Vladimir Maksimovski, Mingzhe Du, Rafaello Sanna, Shreif Abdallah, and Michael L. Scott


Abstract
We present a fully lock-free variant of our recent Montage system for persistent data structures. The variant, nbMontage, adds persistence to almost any nonblocking concurrent structure without introducing significant overhead or blocking of any kind. Like its predecessor, nbMontage is buffered durably linearizable: it guarantees that the state recovered in the wake of a crash will represent a consistent prefix of pre-crash execution. Unlike its predecessor, nbMontage ensures wait-free progress of the persistence frontier, thereby bounding the number of recent updates that may be lost on a crash, and allowing a thread to force an update of the frontier (i.e., to perform a sync operation) without the risk of blocking. As an extra benefit, the helping mechanism employed by our wait-free sync significantly reduces its latency. Performance results for nonblocking queues, skip lists, trees, and hash tables rival custom data structures in the literature - dramatically faster than achieved with prior general-purpose systems, and generally within 50% of equivalent non-persistent structures placed in DRAM.

Cite as

Wentao Cai, Haosen Wen, Vladimir Maksimovski, Mingzhe Du, Rafaello Sanna, Shreif Abdallah, and Michael L. Scott. Fast Nonblocking Persistence for Concurrent Data Structures. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 14:1-14:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{cai_et_al:LIPIcs.DISC.2021.14,
  author =	{Cai, Wentao and Wen, Haosen and Maksimovski, Vladimir and Du, Mingzhe and Sanna, Rafaello and Abdallah, Shreif and Scott, Michael L.},
  title =	{{Fast Nonblocking Persistence for Concurrent Data Structures}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{14:1--14:20},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.14},
  URN =		{urn:nbn:de:0030-drops-148169},
  doi =		{10.4230/LIPIcs.DISC.2021.14},
  annote =	{Keywords: Persistent Memory, Nonblocking Progress, Buffered Durable Linearizability}
}
Document
Massively Parallel Correlation Clustering in Bounded Arboricity Graphs

Authors: Mélanie Cambus, Davin Choo, Havu Miikonen, and Jara Uitto


Abstract
Identifying clusters of similar elements in a set is a common task in data analysis. With the immense growth of data and physical limitations on single processor speed, it is necessary to find efficient parallel algorithms for clustering tasks. In this paper, we study the problem of correlation clustering in bounded arboricity graphs with respect to the Massively Parallel Computation (MPC) model. More specifically, we are given a complete graph where the edges are either positive or negative, indicating whether pairs of vertices are similar or dissimilar. The task is to partition the vertices into clusters with as few disagreements as possible. That is, we want to minimize the number of positive inter-cluster edges and negative intra-cluster edges. Consider an input graph G on n vertices such that the positive edges induce a λ-arboric graph. Our main result is a 3-approximation (in expectation) algorithm to correlation clustering that runs in 𝒪(log λ ⋅ poly(log log n)) MPC rounds in the strongly sublinear memory regime. This is obtained by combining structural properties of correlation clustering on bounded arboricity graphs with the insights of Fischer and Noever (SODA '18) on randomized greedy MIS and the PIVOT algorithm of Ailon, Charikar, and Newman (STOC '05). Combined with known graph matching algorithms, our structural property also implies an exact algorithm and algorithms with worst case (1+ε)-approximation guarantees in the special case of forests, where λ = 1.

Cite as

Mélanie Cambus, Davin Choo, Havu Miikonen, and Jara Uitto. Massively Parallel Correlation Clustering in Bounded Arboricity Graphs. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 15:1-15:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{cambus_et_al:LIPIcs.DISC.2021.15,
  author =	{Cambus, M\'{e}lanie and Choo, Davin and Miikonen, Havu and Uitto, Jara},
  title =	{{Massively Parallel Correlation Clustering in Bounded Arboricity Graphs}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{15:1--15:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.15},
  URN =		{urn:nbn:de:0030-drops-148173},
  doi =		{10.4230/LIPIcs.DISC.2021.15},
  annote =	{Keywords: MPC Algorithm, Correlation Clustering, Bounded Arboricity}
}
Document
Fully Read/Write Fence-Free Work-Stealing with Multiplicity

Authors: Armando Castañeda and Miguel Piña


Abstract
It is known that any algorithm for work-stealing in the standard asynchronous shared memory model must use expensive Read-After-Write synchronization patterns or atomic Read-Modify-Write instructions. There have been proposed algorithms for relaxations in the standard model and algorithms in restricted models that avoid the impossibility result, but only in some operations. This paper considers work-stealing with multiplicity, a relaxation in which every task is taken by at least one operation, with the requirement that any process can extract a task at most once. Two versions of the relaxation are considered and two fully Read/Write algorithms are presented in the standard asynchronous shared memory model, both devoid of Read-After-Write synchronization patterns in all its operations, the second algorithm additionally being fully fence-free, namely, no specific ordering among the algorithm’s instructions is required, beyond what is implied by data dependence. To our knowledge, these are the first algorithms for work-stealing possessing all these properties. Our algorithms are also wait-free solutions of relaxed versions of single-enqueue multi-dequeuer queues. The algorithms are obtained by reducing work-stealing with multiplicity and weak multiplicity to MaxRegister and RangeMaxRegister, a relaxation of MaxRegister which might be of independent interest. An experimental evaluation shows that our fully fence-free algorithm exhibits better performance than Cilk THE, Chase-Lev and Idempotent Work-Stealing algorithms.

Cite as

Armando Castañeda and Miguel Piña. Fully Read/Write Fence-Free Work-Stealing with Multiplicity. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 16:1-16:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{castaneda_et_al:LIPIcs.DISC.2021.16,
  author =	{Casta\~{n}eda, Armando and Pi\~{n}a, Miguel},
  title =	{{Fully Read/Write Fence-Free Work-Stealing with Multiplicity}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{16:1--16:20},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.16},
  URN =		{urn:nbn:de:0030-drops-148181},
  doi =		{10.4230/LIPIcs.DISC.2021.16},
  annote =	{Keywords: Correctness condition, Linearizability, Nonblocking, Relaxed data type, Set-linearizability, Wait-freedom, Work-stealing}
}
Document
Optimal Error-Free Multi-Valued Byzantine Agreement

Authors: Jinyuan Chen


Abstract
Byzantine agreement (BA) is a distributed consensus problem where n processors want to reach agreement on an 𝓁-bit message or value, but up to t of the processors are dishonest or faulty. The challenge of this BA problem lies in achieving agreement despite the presence of dishonest processors who may arbitrarily deviate from the designed protocol. In this work by using coding theory, together with graph theory and linear algebra, we design a coded BA protocol (termed as COOL) that achieves consensus on an 𝓁-bit message with optimal resilience, asymptotically optimal round complexity, and asymptotically optimal communication complexity when 𝓁 ≥ t log t, simultaneously. The proposed COOL is a deterministic BA protocol that is guaranteed to be correct in all executions (error free) and does not rely on cryptographic technique such as signatures, hashing, authentication and secret sharing (signature free). It is secure against computationally unbounded adversary who takes full control over the dishonest processors (information-theoretic secure). The main idea of the proposed COOL is to use a carefully-crafted error correction code that provides an efficient way of exchanging "compressed" information among distributed nodes, while keeping the ability of detecting errors, masking errors, and making a consistent and validated agreement at honest distributed nodes. We show that our results can also be extended to the setting of Byzantine broadcast, aka Byzantine generals problem, where the honest processors want to agree on the message sent by a leader who is potentially dishonest. The results reveal that coding is an effective approach for achieving the fundamental limits of Byzantine agreement and its variants. Our protocol analysis borrows tools from coding theory, graph theory and linear algebra.

Cite as

Jinyuan Chen. Optimal Error-Free Multi-Valued Byzantine Agreement. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 17:1-17:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{chen:LIPIcs.DISC.2021.17,
  author =	{Chen, Jinyuan},
  title =	{{Optimal Error-Free Multi-Valued Byzantine Agreement}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{17:1--17:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.17},
  URN =		{urn:nbn:de:0030-drops-148190},
  doi =		{10.4230/LIPIcs.DISC.2021.17},
  annote =	{Keywords: Byzantine agreement, information-theoretic security, error correction codes}
}
Document
Tame the Wild with Byzantine Linearizability: Reliable Broadcast, Snapshots, and Asset Transfer

Authors: Shir Cohen and Idit Keidar


Abstract
We formalize Byzantine linearizability, a correctness condition that specifies whether a concurrent object with a sequential specification is resilient against Byzantine failures. Using this definition, we systematically study Byzantine-tolerant emulations of various objects from registers. We focus on three useful objects- reliable broadcast, atomic snapshot, and asset transfer. We prove that there exist n-process f-resilient Byzantine linearizable implementations of such objects from registers if and only if f < n/2.

Cite as

Shir Cohen and Idit Keidar. Tame the Wild with Byzantine Linearizability: Reliable Broadcast, Snapshots, and Asset Transfer. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 18:1-18:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{cohen_et_al:LIPIcs.DISC.2021.18,
  author =	{Cohen, Shir and Keidar, Idit},
  title =	{{Tame the Wild with Byzantine Linearizability: Reliable Broadcast, Snapshots, and Asset Transfer}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{18:1--18:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.18},
  URN =		{urn:nbn:de:0030-drops-148203},
  doi =		{10.4230/LIPIcs.DISC.2021.18},
  annote =	{Keywords: Byzantine linearizability, concurrent algorithms, snapshot, asset transfer}
}
Document
Wake up and Join Me! an Energy-Efficient Algorithm for Maximal Matching in Radio Networks

Authors: Varsha Dani, Aayush Gupta, Thomas P. Hayes, and Seth Pettie


Abstract
We consider networks of small, autonomous devices that communicate with each other wirelessly. Minimizing energy usage is an important consideration in designing algorithms for such networks, as battery life is a crucial and limited resource. Working in a model where both sending and listening for messages deplete energy, we consider the problem of finding a maximal matching of the nodes in a radio network of arbitrary and unknown topology. We present a distributed randomized algorithm that produces, with high probability, a maximal matching. The maximum energy cost per node is O(log² n), and the time complexity is O(Δ log n). Here n is any upper bound on the number of nodes, and Δ is any upper bound on the maximum degree; n and Δ are parameters of our algorithm that we assume are known a priori to all the processors. We note that there exist families of graphs for which our bounds on energy cost and time complexity are simultaneously optimal up to polylog factors, so any significant improvement would need additional assumptions about the network topology. We also consider the related problem of assigning, for each node in the network, a neighbor to back up its data in case of eventual node failure. Here, a key goal is to minimize the maximum load, defined as the number of nodes assigned to a single node. We present an efficient decentralized low-energy algorithm that finds a neighbor assignment whose maximum load is at most a polylog(n) factor bigger that the optimum.

Cite as

Varsha Dani, Aayush Gupta, Thomas P. Hayes, and Seth Pettie. Wake up and Join Me! an Energy-Efficient Algorithm for Maximal Matching in Radio Networks. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 19:1-19:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{dani_et_al:LIPIcs.DISC.2021.19,
  author =	{Dani, Varsha and Gupta, Aayush and Hayes, Thomas P. and Pettie, Seth},
  title =	{{Wake up and Join Me! an Energy-Efficient Algorithm for Maximal Matching in Radio Networks}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{19:1--19:14},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.19},
  URN =		{urn:nbn:de:0030-drops-148219},
  doi =		{10.4230/LIPIcs.DISC.2021.19},
  annote =	{Keywords: Distributed Algorithms, Energy-Aware Computation, Radio Networks, Maximal Matching, Sensor Networks}
}
Document
The Canonical Amoebot Model: Algorithms and Concurrency Control

Authors: Joshua J. Daymude, Andréa W. Richa, and Christian Scheideler


Abstract
The amoebot model abstracts active programmable matter as a collection of simple computational elements called amoebots that interact locally to collectively achieve tasks of coordination and movement. Since its introduction (SPAA 2014), a growing body of literature has adapted its assumptions for a variety of problems; however, without a standardized hierarchy of assumptions, precise systematic comparison of results under the amoebot model is difficult. We propose the canonical amoebot model, an updated formalization that distinguishes between core model features and families of assumption variants. A key improvement addressed by the canonical amoebot model is concurrency. Much of the existing literature implicitly assumes amoebot actions are isolated and reliable, reducing analysis to the sequential setting where at most one amoebot is active at a time. However, real programmable matter systems are concurrent. The canonical amoebot model formalizes all amoebot communication as message passing, leveraging adversarial activation models of concurrent executions. Under this granular treatment of time, we take two complementary approaches to concurrent algorithm design. Using hexagon formation as a case study, we first establish a set of sufficient conditions for algorithm correctness under any concurrent execution, embedding concurrency control directly in algorithm design. We then present a concurrency control framework that uses locks to convert amoebot algorithms that terminate in the sequential setting and satisfy certain conventions into algorithms that exhibit equivalent behavior in the concurrent setting. Together, the canonical amoebot model and these complementary approaches to concurrent algorithm design open new directions for distributed computing research on programmable matter.

Cite as

Joshua J. Daymude, Andréa W. Richa, and Christian Scheideler. The Canonical Amoebot Model: Algorithms and Concurrency Control. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 20:1-20:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{daymude_et_al:LIPIcs.DISC.2021.20,
  author =	{Daymude, Joshua J. and Richa, Andr\'{e}a W. and Scheideler, Christian},
  title =	{{The Canonical Amoebot Model: Algorithms and Concurrency Control}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{20:1--20:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.20},
  URN =		{urn:nbn:de:0030-drops-148227},
  doi =		{10.4230/LIPIcs.DISC.2021.20},
  annote =	{Keywords: Programmable matter, self-organization, distributed algorithms, concurrency}
}
Document
Improved Weighted Additive Spanners

Authors: Michael Elkin, Yuval Gitlitz, and Ofer Neiman


Abstract
Graph spanners and emulators are sparse structures that approximately preserve distances of the original graph. While there has been an extensive amount of work on additive spanners, so far little attention was given to weighted graphs. Only very recently [Abu Reyan Ahmed et al., 2020] extended the classical +2 (respectively, +4) spanners for unweighted graphs of size O(n^{3/2}) (resp., O(n^{7/5})) to the weighted setting, where the additive error is +2W (resp., +4W). This means that for every pair u,v, the additive stretch is at most +2W_{u,v}, where W_{u,v} is the maximal edge weight on the shortest u-v path (weights are normalized so that the minimum edge weight is 1). In addition, [Abu Reyan Ahmed et al., 2020] showed a randomized algorithm yielding a +8W_{max} spanner of size O(n^{4/3}), here W_{max} is the maximum edge weight in the entire graph. In this work we improve the latter result by devising a simple deterministic algorithm for a +(6+ε)W spanner for weighted graphs with size O(n^{4/3}) (for any constant ε > 0), thus nearly matching the classical +6 spanner of size O(n^{4/3}) for unweighted graphs. Furthermore, we show a +(2+ε)W subsetwise spanner of size O(n⋅√{|S|}), improving the +4W_{max} result of [Abu Reyan Ahmed et al., 2020] (that had the same size). We also show a simple randomized algorithm for a +4W emulator of size Õ(n^{4/3}). In addition, we show that our technique is applicable for very sparse additive spanners, that have linear size. It is known that such spanners must suffer polynomially large stretch. For weighted graphs, we use a variant of our simple deterministic algorithm that yields a linear size +Õ(√n⋅ W) spanner, and we also obtain a tradeoff between size and stretch. Finally, generalizing the technique of [D. Dor et al., 2000] for unweighted graphs, we devise an efficient randomized algorithm producing a +2W spanner for weighted graphs of size Õ(n^{3/2}) in Õ(n²) time.

Cite as

Michael Elkin, Yuval Gitlitz, and Ofer Neiman. Improved Weighted Additive Spanners. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 21:1-21:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{elkin_et_al:LIPIcs.DISC.2021.21,
  author =	{Elkin, Michael and Gitlitz, Yuval and Neiman, Ofer},
  title =	{{Improved Weighted Additive Spanners}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{21:1--21:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.21},
  URN =		{urn:nbn:de:0030-drops-148232},
  doi =		{10.4230/LIPIcs.DISC.2021.21},
  annote =	{Keywords: Graph theory, Pure additive spanners}
}
Document
Deterministic Size Discovery and Topology Recognition in Radio Networks with Short Labels

Authors: Adam Gańczorz, Tomasz Jurdziński, Mateusz Lewko, and Andrzej Pelc


Abstract
We consider the fundamental problems of size discovery and topology recognition in radio networks modeled by simple undirected connected graphs. Size discovery calls for all nodes to output the number of nodes in the graph, called its size, and in the task of topology recognition each node has to learn the topology of the graph and its position in it. We do not assume collision detection: in case of a collision, node v does not hear anything (except the background noise that it also hears when no neighbor transmits). The time of a deterministic algorithm for each of the above problems is the worst-case number of rounds it takes to solve it. Nodes have labels which are (not necessarily different) binary strings. Each node knows its own label and can use it when executing the algorithm. The length of a labeling scheme is the largest length of a label. For size discovery, we construct a labeling scheme of length O(log logΔ) (which is known to be optimal, even if collision detection is available) and we design an algorithm for this problem using this scheme and working in time O(log² n), where n is the size of the graph. We also show that time complexity O(log² n) is optimal for the problem of size discovery, whenever the labeling scheme is of optimal length O(log logΔ). For topology recognition, we construct a labeling scheme of length O(logΔ), and we design an algorithm for this problem using this scheme and working in time O (DΔ+min(Δ²,n)), where D is the diameter of the graph. We also show that the length of our labeling scheme is asymptotically optimal.

Cite as

Adam Gańczorz, Tomasz Jurdziński, Mateusz Lewko, and Andrzej Pelc. Deterministic Size Discovery and Topology Recognition in Radio Networks with Short Labels. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 22:1-22:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{ganczorz_et_al:LIPIcs.DISC.2021.22,
  author =	{Ga\'{n}czorz, Adam and Jurdzi\'{n}ski, Tomasz and Lewko, Mateusz and Pelc, Andrzej},
  title =	{{Deterministic Size Discovery and Topology Recognition in Radio Networks with Short Labels}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{22:1--22:20},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.22},
  URN =		{urn:nbn:de:0030-drops-148242},
  doi =		{10.4230/LIPIcs.DISC.2021.22},
  annote =	{Keywords: size discovery, topology recognition, radio network, labeling scheme}
}
Document
Broadcast CONGEST Algorithms against Adversarial Edges

Authors: Yael Hitron and Merav Parter


Abstract
We consider the corner-stone broadcast task with an adaptive adversary that controls a fixed number of t edges in the input communication graph. In this model, the adversary sees the entire communication in the network and the random coins of the nodes, while maliciously manipulating the messages sent through a set of t edges (unknown to the nodes). Since the influential work of [Pease, Shostak and Lamport, JACM'80], broadcast algorithms against plentiful adversarial models have been studied in both theory and practice for over more than four decades. Despite this extensive research, there is no round efficient broadcast algorithm for general graphs in the CONGEST model of distributed computing. Even for a single adversarial edge (i.e., t = 1), the state-of-the-art round complexity is polynomial in the number of nodes. We provide the first round-efficient broadcast algorithms against adaptive edge adversaries. Our two key results for n-node graphs of diameter D are as follows: - For t = 1, there is a deterministic algorithm that solves the problem within Õ(D²) rounds, provided that the graph is 3 edge-connected. This round complexity beats the natural barrier of O(D³) rounds, the existential lower bound on the maximal length of 3 edge-disjoint paths between a given pair of nodes in G. This algorithm can be extended to a Õ((tD)^{O(t)})-round algorithm against t adversarial edges in (2t+1) edge-connected graphs. - For expander graphs with edge connectivity of Ω(t²log n), there is a considerably improved broadcast algorithm with O(t log ² n) rounds against t adversarial edges. This algorithm exploits the connectivity and conductance properties of G-subgraphs obtained by employing the Karger’s edge sampling technique. Our algorithms mark a new connection between the areas of fault-tolerant network design and reliable distributed communication.

Cite as

Yael Hitron and Merav Parter. Broadcast CONGEST Algorithms against Adversarial Edges. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 23:1-23:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{hitron_et_al:LIPIcs.DISC.2021.23,
  author =	{Hitron, Yael and Parter, Merav},
  title =	{{Broadcast CONGEST Algorithms against Adversarial Edges}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{23:1--23:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.23},
  URN =		{urn:nbn:de:0030-drops-148256},
  doi =		{10.4230/LIPIcs.DISC.2021.23},
  annote =	{Keywords: CONGEST, Fault-Tolerant Network Design, Edge Connectivity}
}
Document
General CONGEST Compilers against Adversarial Edges

Authors: Yael Hitron and Merav Parter


Abstract
We consider the adversarial CONGEST model of distributed computing in which a fixed number of edges (or nodes) in the graph are controlled by a computationally unbounded adversary that corrupts the computation by sending malicious messages over these (a-priori unknown) controlled edges. As in the standard CONGEST model, communication is synchronous, where per round each processor can send O(log n) bits to each of its neighbors. This paper is concerned with distributed algorithms that are both time efficient (in terms of the number of rounds), as well as, robust against a fixed number of adversarial edges. Unfortunately, the existing algorithms in this setting usually assume that the communication graph is complete (n-clique), and very little is known for graphs with arbitrary topologies. We fill in this gap by extending the methodology of [Parter and Yogev, SODA 2019] and provide a compiler that simulates any CONGEST algorithm 𝒜 (in the reliable setting) into an equivalent algorithm 𝒜' in the adversarial CONGEST model. Specifically, we show the following for every (2f+1) edge-connected graph of diameter D: - For f = 1, there is a general compiler against a single adversarial edge with a compilation overhead of Ô(D³) rounds. This improves upon the Ô(D⁵) round overhead of [Parter and Yogev, SODA 2019] and omits their assumption regarding a fault-free preprocessing phase. - For any constant f, there is a general compiler against f adversarial edges with a compilation overhead of Ô(D^{O(f)}) rounds. The prior compilers of [Parter and Yogev, SODA 2019] were limited to a single adversarial edge. Our compilers are based on a new notion of fault-tolerant cycle covers. The computation of these cycles in the adversarial CONGEST model constitutes the key technical contribution of the paper.

Cite as

Yael Hitron and Merav Parter. General CONGEST Compilers against Adversarial Edges. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 24:1-24:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{hitron_et_al:LIPIcs.DISC.2021.24,
  author =	{Hitron, Yael and Parter, Merav},
  title =	{{General CONGEST Compilers against Adversarial Edges}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{24:1--24:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.24},
  URN =		{urn:nbn:de:0030-drops-148266},
  doi =		{10.4230/LIPIcs.DISC.2021.24},
  annote =	{Keywords: CONGEST, Cycle Covers, Byzantine Adversaries}
}
Document
Fast Arrays: Atomic Arrays with Constant Time Initialization

Authors: Siddhartha Jayanti and Julian Shun


Abstract
Some algorithms require a large array, but only operate on a small fraction of its indices. Examples include adjacency matrices for sparse graphs, hash tables, and van Emde Boas trees. For such algorithms, array initialization can be the most time-consuming operation. Fast arrays were invented to avoid this costly initialization. A fast array is a software implementation of an array, such that the entire array can be initialized in just constant time. While algorithms for sequential fast arrays have been known for a long time, to the best of our knowledge, there are no previous algorithms for concurrent fast arrays. We present the first such algorithms in this paper. Our first algorithm is linearizable and wait-free, uses only linear space, and supports all operations - initialize, read, and write - in constant time. Our second algorithm enhances the first to additionally support all the read-modify-write operations available in hardware (such as compare-and-swap) in constant time.

Cite as

Siddhartha Jayanti and Julian Shun. Fast Arrays: Atomic Arrays with Constant Time Initialization. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 25:1-25:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{jayanti_et_al:LIPIcs.DISC.2021.25,
  author =	{Jayanti, Siddhartha and Shun, Julian},
  title =	{{Fast Arrays: Atomic Arrays with Constant Time Initialization}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{25:1--25:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.25},
  URN =		{urn:nbn:de:0030-drops-148278},
  doi =		{10.4230/LIPIcs.DISC.2021.25},
  annote =	{Keywords: fast array, linearizable, wait-free, asynchronous, multiprocessor, constant time, space efficient, data structure}
}
Document
Byzantine Consensus with Local Multicast Channels

Authors: Muhammad Samir Khan and Nitin H. Vaidya


Abstract
Byzantine consensus is a classical problem in distributed computing. Each node in a synchronous system starts with a binary input. The goal is to reach agreement in the presence of Byzantine faulty nodes. We consider the setting where communication between nodes is modelled via an undirected communication graph. In the classical point-to-point communication model all messages sent on an edge are private between the two endpoints of the edge. This allows a faulty node to equivocate, i.e., lie differently to its different neighbors. Different models have been proposed in the literature that weaken equivocation. In the local broadcast model, every message transmitted by a node is received identically and correctly by all of its neighbors. In the hypergraph model, every message transmitted by a node on a hyperedge is received identically and correctly by all nodes on the hyperedge. Tight network conditions are known for each of the three cases. We introduce a more general model that encompasses all three of these models. In the local multicast model, each node u has one or more local multicast channels. Each channel consists of multiple neighbors of u in the communication graph. When node u sends a message on a channel, it is received identically by all of its neighbors on the channel. For this model, we identify tight network conditions for consensus. We observe how the local multicast model reduces to each of the three models above under specific conditions. In each of the three cases, we relate our network condition to the corresponding known tight conditions. The local multicast model also encompasses other practical network models of interest that have not been explored previously, as elaborated in the paper.

Cite as

Muhammad Samir Khan and Nitin H. Vaidya. Byzantine Consensus with Local Multicast Channels. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 26:1-26:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{khan_et_al:LIPIcs.DISC.2021.26,
  author =	{Khan, Muhammad Samir and Vaidya, Nitin H.},
  title =	{{Byzantine Consensus with Local Multicast Channels}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{26:1--26:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.26},
  URN =		{urn:nbn:de:0030-drops-148285},
  doi =		{10.4230/LIPIcs.DISC.2021.26},
  annote =	{Keywords: Byzantine fault, distributed algorithm, consensus, broadcast, multicast}
}
Document
Singularly Near Optimal Leader Election in Asynchronous Networks

Authors: Shay Kutten, William K. Moses Jr., Gopal Pandurangan, and David Peleg


Abstract
This paper concerns designing distributed algorithms that are singularly optimal, i.e., algorithms that are simultaneously time and message optimal, for the fundamental leader election problem in asynchronous networks. Kutten et al. (JACM 2015) presented a singularly near optimal randomized leader election algorithm for general synchronous networks that ran in O(D) time and used O(m log n) messages (where D, m, and n are the network’s diameter, number of edges and number of nodes, respectively) with high probability. Both bounds are near optimal (up to a logarithmic factor), since Ω(D) and Ω(m) are the respective lower bounds for time and messages for leader election even for synchronous networks and even for (Monte-Carlo) randomized algorithms. On the other hand, for general asynchronous networks, leader election algorithms are only known that are either time or message optimal, but not both. Kutten et al. (DISC 2020) presented a randomized asynchronous leader election algorithm that is singularly near optimal for complete networks, but left open the problem for general networks. This paper shows that singularly near optimal (up to polylogarithmic factors) bounds can be achieved for general asynchronous networks. We present a randomized singularly near optimal leader election algorithm that runs in O(D + log² n) time and O(m log² n) messages with high probability. Our result is the first known distributed leader election algorithm for asynchronous networks that is near optimal with respect to both time and message complexity and improves over a long line of results including the classical results of Gallager et al. (ACM TOPLAS, 1983), Peleg (JPDC, 1989), and Awerbuch (STOC, 89).

Cite as

Shay Kutten, William K. Moses Jr., Gopal Pandurangan, and David Peleg. Singularly Near Optimal Leader Election in Asynchronous Networks. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 27:1-27:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{kutten_et_al:LIPIcs.DISC.2021.27,
  author =	{Kutten, Shay and Moses Jr., William K. and Pandurangan, Gopal and Peleg, David},
  title =	{{Singularly Near Optimal Leader Election in Asynchronous Networks}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{27:1--27:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.27},
  URN =		{urn:nbn:de:0030-drops-148294},
  doi =		{10.4230/LIPIcs.DISC.2021.27},
  annote =	{Keywords: Leader election, Singular optimality, Randomized algorithms, Asynchronous networks, Arbitrary graphs}
}
Document
Permissionless and Asynchronous Asset Transfer

Authors: Petr Kuznetsov, Yvonne-Anne Pignolet, Pavel Ponomarev, and Andrei Tonkikh


Abstract
Most modern asset transfer systems use consensus to maintain a totally ordered chain of transactions. It was recently shown that consensus is not always necessary for implementing asset transfer. More efficient, asynchronous solutions can be built using reliable broadcast instead of consensus. This approach has been originally used in the closed (permissioned) setting. In this paper, we extend it to the open (permissionless) environment. We present {Pastro}, a permissionless and asynchronous asset-transfer implementation, in which quorum systems, traditionally used in reliable broadcast, are replaced with a weighted Proof-of-Stake mechanism. {Pastro} tolerates a dynamic adversary that is able to adaptively corrupt participants based on the assets owned by them.

Cite as

Petr Kuznetsov, Yvonne-Anne Pignolet, Pavel Ponomarev, and Andrei Tonkikh. Permissionless and Asynchronous Asset Transfer. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 28:1-28:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{kuznetsov_et_al:LIPIcs.DISC.2021.28,
  author =	{Kuznetsov, Petr and Pignolet, Yvonne-Anne and Ponomarev, Pavel and Tonkikh, Andrei},
  title =	{{Permissionless and Asynchronous Asset Transfer}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{28:1--28:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.28},
  URN =		{urn:nbn:de:0030-drops-148307},
  doi =		{10.4230/LIPIcs.DISC.2021.28},
  annote =	{Keywords: Asset transfer, permissionless, asynchronous, dynamic adversary}
}
Document
Detectable Sequential Specifications for Recoverable Shared Objects

Authors: Nan Li and Wojciech Golab


Abstract
The recent commercial release of persistent main memory by Intel has sparked intense interest in recoverable concurrent objects. Such objects maintain state in persistent memory, and can be recovered directly following a system-wide crash failure, as opposed to being painstakingly rebuilt using recovery state saved in slower secondary storage. Specifying and implementing recoverable objects is technically challenging on current generation hardware precisely because the top layers of the memory hierarchy (CPU registers and cache) remain volatile, which causes application threads to lose critical execution state during a failure. For example, a thread that completes an operation on a shared object and then crashes may have difficulty determining whether this operation took effect, and if so, what response it returned. Friedman, Herlihy, Marathe, and Petrank (DISC'17) recently proposed that this difficulty can be alleviated by making the recoverable objects detectable, meaning that during recovery, they can resolve the status of an operation that was interrupted by a failure. In this paper, we formalize this important concept using a detectable sequential specification (DSS), which augments an object’s interface with auxiliary methods that threads use to first declare their need for detectability, and then perform detection if needed after a failure. Our contribution is closely related to the nesting-safe recoverable linearizability (NRL) framework of Attiya, Ben-Baruch, and Hendler (PODC'18), which follows an orthogonal approach based on ordinary sequential specifications combined with a novel correctness condition. Compared to NRL, our DSS-based approach is more portable across different models of distributed computation, compatible with several existing linearizability-like correctness conditions, less reliant on assumptions regarding the system, and more flexible in the sense that it allows applications to request detectability on demand. On the other hand, application code assumes full responsibility for nesting DSS-based objects. As a proof of concept, we demonstrate the DSS in action by presenting a detectable recoverable lock-free queue algorithm and evaluating its performance on a multiprocessor equipped with Intel Optane persistent memory.

Cite as

Nan Li and Wojciech Golab. Detectable Sequential Specifications for Recoverable Shared Objects. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 29:1-29:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{li_et_al:LIPIcs.DISC.2021.29,
  author =	{Li, Nan and Golab, Wojciech},
  title =	{{Detectable Sequential Specifications for Recoverable Shared Objects}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{29:1--29:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.29},
  URN =		{urn:nbn:de:0030-drops-148311},
  doi =		{10.4230/LIPIcs.DISC.2021.29},
  annote =	{Keywords: persistent memory, concurrency, fault tolerance, correctness, detectability}
}
Document
Constant RMR Group Mutual Exclusion for Arbitrarily Many Processes and Sessions

Authors: Liat Maor and Gadi Taubenfeld


Abstract
Group mutual exclusion (GME), introduced by Joung in 1998, is a natural synchronization problem that generalizes the classical mutual exclusion and readers and writers problems. In GME a process requests a session before entering its critical section; processes are allowed to be in their critical sections simultaneously provided they have requested the same session. We present a GME algorithm that (1) is the first to achieve a constant Remote Memory Reference (RMR) complexity for both cache coherent and distributed shared memory machines; and (2) is the first that can be accessed by arbitrarily many dynamically allocated processes and with arbitrarily many session names. Neither of the existing GME algorithms satisfies either of these two important properties. In addition, our algorithm has constant space complexity per process and satisfies the two strong fairness properties, first-come-first-served and first-in-first-enabled. Our algorithm uses an atomic instruction set supported by most modern processor architectures, namely: read, write, fetch-and-store and compare-and-swap.

Cite as

Liat Maor and Gadi Taubenfeld. Constant RMR Group Mutual Exclusion for Arbitrarily Many Processes and Sessions. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 30:1-30:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{maor_et_al:LIPIcs.DISC.2021.30,
  author =	{Maor, Liat and Taubenfeld, Gadi},
  title =	{{Constant RMR Group Mutual Exclusion for Arbitrarily Many Processes and Sessions}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{30:1--30:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.30},
  URN =		{urn:nbn:de:0030-drops-148322},
  doi =		{10.4230/LIPIcs.DISC.2021.30},
  annote =	{Keywords: Group mutual exclusion, RMR complexity, unbounded number of processes, fetch\&store (FAS), compare\&swap (CAS)}
}
Document
Efficient CONGEST Algorithms for the Lovász Local Lemma

Authors: Yannic Maus and Jara Uitto


Abstract
We present a poly log log n time randomized CONGEST algorithm for a natural class of Lovász Local Lemma (LLL) instances on constant degree graphs. This implies, among other things, that there are no LCL problems with randomized complexity between Ω(log n) and poly log log n. Furthermore, we provide extensions to the network decomposition algorithms given in the recent breakthrough by Rozhoň and Ghaffari [STOC2020] and the follow up by Ghaffari, Grunau, and Rozhoň [SODA2021]. In particular, we show how to obtain a large distance separated weak network decomposition with a negligible dependency on the range of unique identifiers.

Cite as

Yannic Maus and Jara Uitto. Efficient CONGEST Algorithms for the Lovász Local Lemma. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 31:1-31:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{maus_et_al:LIPIcs.DISC.2021.31,
  author =	{Maus, Yannic and Uitto, Jara},
  title =	{{Efficient CONGEST Algorithms for the Lov\'{a}sz Local Lemma}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{31:1--31:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.31},
  URN =		{urn:nbn:de:0030-drops-148333},
  doi =		{10.4230/LIPIcs.DISC.2021.31},
  annote =	{Keywords: distributed graph algorithms, CONGEST, Lov\'{a}sz Local Lemma, locally checkable labelings}
}
Document
Optimal Communication Complexity of Authenticated Byzantine Agreement

Authors: Atsuki Momose and Ling Ren


Abstract
Byzantine Agreement (BA) is one of the most fundamental problems in distributed computing, and its communication complexity is an important efficiency metric. It is well known that quadratic communication is necessary for BA in the worst case due to a lower bound by Dolev and Reischuk. This lower bound has been shown to be tight for the unauthenticated setting with f < n/3 by Berman et al. but a considerable gap remains for the authenticated setting with n/3 ≤ f < n/2. This paper provides two results towards closing this gap. Both protocols have a quadratic communication complexity and have different trade-offs in resilience and assumptions. The first protocol achieves the optimal resilience of f < n/2 but requires a trusted setup for threshold signature. The second protocol achieves near optimal resilience f ≤ (1/2 - ε)n in the standard PKI model.

Cite as

Atsuki Momose and Ling Ren. Optimal Communication Complexity of Authenticated Byzantine Agreement. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 32:1-32:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{momose_et_al:LIPIcs.DISC.2021.32,
  author =	{Momose, Atsuki and Ren, Ling},
  title =	{{Optimal Communication Complexity of Authenticated Byzantine Agreement}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{32:1--32:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.32},
  URN =		{urn:nbn:de:0030-drops-148341},
  doi =		{10.4230/LIPIcs.DISC.2021.32},
  annote =	{Keywords: Byzantine Agreement, Communication Complexity, Lower Bound}
}
Document
Algorithms for the Minimum Dominating Set Problem in Bounded Arboricity Graphs: Simpler, Faster, and Combinatorial

Authors: Adir Morgan, Shay Solomon, and Nicole Wein


Abstract
We revisit the minimum dominating set problem on graphs with arboricity bounded by α. In the (standard) centralized setting, Bansal and Umboh [Bansal and Umboh, 2017] gave an O(α)-approximation LP rounding algorithm, which also translates into a near-linear time algorithm using general-purpose approximation results for explicit mixed packing and covering or pure covering LPs [Koufogiannakis and Young, 2014; Young, 2014; Allen-Zhu and Orecchia, 2019; Quanrud, 2020]. Moreover, [Bansal and Umboh, 2017] showed that it is NP-hard to achieve an asymptotic improvement for the approximation factor. On the other hand, the previous two non-LP-based algorithms, by Lenzen and Wattenhofer [Christoph Lenzen and Roger Wattenhofer, 2010], and Jones et al. [Jones et al., 2013], achieve an approximation factor of O(α²) in linear time. There is a similar situation in the distributed setting: While there is an O(log² n)-round LP-based O(α)-approximation algorithm implied in [Kuhn et al., 2006], the best non-LP-based algorithm by Lenzen and Wattenhofer [Christoph Lenzen and Roger Wattenhofer, 2010] is an implementation of their centralized algorithm, providing an O(α²)-approximation within O(log n) rounds. We address the questions of whether one can achieve an O(α)-approximation algorithm that is elementary, i.e., not based on any LP-based methods, either in the centralized setting or in the distributed setting. We resolve both questions in the affirmative, and en route achieve algorithms that are faster than the state-of-the-art LP-based algorithms. Our contribution is two-fold: 1) In the centralized setting, we provide a surprisingly simple combinatorial algorithm that is asymptotically optimal in terms of both approximation factor and running time: an O(α)-approximation in linear time. The previous state-of-the-art O(α)-approximation algorithms are (1) LP-based, (2) more complicated, and (3) have super-linear running time. 2) Based on our centralized algorithm, we design a distributed combinatorial O(α)-approximation algorithm in the CONGEST model that runs in O(αlog n) rounds with high probability. Not only does this result provide the first nontrivial non-LP-based distributed o(α²)-approximation algorithm for this problem, it also outperforms the best LP-based distributed algorithm for a wide range of parameters.

Cite as

Adir Morgan, Shay Solomon, and Nicole Wein. Algorithms for the Minimum Dominating Set Problem in Bounded Arboricity Graphs: Simpler, Faster, and Combinatorial. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 33:1-33:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{morgan_et_al:LIPIcs.DISC.2021.33,
  author =	{Morgan, Adir and Solomon, Shay and Wein, Nicole},
  title =	{{Algorithms for the Minimum Dominating Set Problem in Bounded Arboricity Graphs: Simpler, Faster, and Combinatorial}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{33:1--33:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.33},
  URN =		{urn:nbn:de:0030-drops-148353},
  doi =		{10.4230/LIPIcs.DISC.2021.33},
  annote =	{Keywords: Graph Algorithms, Dominating Set, Bounded Arboricity, Linear time algorithms}
}
Document
Smoothed Analysis of Population Protocols

Authors: Gregory Schwartzman and Yuichi Sudo


Abstract
In this work, we initiate the study of smoothed analysis of population protocols. We consider a population protocol model where an adaptive adversary dictates the interactions between agents, but with probability p every such interaction may change into an interaction between two agents chosen uniformly at random. That is, p-fraction of the interactions are random, while (1-p)-fraction are adversarial. The aim of our model is to bridge the gap between a uniformly random scheduler (which is too idealistic) and an adversarial scheduler (which is too strict). We focus on the fundamental problem of leader election in population protocols. We show that, for a population of size n, the leader election problem can be solved in O(p^{-2}n log³ n) steps with high probability, using O((log² n) ⋅ (log (n/p))) states per agent, for all values of p ≤ 1. Although our result does not match the best known running time of O(n log n) for the uniformly random scheduler (p = 1), we are able to present a smooth transition between a running time of O(n polylog n) for p = 1 and an infinite running time for the adversarial scheduler (p = 0), where the problem cannot be solved. The key technical contribution of our work is a novel phase clock algorithm for our model. This is a key primitive for much-studied fundamental population protocol algorithms (leader election, majority), and we believe it is of independent interest.

Cite as

Gregory Schwartzman and Yuichi Sudo. Smoothed Analysis of Population Protocols. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 34:1-34:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{schwartzman_et_al:LIPIcs.DISC.2021.34,
  author =	{Schwartzman, Gregory and Sudo, Yuichi},
  title =	{{Smoothed Analysis of Population Protocols}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{34:1--34:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.34},
  URN =		{urn:nbn:de:0030-drops-148362},
  doi =		{10.4230/LIPIcs.DISC.2021.34},
  annote =	{Keywords: Population protocols, Smoothed analysis, Leader election}
}
Document
VBR: Version Based Reclamation

Authors: Gali Sheffi, Maurice Herlihy, and Erez Petrank


Abstract
Safe lock-free memory reclamation is a difficult problem. Existing solutions follow three basic methods (or their combinations): epoch based reclamation, hazard pointers, and optimistic reclamation. Epoch-based methods are fast, but do not guarantee lock-freedom. Hazard pointer solutions are lock-free but typically do not provide high performance. Optimistic methods are lock-free and fast, but previous optimistic methods did not go all the way. While reads were executed optimistically, writes were protected by hazard pointers. In this work we present a new reclamation scheme called version based reclamation (VBR), which provides a full optimistic solution to lock-free memory reclamation, obtaining lock-freedom and high efficiency. Speculative execution is known as a fundamental tool for improving performance in various areas of computer science, and indeed evaluation with a lock-free linked-list, hash-table and skip-list shows that VBR outperforms state-of-the-art existing solutions.

Cite as

Gali Sheffi, Maurice Herlihy, and Erez Petrank. VBR: Version Based Reclamation. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 35:1-35:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{sheffi_et_al:LIPIcs.DISC.2021.35,
  author =	{Sheffi, Gali and Herlihy, Maurice and Petrank, Erez},
  title =	{{VBR: Version Based Reclamation}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{35:1--35:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.35},
  URN =		{urn:nbn:de:0030-drops-148374},
  doi =		{10.4230/LIPIcs.DISC.2021.35},
  annote =	{Keywords: Safe memory reclamation, concurrency, linearizability, lock-freedom}
}
Document
Extension-Based Proofs for Synchronous Message Passing

Authors: Yilun Sheng and Faith Ellen


Abstract
There is no wait-free algorithm that solves k-set agreement among n ≥ k+1 processes in asynchronous systems where processes communicate using only registers. However, proofs of this result for k ≥ 2 are complicated and involve topological reasoning. To explain why such sophisticated arguments are necessary, Alistarh, Aspnes, Ellen, Gelashvili, and Zhu recently introduced extension-based proofs, which generalize valency arguments, and proved that there are no extension-based proofs of this result. In the synchronous message passing model, k-set agreement is solvable, but there is a lower bound of t rounds for any k-set agreement algorithm among n > kt processes when at most k processes can crash each round. The proof of this result for k ≥ 2 is also a complicated topological argument. We define a notion of extension-based proofs for this model and we show there are no extension-based proofs that t rounds are necessary for any k-set agreement algorithm among n = kt+1 processes, for k ≥ 2 and t > 2, when at most k processes can crash each round. In particular, our result shows that no valency argument can prove this lower bound.

Cite as

Yilun Sheng and Faith Ellen. Extension-Based Proofs for Synchronous Message Passing. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 36:1-36:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{sheng_et_al:LIPIcs.DISC.2021.36,
  author =	{Sheng, Yilun and Ellen, Faith},
  title =	{{Extension-Based Proofs for Synchronous Message Passing}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{36:1--36:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.36},
  URN =		{urn:nbn:de:0030-drops-148380},
  doi =		{10.4230/LIPIcs.DISC.2021.36},
  annote =	{Keywords: Set agreement, lower bounds, valency arguments}
}
Document
Truthful Information Dissemination in General Asynchronous Networks

Authors: Lior Solodkin and Rotem Oshman


Abstract
We give a protocol for information dissemination in asynchronous networks of rational players, where each player may have its own desires and preferences as to the outcome of the protocol, and players may deviate from the protocol if doing so achieves their goals. We show that under minimalistic assumptions, it is possible to solve the information dissemination problem in a truthful manner, such that no participant has an incentive to deviate from the protocol we design. Our protocol works in any asynchronous network, provided the network graph is at least 2-connected. We complement the protocol with two impossibility results, showing that 2-connectivity is necessary, and also that our protocol achieves optimal bit complexity. As an application, we show that truthful information dissemination can be used to implement a certain class of communication equilibria, which are equilibria that are typically reached by interacting with a trusted third party. Recent work has shown that communication equilibria can be implemented in synchronous networks, or in asynchronous, complete networks; we show that in some useful cases, our protocol yields a lightweight mechanism for implementing communication equilibria in any 2-connected asynchronous network.

Cite as

Lior Solodkin and Rotem Oshman. Truthful Information Dissemination in General Asynchronous Networks. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 37:1-37:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{solodkin_et_al:LIPIcs.DISC.2021.37,
  author =	{Solodkin, Lior and Oshman, Rotem},
  title =	{{Truthful Information Dissemination in General Asynchronous Networks}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{37:1--37:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.37},
  URN =		{urn:nbn:de:0030-drops-148398},
  doi =		{10.4230/LIPIcs.DISC.2021.37},
  annote =	{Keywords: game theory, asynchronous networks, information dissemination}
}
Document
In Search for an Optimal Authenticated Byzantine Agreement

Authors: Alexander Spiegelman


Abstract
In this paper, we challenge the conventional approach of state machine replication systems to design deterministic agreement protocols in the eventually synchronous communication model. We first prove that no such protocol can guarantee bounded communication cost before the global stabilization time and propose a different approach that hopes for the best (synchrony) but prepares for the worst (asynchrony). Accordingly, we design an optimistic byzantine agreement protocol that first tries an efficient deterministic algorithm that relies on synchrony for termination only, and then, only if an agreement was not reached due to asynchrony, the protocol uses a randomized asynchronous protocol for fallback that guarantees termination with probability 1. We formally prove that our protocol achieves optimal communication complexity under all network conditions and failure scenarios. We first prove a lower bound of Ω(ft+ t) for synchronous deterministic byzantine agreement protocols, where t is the failure threshold, and f is the actual number of failures. Then, we present a tight upper bound and use it for the synchronous part of the optimistic protocol. Finally, for the asynchronous fallback, we use a variant of the (optimal) VABA protocol, which we reconstruct to safely combine it with the synchronous part. We believe that our adaptive to failures synchronous byzantine agreement protocol has an independent interest since it is the first protocol we are aware of which communication complexity optimally depends on the actual number of failures.

Cite as

Alexander Spiegelman. In Search for an Optimal Authenticated Byzantine Agreement. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 38:1-38:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{spiegelman:LIPIcs.DISC.2021.38,
  author =	{Spiegelman, Alexander},
  title =	{{In Search for an Optimal Authenticated Byzantine Agreement}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{38:1--38:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.38},
  URN =		{urn:nbn:de:0030-drops-148401},
  doi =		{10.4230/LIPIcs.DISC.2021.38},
  annote =	{Keywords: Byzantine agreement, Optimistic, Asynchronous fallback}
}
Document
The Power of Random Symmetry-Breaking in Nakamoto Consensus

Authors: Lili Su, Quanquan C. Liu, and Neha Narula


Abstract
Nakamoto consensus underlies the security of many of the world’s largest cryptocurrencies, such as Bitcoin and Ethereum. Common lore is that Nakamoto consensus only achieves consistency and liveness under a regime where the difficulty of its underlying mining puzzle is very high, negatively impacting overall throughput and latency. In this work, we study Nakamoto consensus under a wide range of puzzle difficulties, including very easy puzzles. We first analyze an adversary-free setting and show that, surprisingly, the common prefix of the blockchain grows quickly even with easy puzzles. In a setting with adversaries, we provide a small backwards-compatible change to Nakamoto consensus to achieve consistency and liveness with easy puzzles. Our insight relies on a careful choice of symmetry-breaking strategy, which was significantly underestimated in prior work. We introduce a new method - coalescing random walks - to analyzing the correctness of Nakamoto consensus under the uniformly-at-random symmetry-breaking strategy. This method is more powerful than existing analysis methods that focus on bounding the number of convergence opportunities.

Cite as

Lili Su, Quanquan C. Liu, and Neha Narula. The Power of Random Symmetry-Breaking in Nakamoto Consensus. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 39:1-39:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{su_et_al:LIPIcs.DISC.2021.39,
  author =	{Su, Lili and Liu, Quanquan C. and Narula, Neha},
  title =	{{The Power of Random Symmetry-Breaking in Nakamoto Consensus}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{39:1--39:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.39},
  URN =		{urn:nbn:de:0030-drops-148413},
  doi =		{10.4230/LIPIcs.DISC.2021.39},
  annote =	{Keywords: Nakamoto consensus, Byzantine consensus, blockchain, symmetry-breaking, coalescing random walks}
}
Document
Time-Optimal Loosely-Stabilizing Leader Election in Population Protocols

Authors: Yuichi Sudo, Ryota Eguchi, Taisuke Izumi, and Toshimitsu Masuzawa


Abstract
We consider the leader election problem in the population protocol model. In pragmatic settings of population protocols, self-stabilization is a highly desired feature owing to its fault resilience and the benefit of initialization freedom. However, the design of self-stabilizing leader election is possible only under a strong assumption (i.e., the knowledge of the exact size of a network) and rich computational resource (i.e., the number of states). Loose-stabilization is a promising relaxed concept of self-stabilization to address the aforementioned issue. Loose-stabilization guarantees that starting from any configuration, the network will reach a safe configuration where a single leader exists within a short time, and thereafter it will maintain the single leader for a long time, but not necessarily forever. The main contribution of this paper is giving a time-optimal loosely-stabilizing leader election protocol. The proposed protocol with design parameter τ ≥ 1 attains O(τ log n) parallel convergence time and Ω(n^τ) parallel holding time (i.e., the length of the period keeping the unique leader), both in expectation. This protocol is time-optimal in the sense of both the convergence and holding times in expectation because any loosely-stabilizing leader election protocol with the same length of the holding time is known to require Ω(τ log n) parallel time.

Cite as

Yuichi Sudo, Ryota Eguchi, Taisuke Izumi, and Toshimitsu Masuzawa. Time-Optimal Loosely-Stabilizing Leader Election in Population Protocols. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 40:1-40:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{sudo_et_al:LIPIcs.DISC.2021.40,
  author =	{Sudo, Yuichi and Eguchi, Ryota and Izumi, Taisuke and Masuzawa, Toshimitsu},
  title =	{{Time-Optimal Loosely-Stabilizing Leader Election in Population Protocols}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{40:1--40:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.40},
  URN =		{urn:nbn:de:0030-drops-148427},
  doi =		{10.4230/LIPIcs.DISC.2021.40},
  annote =	{Keywords: population protocols, leader election, loose-stabilization, self-stabilization}
}
Document
Efficient Distribution of Quantum Circuits

Authors: Ranjani G Sundaram, Himanshu Gupta, and C. R. Ramakrishnan


Abstract
Quantum computing hardware is improving in robustness, but individual computers still have small number of qubits (for storing quantum information). Computations needing a large number of qubits can only be performed by distributing them over a network of smaller quantum computers. In this paper, we consider the problem of distributing a quantum computation, represented as a quantum circuit, over a homogeneous network of quantum computers, minimizing the number of communication operations needed to complete every step of the computation. We propose a two-step solution: dividing the given circuit’s qubits among the computers in the network, and scheduling communication operations, called migrations, to share quantum information among the computers to ensure that every operation can be performed locally. While the first step is an intractable problem, we present a polynomial-time solution for the second step in a special setting, and a O(log n)-approximate solution in the general setting. We provide empirical results which show that our two-step solution outperforms existing heuristic for this problem by a significant margin (up to 90%, in some cases).

Cite as

Ranjani G Sundaram, Himanshu Gupta, and C. R. Ramakrishnan. Efficient Distribution of Quantum Circuits. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 41:1-41:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{gsundaram_et_al:LIPIcs.DISC.2021.41,
  author =	{G Sundaram, Ranjani and Gupta, Himanshu and Ramakrishnan, C. R.},
  title =	{{Efficient Distribution of Quantum Circuits}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{41:1--41:20},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.41},
  URN =		{urn:nbn:de:0030-drops-148434},
  doi =		{10.4230/LIPIcs.DISC.2021.41},
  annote =	{Keywords: Distributed Quantum Computing, Hypergraph Min-Cut}
}
Document
Game Theoretical Framework for Analyzing Blockchains Robustness

Authors: Paolo Zappalà, Marianna Belotti, Maria Potop-Butucaru, and Stefano Secci


Abstract
In this paper we propose a game theoretical framework in order to formally characterize the robustness of blockchains systems in terms of resilience to rational deviations and immunity to Byzantine behaviors. Our framework includes necessary and sufficient conditions for checking the immunity and resilience of games and an original technique for composing games that preserves the robustness of individual games. We prove the practical interest of our formal framework by characterizing the robustness of various blockchain protocols: Bitcoin (the most popular permissionless blockchain), Tendermint (the first permissioned blockchain used by the practitioners), Lightning Network, a side-chain protocol and a cross-chain swap protocol. For each one of the studied protocols we identify upper and lower bounds with respect to their resilience and immunity (expressed as no worse payoff than the initial state) face to rational and Byzantine behaviors.

Cite as

Paolo Zappalà, Marianna Belotti, Maria Potop-Butucaru, and Stefano Secci. Game Theoretical Framework for Analyzing Blockchains Robustness. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 42:1-42:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{zappala_et_al:LIPIcs.DISC.2021.42,
  author =	{Zappal\`{a}, Paolo and Belotti, Marianna and Potop-Butucaru, Maria and Secci, Stefano},
  title =	{{Game Theoretical Framework for Analyzing Blockchains Robustness}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{42:1--42:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.42},
  URN =		{urn:nbn:de:0030-drops-148440},
  doi =		{10.4230/LIPIcs.DISC.2021.42},
  annote =	{Keywords: Blockchain protocols, Distributed algorithms, Game-theoretical modeling, Fault tolerance, Failure robustness}
}
Document
Brief Announcement
Brief Announcement: Fast Graphical Population Protocols

Authors: Dan Alistarh, Rati Gelashvili, and Joel Rybicki


Abstract
Let G be a graph on n nodes. In the stochastic population protocol model, a collection of n indistinguishable, resource-limited nodes collectively solve tasks via pairwise interactions. In each interaction, two randomly chosen neighbors first read each other’s states, and then update their local states. A rich line of research has established tight upper and lower bounds on the complexity of fundamental tasks, such as majority and leader election, in this model, when G is a clique. Specifically, in the clique, these tasks can be solved fast, i.e., in n polylog n pairwise interactions, with high probability, using at most polylog n states per node. In this work, we consider the more general setting where G is an arbitrary graph, and present a technique for simulating protocols designed for fully-connected networks in any connected regular graph. Our main result is a simulation that is efficient on many interesting graph families: roughly, the simulation overhead is polylogarithmic in the number of nodes, and quadratic in the conductance of the graph. As an example, this implies that, in any regular graph with conductance φ, both leader election and exact majority can be solved in φ^{-2} ⋅ n polylog n pairwise interactions, with high probability, using at most φ^{-2} ⋅ polylog n states per node. This shows that there are fast and space-efficient population protocols for leader election and exact majority on graphs with good expansion properties.

Cite as

Dan Alistarh, Rati Gelashvili, and Joel Rybicki. Brief Announcement: Fast Graphical Population Protocols. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 43:1-43:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{alistarh_et_al:LIPIcs.DISC.2021.43,
  author =	{Alistarh, Dan and Gelashvili, Rati and Rybicki, Joel},
  title =	{{Brief Announcement: Fast Graphical Population Protocols}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{43:1--43:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.43},
  URN =		{urn:nbn:de:0030-drops-148451},
  doi =		{10.4230/LIPIcs.DISC.2021.43},
  annote =	{Keywords: population protocols, leader election, majority}
}
Document
Brief Announcement
Brief Announcement: How to Trust Strangers - Composition of Byzantine Quorum Systems

Authors: Orestis Alpos, Christian Cachin, and Luca Zanolini


Abstract
Trust is the basis of any distributed, fault-tolerant, or secure system. A trust assumption specifies the failures that a system, such as a blockchain network, can tolerate and determines the conditions under which it operates correctly. In systems subject to Byzantine faults, the trust assumption is usually specified through sets of processes that may fail together. Trust has traditionally been symmetric, such that all processes in the system adhere to the same, global assumption about potential faults. Recently, asymmetric trust models have also been considered, especially in the context of blockchains, where every participant is free to choose who to trust. In both cases, it is an open question how to compose trust assumptions. Consider two or more systems, run by different and possibly disjoint sets of participants, with different assumptions about faults: how can they work together? This work answers this question for the first time and offers composition rules for symmetric and for asymmetric quorum systems. These rules are static and do not require interaction or agreement on the new trust assumption among the participants. Moreover, they ensure that if the original systems allow for running a particular protocol (guaranteeing consistency and availability), then so will the joint system. At the same time, the composed system tolerates as many faults as possible, subject to the underlying consistency and availability properties. Reaching consensus with asymmetric trust in the model of personal Byzantine quorum systems (Losa et al., DISC 2019) was shown to be impossible, if the trust assumptions of the processes diverge from each other. With asymmetric quorum systems, and by applying our composition rule, we show how consensus is actually possible, even with the combination of disjoint sets of processes.

Cite as

Orestis Alpos, Christian Cachin, and Luca Zanolini. Brief Announcement: How to Trust Strangers - Composition of Byzantine Quorum Systems. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 44:1-44:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{alpos_et_al:LIPIcs.DISC.2021.44,
  author =	{Alpos, Orestis and Cachin, Christian and Zanolini, Luca},
  title =	{{Brief Announcement: How to Trust Strangers - Composition of Byzantine Quorum Systems}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{44:1--44:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.44},
  URN =		{urn:nbn:de:0030-drops-148468},
  doi =		{10.4230/LIPIcs.DISC.2021.44},
  annote =	{Keywords: Byzantine quorum systems, composition of quorum systems, trust models, asymmetric trust}
}
Document
Brief Announcement
Brief Announcement: Using Nesting to Push the Limits of Transactional Data Structure Libraries

Authors: Gal Assa, Hagar Meir, Guy Golan-Gueta, Idit Keidar, and Alexander Spiegelman


Abstract
Transactional data structure libraries (TDSL) combine the ease-of-programming of transactions with the high performance and scalability of custom-tailored concurrent data structures. They can be very efficient thanks to their ability to exploit data structure semantics in order to reduce overhead, aborts, and wasted work compared to general-purpose software transactional memory. However, TDSLs were not previously used for complex use-cases involving long transactions and a variety of data structures. In this paper, we boost the performance and usability of a TDSL, towards allowing it to support complex applications. A key idea is nesting. Nested transactions create checkpoints within a longer transaction, so as to limit the scope of abort, without changing the semantics of the original transaction. We build a Java TDSL with built-in support for nested transactions over a number of data structures. We conduct a case study of a complex network intrusion detection system that invests a significant amount of work to process each packet. Our study shows that our library outperforms publicly available STMs twofold without nesting, and by up to 16x when nesting is used.

Cite as

Gal Assa, Hagar Meir, Guy Golan-Gueta, Idit Keidar, and Alexander Spiegelman. Brief Announcement: Using Nesting to Push the Limits of Transactional Data Structure Libraries. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 45:1-45:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{assa_et_al:LIPIcs.DISC.2021.45,
  author =	{Assa, Gal and Meir, Hagar and Golan-Gueta, Guy and Keidar, Idit and Spiegelman, Alexander},
  title =	{{Brief Announcement: Using Nesting to Push the Limits of Transactional Data Structure Libraries}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{45:1--45:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.45},
  URN =		{urn:nbn:de:0030-drops-148479},
  doi =		{10.4230/LIPIcs.DISC.2021.45},
  annote =	{Keywords: Transactional Libraries}
}
Document
Brief Announcement
Brief Announcement: Twins – BFT Systems Made Robust

Authors: Shehar Bano, Alberto Sonnino, Andrey Chursin, Dmitri Perelman, Zekun Li, Avery Ching, and Dahlia Malkhi


Abstract
Twins is an effective strategy for generating test scenarios with Byzantine [Lamport et al., 1982] nodes in order to find flaws in Byzantine Fault Tolerant (BFT) systems. Twins finds flaws in the design or implementation of BFT protocols that may cause correctness issues. The main idea of Twins is the following: running twin instances of a node that use correct, unmodified code and share the same network identity and credentials allows to emulate most interesting Byzantine behaviors. Because a twin executes normal, unmodified node code, building Twins only requires a thin wrapper over an existing distributed system designed for Byzantine tolerance. To emulate material, interesting scenarios with Byzantine nodes, it instantiates one or more twin copies of the node, giving the twins the same identities and network credentials as the original node. To the rest of the system, the node and all its twins appear indistinguishable from a single node behaving in a "questionable" manner. This approach generates many interesting Byzantine behaviors, including equivocation, double voting, and losing internal state, while forgoing uninteresting behavior scenarios that can be filtered at the transport layer, such as producing semantically invalid messages. Building on configurations with twin nodes, Twins systematically generates scenarios with Byzantine nodes via enumeration over protocol rounds and communication patterns among nodes. Despite this being inherently exponential, one new flaw and several known flaws were materialized by Twins in the arena of BFT consensus protocols. In all cases, protocols break within fewer than a dozen protocol rounds, hence it is realistic for the Twins approach to expose the problems. In two of these cases, it took the community more than a decade to discover protocol flaws that Twins would have surfaced within minutes. Additionally, Twins has been incorporated into the continuous release testing process of a production setting (DiemBFT) in which it can execute 44M Twins-generated scenarios daily.

Cite as

Shehar Bano, Alberto Sonnino, Andrey Chursin, Dmitri Perelman, Zekun Li, Avery Ching, and Dahlia Malkhi. Brief Announcement: Twins – BFT Systems Made Robust. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 46:1-46:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{bano_et_al:LIPIcs.DISC.2021.46,
  author =	{Bano, Shehar and Sonnino, Alberto and Chursin, Andrey and Perelman, Dmitri and Li, Zekun and Ching, Avery and Malkhi, Dahlia},
  title =	{{Brief Announcement: Twins – BFT Systems Made Robust}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{46:1--46:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.46},
  URN =		{urn:nbn:de:0030-drops-148485},
  doi =		{10.4230/LIPIcs.DISC.2021.46},
  annote =	{Keywords: Distributed Systems, Byzantine Fault Tolerance, Real-World Deployment}
}
Document
Brief Announcement
Brief Annoucement: On Extending Brandt’s Speedup Theorem from LOCAL to Round-Based Full-Information Models

Authors: Paul Bastide and Pierre Fraigniaud


Abstract
Given any task Π, Brandt’s speedup theorem (PODC 2019) provides a mechanical way to design another task Π' on the same input-set as Π such that, for any t ≥ 1, Π is solvable in t rounds in the LOCAL model if and only if Π' is solvable in t-1 rounds in the LOCAL model. We dissect the construction in Brandt’s speedup theorem for expressing it in the broader framework of all round-based models supporting full information protocols, which includes models as different as asynchronous wait-free shared-memory computing with iterated immediate snapshots, and synchronous failure-free network computing.

Cite as

Paul Bastide and Pierre Fraigniaud. Brief Annoucement: On Extending Brandt’s Speedup Theorem from LOCAL to Round-Based Full-Information Models. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 47:1-47:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{bastide_et_al:LIPIcs.DISC.2021.47,
  author =	{Bastide, Paul and Fraigniaud, Pierre},
  title =	{{Brief Annoucement: On Extending Brandt’s Speedup Theorem from LOCAL to Round-Based Full-Information Models}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{47:1--47:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.47},
  URN =		{urn:nbn:de:0030-drops-148492},
  doi =		{10.4230/LIPIcs.DISC.2021.47},
  annote =	{Keywords: Local Checkability, Distributed Complexity and Computability}
}
Document
Brief Announcement
Brief Announcement: Automating and Mechanising Cutoff Proofs for Parameterized Verification of Distributed Protocols

Authors: Shreesha G. Bhat and Kartik Nagar


Abstract
We propose a framework to automate and mechanize simulation-based proofs of cutoffs for parameterized verification of distributed protocols. We propose a strategy to derive the simulation relation given the cutoff instance and encode the correctness of the simulation relation as a formula in first-order logic. We have successfully applied our approach on a number of distributed protocols.

Cite as

Shreesha G. Bhat and Kartik Nagar. Brief Announcement: Automating and Mechanising Cutoff Proofs for Parameterized Verification of Distributed Protocols. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 48:1-48:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{bhat_et_al:LIPIcs.DISC.2021.48,
  author =	{Bhat, Shreesha G. and Nagar, Kartik},
  title =	{{Brief Announcement: Automating and Mechanising Cutoff Proofs for Parameterized Verification of Distributed Protocols}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{48:1--48:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.48},
  URN =		{urn:nbn:de:0030-drops-148505},
  doi =		{10.4230/LIPIcs.DISC.2021.48},
  annote =	{Keywords: Formal Methods, Automated Verification, Distributed Protocols}
}
Document
Brief Announcement
Brief Announcement: Local Certification of Graph Decompositions and Applications to Minor-Free Classes

Authors: Nicolas Bousquet, Laurent Feuilloley, and Théo Pierron


Abstract
Local certification consists in assigning labels to the nodes of a network to certify that some given property is satisfied, in such a way that the labels can be checked locally. In the last few years, certification of graph classes received a considerable attention. The goal is to certify that a graph G belongs to a given graph class 𝒢. Such certifications with labels of size O(log n) (where n is the size of the network) exist for trees, planar graphs and graphs embedded on surfaces. Feuilloley et al. ask if this can be extended to any class of graphs defined by a finite set of forbidden minors. In this paper, we develop new decomposition tools for graph certification, and apply them to show that for every small enough minor H, H-minor-free graphs can indeed be certified with labels of size O(log n). We also show matching lower bounds with a new simple proof technique.

Cite as

Nicolas Bousquet, Laurent Feuilloley, and Théo Pierron. Brief Announcement: Local Certification of Graph Decompositions and Applications to Minor-Free Classes. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 49:1-49:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{bousquet_et_al:LIPIcs.DISC.2021.49,
  author =	{Bousquet, Nicolas and Feuilloley, Laurent and Pierron, Th\'{e}o},
  title =	{{Brief Announcement: Local Certification of Graph Decompositions and Applications to Minor-Free Classes}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{49:1--49:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.49},
  URN =		{urn:nbn:de:0030-drops-148515},
  doi =		{10.4230/LIPIcs.DISC.2021.49},
  annote =	{Keywords: Local certification, proof-labeling schemes, locally checkable proofs, graph decompositions, minor-free graphs}
}
Document
Brief Announcement
Brief Announcement: Memory Efficient Massively Parallel Algorithms for LCL Problems on Trees

Authors: Sebastian Brandt, Rustam Latypov, and Jara Uitto


Abstract
We establish scalable Massively Parallel Computation (MPC) algorithms for a family of fundamental graph problems on trees. We give a general method that, for a wide range of LCL problems, turns their message passing counterparts into exponentially faster algorithms in the sublinear MPC model. In particular, we show that any LCL on trees that has a deterministic complexity of O(n) in the LOCAL model can be sped up to O(log n) (high-complexity regime) in the sublinear MPC model and similarly n^{o(1)} to O(log log n) (intermediate-complexity regime). We emphasize, that we work on bounded degree trees and all of our algorithms work in the sublinear MPC model, where local memory is O(n^δ) for δ < 1 and global memory is O(m). For the high-complexity regime, one key ingredient is a novel pointer-chain technique and analysis that allows us to solve any solvable LCL on trees with a sublinear MPC algorithm with complexity O(log n). For the intermediate-complexity regime, we adapt the approach by Chang and Pettie [FOCS'17], who gave a canonical algorithm for solving LCL problems on trees in the LOCAL model. For the special case of 3-coloring trees, which is a natural LCL problem, we provide a conditional Ω(log log n) lower bound, implying that solving LCL problems on trees with deterministic LOCAL complexity n^{o(1)} requires Θ(log log n) deterministic time in the sublinear MPC model when using a natural family of component-stable algorithms.

Cite as

Sebastian Brandt, Rustam Latypov, and Jara Uitto. Brief Announcement: Memory Efficient Massively Parallel Algorithms for LCL Problems on Trees. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 50:1-50:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{brandt_et_al:LIPIcs.DISC.2021.50,
  author =	{Brandt, Sebastian and Latypov, Rustam and Uitto, Jara},
  title =	{{Brief Announcement: Memory Efficient Massively Parallel Algorithms for LCL Problems on Trees}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{50:1--50:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.50},
  URN =		{urn:nbn:de:0030-drops-148521},
  doi =		{10.4230/LIPIcs.DISC.2021.50},
  annote =	{Keywords: Distributed computing, Locally checkable labeling problems, Trees, Massively Parallel Computation, Sublinear memory, 3-coloring}
}
Document
Brief Announcement
Brief Announcement: Revisiting Signature-Free Asynchronous Byzantine Consensus

Authors: Christian Cachin and Luca Zanolini


Abstract
Among asynchronous, randomized, and signature-free implementations of consensus, the protocols of Mostéfaoui et al. (PODC 2014 and JACM 2015) represent a landmark result, which has been extended later and taken up in practical systems. The protocols achieve optimal resilience and take, in expectation, only a constant expected number of rounds and have quadratic message complexity. Randomization is provided through a common-coin primitive. However, the first version of this simple and appealing protocol suffers from a little-known liveness issue due to asynchrony. The JACM 2015 version avoids the problem, but is considerably more complex. This work revisits the original protocol of PODC 2014 and points out in detail why it may not progress. A fix for the protocol is presented, which does not affect any of its properties, but lets it regain the original simplicity in asynchronous networks enhanced with a common-coin protocol.

Cite as

Christian Cachin and Luca Zanolini. Brief Announcement: Revisiting Signature-Free Asynchronous Byzantine Consensus. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 51:1-51:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{cachin_et_al:LIPIcs.DISC.2021.51,
  author =	{Cachin, Christian and Zanolini, Luca},
  title =	{{Brief Announcement: Revisiting Signature-Free Asynchronous Byzantine Consensus}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{51:1--51:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.51},
  URN =		{urn:nbn:de:0030-drops-148535},
  doi =		{10.4230/LIPIcs.DISC.2021.51},
  annote =	{Keywords: Randomized consensus}
}
Document
Brief Announcement
Brief Announcement: Non-Blocking Dynamic Unbounded Graphs with Worst-Case Amortized Bounds

Authors: Bapi Chatterjee, Sathya Peri, and Muktikanta Sa


Abstract
This paper reports a new concurrent graph data structure that supports updates of both edges and vertices and queries: Breadth-first search, Single-source shortest-path, and Betweenness centrality. The operations are provably linearizable and non-blocking.

Cite as

Bapi Chatterjee, Sathya Peri, and Muktikanta Sa. Brief Announcement: Non-Blocking Dynamic Unbounded Graphs with Worst-Case Amortized Bounds. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 52:1-52:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{chatterjee_et_al:LIPIcs.DISC.2021.52,
  author =	{Chatterjee, Bapi and Peri, Sathya and Sa, Muktikanta},
  title =	{{Brief Announcement: Non-Blocking Dynamic Unbounded Graphs with Worst-Case Amortized Bounds}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{52:1--52:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.52},
  URN =		{urn:nbn:de:0030-drops-148549},
  doi =		{10.4230/LIPIcs.DISC.2021.52},
  annote =	{Keywords: concurrent data structure, linearizability, non-blocking, directed graph, breadth-first search, single-source shortest-path, betweenness centrality}
}
Document
Brief Announcement
Brief Announcement: Auditable Register Emulations

Authors: Vinicius Vielmo Cogo and Alysson Bessani


Abstract
We initiate the study of auditable storage emulations, which provide the capability for an auditor to report the previously executed reads in a register. We define the notion of auditable register and its properties, and establish tight bounds and impossibility results for auditable storage emulations in the presence of faulty base storage objects. Our formulation considers registers that securely store data using information dispersal (each base object stores only a block of the written value) and supporting fast reads (that complete in one communication round-trip). In such a scenario, given a maximum number f of faulty storage objects and a minimum number τ of data blocks required to recover a stored value, we prove that (R1) auditability is impossible if τ ≤ 2f; (R2) implementing a weak form of auditability requires τ ≥ 3f+1; and (R3) a stronger form of auditability is impossible. We also show that (R4) signing read requests generically overcomes the lower bound of weak auditability, while (R5 and R6) totally ordering operations or using non-fast reads enables strong auditability. These results establish that practical storage emulations need f to 2f additional objects compared to their original lower bounds to support auditability.

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Vinicius Vielmo Cogo and Alysson Bessani. Brief Announcement: Auditable Register Emulations. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 53:1-53:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{cogo_et_al:LIPIcs.DISC.2021.53,
  author =	{Cogo, Vinicius Vielmo and Bessani, Alysson},
  title =	{{Brief Announcement: Auditable Register Emulations}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{53:1--53:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.53},
  URN =		{urn:nbn:de:0030-drops-148558},
  doi =		{10.4230/LIPIcs.DISC.2021.53},
  annote =	{Keywords: Auditability, Secure Storage, Information Dispersal}
}
Document
Brief Announcement
Brief Announcement: Accountability and Reconfiguration — Self-Healing Lattice Agreement

Authors: Luciano Freitas de Souza, Petr Kuznetsov, Thibault Rieutord, and Sara Tucci-Piergiovanni


Abstract
An accountable distributed system provides means to detect deviations of system components from their expected behavior. It is natural to complement fault detection with a reconfiguration mechanism, so that the system could heal itself, by replacing malfunctioning parts with new ones. In this paper, we describe a framework that can be used to implement a large class of accountable and reconfigurable replicated services. We build atop the fundamental lattice agreement abstraction lying at the core of storage systems and cryptocurrencies. Our asynchronous implementation of accountable lattice agreement ensures that every violation of consistency is followed by an undeniable evidence of misbehavior of a faulty replica. The system can then be seamlessly reconfigured by evicting faulty replicas, adding new ones and merging inconsistent states. We believe that this paper opens a direction towards asynchronous "self-healing" systems that combine accountability and reconfiguration.

Cite as

Luciano Freitas de Souza, Petr Kuznetsov, Thibault Rieutord, and Sara Tucci-Piergiovanni. Brief Announcement: Accountability and Reconfiguration — Self-Healing Lattice Agreement. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 54:1-54:5, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{desouza_et_al:LIPIcs.DISC.2021.54,
  author =	{de Souza, Luciano Freitas and Kuznetsov, Petr and Rieutord, Thibault and Tucci-Piergiovanni, Sara},
  title =	{{Brief Announcement: Accountability and Reconfiguration — Self-Healing Lattice Agreement}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{54:1--54:5},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.54},
  URN =		{urn:nbn:de:0030-drops-148565},
  doi =		{10.4230/LIPIcs.DISC.2021.54},
  annote =	{Keywords: Reconfiguration, accountability, asynchronous, lattice agreement}
}
Document
Brief Announcement
Brief Announcement: On Strong Observational Refinement and Forward Simulation

Authors: John Derrick, Simon Doherty, Brijesh Dongol, Gerhard Schellhorn, and Heike Wehrheim


Abstract
Hyperproperties are correctness conditions for labelled transition systems that are more expressive than traditional trace properties, with particular relevance to security. Recently, Attiya and Enea studied a notion of strong observational refinement that preserves all hyperproperties. They analyse the correspondence between forward simulation and strong observational refinement in a setting with finite traces only. We study this correspondence in a setting with both finite and infinite traces. In particular, we show that forward simulation does not preserve hyperliveness properties in this setting. We extend the forward simulation proof obligation with a progress condition, and prove that this progressive forward simulation does imply strong observational refinement.

Cite as

John Derrick, Simon Doherty, Brijesh Dongol, Gerhard Schellhorn, and Heike Wehrheim. Brief Announcement: On Strong Observational Refinement and Forward Simulation. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 55:1-55:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{derrick_et_al:LIPIcs.DISC.2021.55,
  author =	{Derrick, John and Doherty, Simon and Dongol, Brijesh and Schellhorn, Gerhard and Wehrheim, Heike},
  title =	{{Brief Announcement: On Strong Observational Refinement and Forward Simulation}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{55:1--55:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.55},
  URN =		{urn:nbn:de:0030-drops-148575},
  doi =		{10.4230/LIPIcs.DISC.2021.55},
  annote =	{Keywords: Strong Observational Refinement, Hyperproperties, Forward Simulation}
}
Document
Brief Announcement
Brief Announcement: Persistent Software Combining

Authors: Panagiota Fatourou, Nikolaos D. Kallimanis, and Eleftherios Kosmas


Abstract
We study the performance power of software combining in designing recoverable algorithms and data structures. We present two recoverable synchronization protocols, one blocking and another wait-free, which illustrate how to use software combining to achieve both low persistence and synchronization cost. Our experiments show that these protocols outperform by far state-of-the-art recoverable universal constructions and transactional memory systems. We built recoverable queues and stacks, based on these protocols, that exhibit much better performance than previous such implementations.

Cite as

Panagiota Fatourou, Nikolaos D. Kallimanis, and Eleftherios Kosmas. Brief Announcement: Persistent Software Combining. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 56:1-56:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{fatourou_et_al:LIPIcs.DISC.2021.56,
  author =	{Fatourou, Panagiota and Kallimanis, Nikolaos D. and Kosmas, Eleftherios},
  title =	{{Brief Announcement: Persistent Software Combining}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{56:1--56:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.56},
  URN =		{urn:nbn:de:0030-drops-148580},
  doi =		{10.4230/LIPIcs.DISC.2021.56},
  annote =	{Keywords: Persistent objects, recoverable algorithms, durability, synchronization protocols, software combining, universal constructions, wait-freedom, stacks, queues}
}
Document
Brief Announcement
Brief Announcement: Probabilistic Indistinguishability and The Quality of Validity in Byzantine Agreement

Authors: Guy Goren, Yoram Moses, and Alexander Spiegelman


Abstract
Lower bounds and impossibility results in distributed computing are both intellectually challenging and practically important. Hundreds if not thousands of proofs appear in the literature, but surprisingly, the vast majority of them apply to deterministic algorithms only. Probabilistic protocols have been around for at least four decades and are receiving a lot of attention with the emergence of blockchain systems. Nonetheless, we are aware of only a handful of randomized lower bounds. In this work we provide a formal framework for reasoning about randomized distributed algorithms. We generalize the notion of indistinguishability, the most useful tool in deterministic lower bounds, to apply to a probabilistic setting. We apply this framework to prove a result of independent interest. Namely, we completely characterize the quality of decisions that protocols for a randomized multi-valued Consensus problem can guarantee in an asynchronous environment with Byzantine faults. We use the new notion to prove a lower bound on the guaranteed probability that honest parties will not decide on a possibly bogus value proposed by a malicious party. Finally, we show that the bound is tight by providing a protocol that matches it. This brief announcement consists of an introduction to the full paper [Guy Goren et al., 2020] by the same title. The interested reader is advised to consult the full paper for a detailed exposition.

Cite as

Guy Goren, Yoram Moses, and Alexander Spiegelman. Brief Announcement: Probabilistic Indistinguishability and The Quality of Validity in Byzantine Agreement. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 57:1-57:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{goren_et_al:LIPIcs.DISC.2021.57,
  author =	{Goren, Guy and Moses, Yoram and Spiegelman, Alexander},
  title =	{{Brief Announcement: Probabilistic Indistinguishability and The Quality of Validity in Byzantine Agreement}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{57:1--57:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.57},
  URN =		{urn:nbn:de:0030-drops-148596},
  doi =		{10.4230/LIPIcs.DISC.2021.57},
  annote =	{Keywords: Indistinguishability, probabilistic lower bounds, Byzantine agreement}
}
Document
Brief Announcement
Brief Announcement: Sinkless Orientation Is Hard Also in the Supported LOCAL Model

Authors: Janne H. Korhonen, Ami Paz, Joel Rybicki, Stefan Schmid, and Jukka Suomela


Abstract
We show that any algorithm that solves the sinkless orientation problem in the supported LOCAL model requires Ω(log n) rounds, and this is tight. The supported LOCAL is at least as strong as the usual LOCAL model, and as a corollary this also gives a new, short and elementary proof that shows that the round complexity of the sinkless orientation problem in the deterministic LOCAL model is Ω(log n).

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Janne H. Korhonen, Ami Paz, Joel Rybicki, Stefan Schmid, and Jukka Suomela. Brief Announcement: Sinkless Orientation Is Hard Also in the Supported LOCAL Model. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 58:1-58:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{korhonen_et_al:LIPIcs.DISC.2021.58,
  author =	{Korhonen, Janne H. and Paz, Ami and Rybicki, Joel and Schmid, Stefan and Suomela, Jukka},
  title =	{{Brief Announcement: Sinkless Orientation Is Hard Also in the Supported LOCAL Model}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{58:1--58:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.58},
  URN =		{urn:nbn:de:0030-drops-148609},
  doi =		{10.4230/LIPIcs.DISC.2021.58},
  annote =	{Keywords: Supported LOCAL model, sinkless orientation, round elimination}
}
Document
Brief Announcement
Brief Announcement: Simple Majority Consensus in Networks with Unreliable Communication

Authors: Ariel Livshits, Yonatan Shadmi, and Ran Tamir (Averbuch)


Abstract
In this work, we consider a synchronous model of n faultless agents, with a complete communication graph and messages that are lost with some constant probability q ∈ (0,1). In this model we show that there exists a protocol, called the Simple Majority Protocol, that solves consensus in 3 communication rounds with probability of agreement converging to 1 as n → ∞. We also prove that 3 communication rounds are necessary for the SMP to achieve consensus, with high probability.

Cite as

Ariel Livshits, Yonatan Shadmi, and Ran Tamir (Averbuch). Brief Announcement: Simple Majority Consensus in Networks with Unreliable Communication. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 59:1-59:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{livshits_et_al:LIPIcs.DISC.2021.59,
  author =	{Livshits, Ariel and Shadmi, Yonatan and Tamir (Averbuch), Ran},
  title =	{{Brief Announcement: Simple Majority Consensus in Networks with Unreliable Communication}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{59:1--59:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.59},
  URN =		{urn:nbn:de:0030-drops-148617},
  doi =		{10.4230/LIPIcs.DISC.2021.59},
  annote =	{Keywords: Majority consensus, probabilistic message loss, distributed systems}
}
Document
Brief Announcement
Brief Announcement: Crystalline: Fast and Memory Efficient Wait-Free Reclamation

Authors: Ruslan Nikolaev and Binoy Ravindran


Abstract
We present a new wait-free memory reclamation scheme, Crystalline, that simultaneously addresses the challenges of high performance, high memory efficiency, and wait-freedom. Crystalline guarantees complete wait-freedom even when threads are dynamically recycled, asynchronously reclaims memory in the sense that any thread can reclaim memory retired by any other thread, and ensures (an almost) balanced reclamation workload across all threads. The latter two properties result in Crystalline’s high performance and high memory efficiency, a difficult trade-off for most existing schemes. Our evaluations show that Crystalline exhibits outstanding scalability and memory efficiency, and achieves superior throughput than state-of-the-art reclamation schemes as the number of threads grows.

Cite as

Ruslan Nikolaev and Binoy Ravindran. Brief Announcement: Crystalline: Fast and Memory Efficient Wait-Free Reclamation. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 60:1-60:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{nikolaev_et_al:LIPIcs.DISC.2021.60,
  author =	{Nikolaev, Ruslan and Ravindran, Binoy},
  title =	{{Brief Announcement: Crystalline: Fast and Memory Efficient Wait-Free Reclamation}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{60:1--60:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.60},
  URN =		{urn:nbn:de:0030-drops-148626},
  doi =		{10.4230/LIPIcs.DISC.2021.60},
  annote =	{Keywords: memory reclamation, wait-free, reference counting, hazard pointers}
}
Document
Brief Announcement
Brief Announcement: Design and Verification of a Logless Dynamic Reconfiguration Protocol in MongoDB Replication

Authors: William Schultz, Siyuan Zhou, and Stavros Tripakis


Abstract
We introduce a novel dynamic reconfiguration protocol for the MongoDB replication system that extends and generalizes the single server reconfiguration protocol of the Raft consensus algorithm. Our protocol decouples the processing of configuration changes from the main database operation log, which allows reconfigurations to proceed in cases when the main log is prevented from processing new operations. Additionally, this decoupling allows for configuration state to be managed by a logless replicated state machine, storing only the latest version of the configuration and avoiding the complexities of a log-based protocol. We present a formal specification of the protocol in TLA+, initial verification results of model checking its safety properties, and an experimental evaluation of how reconfigurations are able to quickly restore a system to healthy operation when node failures have stalled the main operation log. This announcement is a short version and the full paper is available at [Schultz et al., 2021].

Cite as

William Schultz, Siyuan Zhou, and Stavros Tripakis. Brief Announcement: Design and Verification of a Logless Dynamic Reconfiguration Protocol in MongoDB Replication. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 61:1-61:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{schultz_et_al:LIPIcs.DISC.2021.61,
  author =	{Schultz, William and Zhou, Siyuan and Tripakis, Stavros},
  title =	{{Brief Announcement: Design and Verification of a Logless Dynamic Reconfiguration Protocol in MongoDB Replication}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{61:1--61:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.61},
  URN =		{urn:nbn:de:0030-drops-148636},
  doi =		{10.4230/LIPIcs.DISC.2021.61},
  annote =	{Keywords: Reconfiguration, Consensus, State Machine Replication}
}
Document
Brief Announcement
Brief Announcement: Communication-Efficient BFT Using Small Trusted Hardware to Tolerate Minority Corruption

Authors: Sravya Yandamuri, Ittai Abraham, Kartik Nayak, and Michael Reiter


Abstract
Small trusted hardware primitives can improve fault tolerance of Byzantine Fault Tolerant (BFT) protocols to one-half faults. However, existing works achieve this at the cost of increased communication complexity. In this work, we explore the design of communication-efficient BFT protocols that can boost fault tolerance to one-half without worsening communication complexity. Our results include a version of HotStuff that retains linear communication complexity in each view and a version of the VABA protocol with quadratic communication, both leveraging trusted hardware to tolerate a minority of corruptions. As a building block, we present communication-efficient provable broadcast, a core broadcast primitive with increased fault tolerance. Our results use expander graphs to achieve efficient communication in a manner that may be of independent interest.

Cite as

Sravya Yandamuri, Ittai Abraham, Kartik Nayak, and Michael Reiter. Brief Announcement: Communication-Efficient BFT Using Small Trusted Hardware to Tolerate Minority Corruption. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 62:1-62:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{yandamuri_et_al:LIPIcs.DISC.2021.62,
  author =	{Yandamuri, Sravya and Abraham, Ittai and Nayak, Kartik and Reiter, Michael},
  title =	{{Brief Announcement: Communication-Efficient BFT Using Small Trusted Hardware to Tolerate Minority Corruption}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{62:1--62:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.62},
  URN =		{urn:nbn:de:0030-drops-148647},
  doi =		{10.4230/LIPIcs.DISC.2021.62},
  annote =	{Keywords: communication complexity, consensus, trusted hardware}
}
Document
Brief Announcement
Brief Announcement: Ordered Reliable Broadcast and Fast Ordered Byzantine Consensus for Cryptocurrency

Authors: Pouriya Zarbafian and Vincent Gramoli


Abstract
The problem of transaction reordering in blockchains, also known as the blockchain anomaly [Christopher Natoli and Vincent Gramoli, 2016], can lead to fairness limitations [Kelkar et al., 2020] and front-running activities [Philip Daian et al., 2020] in cryptocurrency. To cope with this problem despite f < n/3 byzantine processes, Zhang et al. [Zhang et al., 2020] have introduced the ordering linearizability property ensuring that if two transactions or commands are perceived by all correct processes in the same order, then they are executed in this order. They proposed a generic distributed protocol that first orders commands and then runs a leader-based consensus protocol to agree on these orders, hence requiring at least 11 message delays. In this paper, we parallelize the ordering with the execution of the consensus to require only 6 message delays. For the ordering, we introduce the ordered reliable broadcast primitive suitable for broadcast-based cryptocurrencies (e.g., [Daniel Collins et al., 2020]). For the agreement, we build upon the DBFT leaderless consensus protocol [Tyler Crain et al., 2018] that was recently formally verified [Bertrand et al., 2021]. The combination is thus suitable to ensure ordering linearizability in consensus-based cryptocurrencies (e.g., [Tyler Crain et al., 2021]).

Cite as

Pouriya Zarbafian and Vincent Gramoli. Brief Announcement: Ordered Reliable Broadcast and Fast Ordered Byzantine Consensus for Cryptocurrency. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 63:1-63:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{zarbafian_et_al:LIPIcs.DISC.2021.63,
  author =	{Zarbafian, Pouriya and Gramoli, Vincent},
  title =	{{Brief Announcement: Ordered Reliable Broadcast and Fast Ordered Byzantine Consensus for Cryptocurrency}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{63:1--63:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.63},
  URN =		{urn:nbn:de:0030-drops-148655},
  doi =		{10.4230/LIPIcs.DISC.2021.63},
  annote =	{Keywords: distributed algorithm, consensus, reliable broadcast, byzantine fault tolerance, linearizability, blockchain}
}

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