72 Search Results for "Scheideler, Christian"


Volume

LIPIcs, Volume 246

36th International Symposium on Distributed Computing (DISC 2022)

DISC 2022, October 25-27, 2022, Augusta, Georgia, USA

Editors: Christian Scheideler

Document
Complete Volume
LIPIcs, Volume 246, DISC 2022, Complete Volume

Authors: Christian Scheideler

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


Abstract
LIPIcs, Volume 246, DISC 2022, Complete Volume

Cite as

36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 1-790, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@Proceedings{scheideler:LIPIcs.DISC.2022,
  title =	{{LIPIcs, Volume 246, DISC 2022, Complete Volume}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{1--790},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022},
  URN =		{urn:nbn:de:0030-drops-171908},
  doi =		{10.4230/LIPIcs.DISC.2022},
  annote =	{Keywords: LIPIcs, Volume 246, DISC 2022, Complete Volume}
}
Document
Front Matter
Front Matter, Table of Contents, Preface, Conference Organization

Authors: Christian Scheideler

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


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

Cite as

36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 0:i-0:xx, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@InProceedings{scheideler:LIPIcs.DISC.2022.0,
  author =	{Scheideler, Christian},
  title =	{{Front Matter, Table of Contents, Preface, Conference Organization}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{0:i--0:xx},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.0},
  URN =		{urn:nbn:de:0030-drops-171917},
  doi =		{10.4230/LIPIcs.DISC.2022.0},
  annote =	{Keywords: Front Matter, Table of Contents, Preface, Conference Organization}
}
Document
Invited Talk
Graph Coloring, Palette Sparsification, and Beyond (Invited Talk)

Authors: Sepehr Assadi

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


Abstract
Graph coloring is a central problem in graph theory and has numerous applications in diverse areas of computer science. An important and well-studied case of graph coloring problems is the (Δ+1) (vertex) coloring problem where Δ is the maximum degree of the graph. Not only does every graph admit a (Δ + 1) coloring, but in fact we can find one quite easily in linear time and space via a greedy algorithm. But are there more efficient algorithms for (Δ+1) coloring that can process massive graphs that even this algorithm cannot handle? This talk overviews recent results that answer this question in affirmative across a variety of models dedicated to processing massive graphs - streaming, sublinear-time, massively parallel computation, distributed communication, etc. - via a single unified approach: Palette Sparsification. We survey the ideas behind these results and techniques, their generalizations to various other coloring problems and even beyond (e.g., to clustering problems), as well as their natural limitations. The talk is based on a series of joint works with Noga Alon, Andrew Chen, Yu Chen, Sanjeev Khanna, Pankaj Kumar, Parth Mittal, Glenn Sun, and Chen Wang.

Cite as

Sepehr Assadi. Graph Coloring, Palette Sparsification, and Beyond (Invited Talk). In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, p. 1:1, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@InProceedings{assadi:LIPIcs.DISC.2022.1,
  author =	{Assadi, Sepehr},
  title =	{{Graph Coloring, Palette Sparsification, and Beyond}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{1:1--1:1},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.1},
  URN =		{urn:nbn:de:0030-drops-171920},
  doi =		{10.4230/LIPIcs.DISC.2022.1},
  annote =	{Keywords: Graph coloring, Palette Sparsification, Sublinear Algorithms}
}
Document
Invited Talk
Managing the Cyber Risk in a Decoupled World: Does This Bring Potential Opportunities in Computer Science? (Invited Talk)

Authors: Roberto Baldoni

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


Abstract
The last thirty years witnessed the growth of both globalization and digital transformation, characterized by information systems becoming interconnected and distributed on a worldwide scale with IT aimed to become a commodity. Cloud computing and blockchain being examples of such robust and distributed technologies which have been the main driver of this globalization process. Global technologies and infrastructures paved the way to organic and highly frequent interactions between millions of companies and organizations in multiple countries almost irrespective of geopolitical implications establishing global and complex interconnected supply chains whose aim was mainly keeping software/devices costs low. This created a virtuous loop that generated an exponential increase of countries' digitalization process and globalized industries. Like energy, IT progressively became a strategic geopolitical factor as the nation’s vital services implementation went digital. As a consequence, governments realized IT cannot be a simple commodity and that they have to manage the cyber risk associated with procured IT in strategic sectors like, for instance, telecommunication, finance and transportation. Governments have to understand and mitigate IT risks coming from these globalized supply chains against operations of potential powerful adversaries. Even a single supply chain dependency can be a risk, also from a national security perspective, when such dependency is established by a provider/vendor under the direct political influence of an untrusted nation or a trusted provider/vendor victim of a state-backed cyber attack. The recent Ukrainian crisis and the large degree of tension between US and China are amplifying risks coming from globalized supply chains in a world that is politically liquid polarizing in at least two blocks. In addition, the globalization process has shown its natural limits and frailty culminating with the global supply chain crisis created by the effect of the covid-19 pandemic and extreme events due to climate change. Paradoxically, experience shows the main drawback of globalized supply chains is the centralization of certain key manufacturing in restricted geographical areas, this is the case for the infamous chip shortage. This centralization poses risks if a critical portion of these key manufacturing are owned by untrusted actors. A parallel can be seen in the permissionless blockchain technologies based on Proof-of-Work, where the decentralized worldwide spirit has mercilessly converged to a more convenient but weaker almost centralized system which makes it easier for a powerful adversary to take control of the whole blockchain. The likely trends of the next few years will be a progressive decoupling of supply chains particularly for all software/hardware manufacturing employed into vital services of a nation. This will be a long and non-economically neutral process that will bring in a medium term towards the composition of “friendshoring” or “almost domestic” supply chains where developing robust technologies and algorithms compliant to society values. This is expected to increase the number, the magnitude and complexity of cyber attacks coming from other geopolitical blocks for espionage or terroristic reasons in a continuous hybrid warfare scenario. Computer scientists and engineers will have to cope with the new challenges within this decoupled world. The keynote will be an attempt to shed some light on what this could imply in terms of technology, computing paradigms and nation IT capability.

Cite as

Roberto Baldoni. Managing the Cyber Risk in a Decoupled World: Does This Bring Potential Opportunities in Computer Science? (Invited Talk). In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, p. 2:1, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@InProceedings{baldoni:LIPIcs.DISC.2022.2,
  author =	{Baldoni, Roberto},
  title =	{{Managing the Cyber Risk in a Decoupled World: Does This Bring Potential Opportunities in Computer Science?}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{2:1--2:1},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.2},
  URN =		{urn:nbn:de:0030-drops-171931},
  doi =		{10.4230/LIPIcs.DISC.2022.2},
  annote =	{Keywords: Supply chain decoupling, technology risk, cyber attacks, computing paradigms and methodologies}
}
Document
Invited Talk
Using Linearizable Objects in Randomized Concurrent Programs (Invited Talk)

Authors: Jennifer L. Welch

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


Abstract
Atomic shared objects, whose operations take place instantaneously, are a powerful technique for designing complex concurrent programs. Since they are not always available, they are typically substituted with software implementations. A prominent condition relating these implementations to their atomic specifications is linearizability, which preserves safety properties of programs using them. However linearizability does not preserve hyper-properties, which include probabilistic guarantees about randomized programs. A more restrictive property, strong linearizability, does preserve hyper-properties but it is impossible to achieve in many situations. In particular, we show that there are no strongly linearizable implementations of multi-writer registers or snapshot objects in message-passing systems. On the other hand, we show that a wide class of linearizable implementations, including well-known ones for registers and snapshots, can be modified to approximate the probabilistic guarantees of randomized programs when using atomic objects. This is joint work with Hagit Attiya and Constantin Enea.

Cite as

Jennifer L. Welch. Using Linearizable Objects in Randomized Concurrent Programs (Invited Talk). In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, p. 3:1, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@InProceedings{welch:LIPIcs.DISC.2022.3,
  author =	{Welch, Jennifer L.},
  title =	{{Using Linearizable Objects in Randomized Concurrent Programs}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{3:1--3:1},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.3},
  URN =		{urn:nbn:de:0030-drops-171946},
  doi =		{10.4230/LIPIcs.DISC.2022.3},
  annote =	{Keywords: Concurrent objects, strong linearizability, impossibility proofs, message-passing systems, randomized algorithms}
}
Document
Good-Case Early-Stopping Latency of Synchronous Byzantine Reliable Broadcast: The Deterministic Case

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

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


Abstract
This paper considers the good-case latency of Byzantine Reliable Broadcast (BRB), i.e., the time taken by correct processes to deliver a message when the initial sender is correct, and an essential property for practical distributed systems. Although significant strides have been made in recent years on this question, progress has mainly focused on either asynchronous or randomized algorithms. By contrast, the good-case latency of deterministic synchronous BRB under a majority of Byzantine faults has been little studied. In particular, it was not known whether a good-case latency below the worst-case bound of t+1 rounds could be obtained under a Byzantine majority. In this work, we answer this open question positively and propose a deterministic synchronous Byzantine reliable broadcast that achieves a good-case latency of max(2,t+3-c) rounds, where t is the upper bound on the number of Byzantine processes, and c the number of effectively correct processes.

Cite as

Timothé Albouy, Davide Frey, Michel Raynal, and François Taïani. Good-Case Early-Stopping Latency of Synchronous Byzantine Reliable Broadcast: The Deterministic Case. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 4:1-4:22, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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

Authors: Gal Amram, Avi Hayoun, Lior Mizrahi, and Gera Weiss

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


Abstract
We analyze correctness of implementations of the snapshot data structure in terms of linearizability. We show that such implementations can be verified in polynomial time. Additionally, we identify a set of representative executions for testing and show that the correctness of each of these executions can be validated in linear time. These results present a significant speedup considering that verifying linearizability of implementations of concurrent data structures, in general, is EXPSPACE-complete in the number of program-states, and testing linearizability is NP-complete in the length of the tested execution. The crux of our approach is identifying a class of executions, which we call simple, such that a snapshot implementation is linearizable if and only if all of its simple executions are linearizable. We then divide all possible non-linearizable simple executions into three categories and construct a small automaton that recognizes each category. We describe two implementations (one for verification and one for testing) of an automata-based approach that we develop based on this result and an evaluation that demonstrates significant improvements over existing tools. For verification, we show that restricting a state-of-the-art tool to analyzing only simple executions saves resources and allows the analysis of more complex cases. Specifically, restricting attention to simple executions finds bugs in 27 instances, whereas, without this restriction, we were only able to find 14 of the 30 bugs in the instances we examined. We also show that our technique accelerates testing performance significantly. Specifically, our implementation solves the complete set of 900 problems we generated, whereas the state-of-the-art linearizability testing tool solves only 554 problems.

Cite as

Gal Amram, Avi Hayoun, Lior Mizrahi, and Gera Weiss. Polynomial-Time Verification and Testing of Implementations of the Snapshot Data Structure. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 5:1-5:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@InProceedings{amram_et_al:LIPIcs.DISC.2022.5,
  author =	{Amram, Gal and Hayoun, Avi and Mizrahi, Lior and Weiss, Gera},
  title =	{{Polynomial-Time Verification and Testing of Implementations of the Snapshot Data Structure}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{5:1--5:20},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.5},
  URN =		{urn:nbn:de:0030-drops-171964},
  doi =		{10.4230/LIPIcs.DISC.2022.5},
  annote =	{Keywords: Snapshot, Linearizability, Verification, Formal Methods}
}
Document
Almost Universally Optimal Distributed Laplacian Solvers via Low-Congestion Shortcuts

Authors: Ioannis Anagnostides, Christoph Lenzen, Bernhard Haeupler, Goran Zuzic, and Themis Gouleakis

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


Abstract
In this paper, we refine the (almost) existentially optimal distributed Laplacian solver recently developed by Forster, Goranci, Liu, Peng, Sun, and Ye (FOCS `21) into an (almost) universally optimal distributed Laplacian solver. Specifically, when the topology is known (i.e., the Supported-CONGEST model), we show that any Laplacian system on an n-node graph with shortcut quality SQ(G) can be solved after n^{o(1)} SQ(G) log(1/ε) rounds, where ε is the required accuracy. This almost matches our lower bound that guarantees that any correct algorithm on G requires Ω̃(SQ(G)) rounds, even for a crude solution with ε ≤ 1/2. Several important implications hold in the unknown-topology (i.e., standard CONGEST) case: for excluded-minor graphs we get an almost universally optimal algorithm that terminates in D ⋅ n^{o(1)} log(1/ε) rounds, where D is the hop-diameter of the network; as well as n^{o(1)} log (1/ε)-round algorithms for the case of SQ(G) ≤ n^{o(1)}, which holds for most networks of interest. Conditioned on improvements in state-of-the-art constructions of low-congestion shortcuts, the CONGEST results will match the Supported-CONGEST ones. Moreover, following a recent line of work in distributed algorithms, we consider a hybrid communication model which enhances CONGEST with limited global power in the form of the node-capacitated clique (NCC) model. In this model, we show the existence of a Laplacian solver with round complexity n^{o(1)} log(1/ε). The unifying thread of these results, and our main technical contribution, is the study of a novel ρ-congested generalization of the standard part-wise aggregation problem. We develop near-optimal algorithms for this primitive in the Supported-CONGEST model, almost-optimal algorithms in (standard) CONGEST (with the additional overhead due to standard barriers), as well as a simple algorithm for bounded-treewidth graphs with a quadratic dependence on the congestion ρ. This primitive can be readily used to accelerate the Laplacian solver of Forster, Goranci, Liu, Peng, Sun, and Ye, and we believe it will find further independent applications in the future.

Cite as

Ioannis Anagnostides, Christoph Lenzen, Bernhard Haeupler, Goran Zuzic, and Themis Gouleakis. Almost Universally Optimal Distributed Laplacian Solvers via Low-Congestion Shortcuts. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 6:1-6:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@InProceedings{anagnostides_et_al:LIPIcs.DISC.2022.6,
  author =	{Anagnostides, Ioannis and Lenzen, Christoph and Haeupler, Bernhard and Zuzic, Goran and Gouleakis, Themis},
  title =	{{Almost Universally Optimal Distributed Laplacian Solvers via Low-Congestion Shortcuts}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{6:1--6:20},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.6},
  URN =		{urn:nbn:de:0030-drops-171978},
  doi =		{10.4230/LIPIcs.DISC.2022.6},
  annote =	{Keywords: Distributed algorithms, Laplacian solvers, low-congestion shortcuts}
}
Document
Byzantine Connectivity Testing in the Congested Clique

Authors: John Augustine, Anisur Rahaman Molla, Gopal Pandurangan, and Yadu Vasudev

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


Abstract
We initiate the study of distributed graph algorithms under the presence of Byzantine nodes. We consider the fundamental problem of testing the connectivity of a graph in the congested clique model in a Byzantine setting. We are given a n-vertex (arbitrary) graph G embedded in a n-node congested clique where an arbitrary subset of B nodes of the clique of size up to (1/3-ε)n (for any arbitrary small constant ε > 0) can be Byzantine. We consider the full information model where Byzantine nodes can behave arbitrarily, collude with each other, and have unlimited computational power and full knowledge of the states and actions of the honest nodes, including random choices made up to the current round. Our main result is an efficient randomized distributed algorithm that is able to correctly distinguish between two contrasting cases: (1) the graph G⧵ B (i.e., the graph induced by the removal of the vertices assigned to the Byzantine nodes in the clique) is connected or (2) the graph G is far from connected, i.e., it has at least 2|B|+1 connected components. Our algorithm runs in O(polylog n) rounds in the congested clique model and guarantees that all honest nodes will decide on the correct case with high probability. Since Byzantine nodes can lie about the vertices assigned to them, we show that this is essentially the best possible that can be done by any algorithm. Our result can be viewed also in the spirit of property testing, where our algorithm is able to distinguish between two contrasting cases while giving no guarantees if the graph falls in the grey area (i.e., neither of the cases occur). Our work is a step towards robust and secure distributed graph computation that can output meaningful results even in the presence of a large number of faulty or malicious nodes.

Cite as

John Augustine, Anisur Rahaman Molla, Gopal Pandurangan, and Yadu Vasudev. Byzantine Connectivity Testing in the Congested Clique. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 7:1-7:21, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@InProceedings{augustine_et_al:LIPIcs.DISC.2022.7,
  author =	{Augustine, John and Molla, Anisur Rahaman and Pandurangan, Gopal and Vasudev, Yadu},
  title =	{{Byzantine Connectivity Testing in the Congested Clique}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{7:1--7:21},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.7},
  URN =		{urn:nbn:de:0030-drops-171987},
  doi =		{10.4230/LIPIcs.DISC.2022.7},
  annote =	{Keywords: Byzantine protocols, distributed graph algorithms, congested clique, graph connectivity, fault-tolerant computation, randomized algorithms}
}
Document
Efficient Classification of Locally Checkable Problems in Regular Trees

Authors: Alkida Balliu, Sebastian Brandt, Yi-Jun Chang, Dennis Olivetti, Jan Studený, and Jukka Suomela

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


Abstract
We give practical, efficient algorithms that automatically determine the asymptotic distributed round complexity of a given locally checkable graph problem in the [Θ(log n), Θ(n)] region, in two settings. We present one algorithm for unrooted regular trees and another algorithm for rooted regular trees. The algorithms take the description of a locally checkable labeling problem as input, and the running time is polynomial in the size of the problem description. The algorithms decide if the problem is solvable in O(log n) rounds. If not, it is known that the complexity has to be Θ(n^{1/k}) for some k = 1, 2, ..., and in this case the algorithms also output the right value of the exponent k. In rooted trees in the O(log n) case we can then further determine the exact complexity class by using algorithms from prior work; for unrooted trees the more fine-grained classification in the O(log n) region remains an open question.

Cite as

Alkida Balliu, Sebastian Brandt, Yi-Jun Chang, Dennis Olivetti, Jan Studený, and Jukka Suomela. Efficient Classification of Locally Checkable Problems in Regular Trees. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 8:1-8:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@InProceedings{balliu_et_al:LIPIcs.DISC.2022.8,
  author =	{Balliu, Alkida and Brandt, Sebastian and Chang, Yi-Jun and Olivetti, Dennis and Studen\'{y}, Jan and Suomela, Jukka},
  title =	{{Efficient Classification of Locally Checkable Problems in Regular Trees}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{8:1--8:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.8},
  URN =		{urn:nbn:de:0030-drops-171993},
  doi =		{10.4230/LIPIcs.DISC.2022.8},
  annote =	{Keywords: locally checkable labeling, locality, distributed computational complexity}
}
Document
Exponential Speedup over Locality in MPC with Optimal Memory

Authors: Alkida Balliu, Sebastian Brandt, Manuela Fischer, Rustam Latypov, Yannic Maus, Dennis Olivetti, and Jara Uitto

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


Abstract
Locally Checkable Labeling (LCL) problems are graph problems in which a solution is correct if it satisfies some given constraints in the local neighborhood of each node. Example problems in this class include maximal matching, maximal independent set, and coloring problems. A successful line of research has been studying the complexities of LCL problems on paths/cycles, trees, and general graphs, providing many interesting results for the LOCAL model of distributed computing. In this work, we initiate the study of LCL problems in the low-space Massively Parallel Computation (MPC) model. In particular, on forests, we provide a method that, given the complexity of an LCL problem in the LOCAL model, automatically provides an exponentially faster algorithm for the low-space MPC setting that uses optimal global memory, that is, truly linear. While restricting to forests may seem to weaken the result, we emphasize that all known (conditional) lower bounds for the MPC setting are obtained by lifting lower bounds obtained in the distributed setting in tree-like networks (either forests or high girth graphs), and hence the problems that we study are challenging already on forests. Moreover, the most important technical feature of our algorithms is that they use optimal global memory, that is, memory linear in the number of edges of the graph. In contrast, most of the state-of-the-art algorithms use more than linear global memory. Further, they typically start with a dense graph, sparsify it, and then solve the problem on the residual graph, exploiting the relative increase in global memory. On forests, this is not possible, because the given graph is already as sparse as it can be, and using optimal memory requires new solutions.

Cite as

Alkida Balliu, Sebastian Brandt, Manuela Fischer, Rustam Latypov, Yannic Maus, Dennis Olivetti, and Jara Uitto. Exponential Speedup over Locality in MPC with Optimal Memory. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 9:1-9:21, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@InProceedings{balliu_et_al:LIPIcs.DISC.2022.9,
  author =	{Balliu, Alkida and Brandt, Sebastian and Fischer, Manuela and Latypov, Rustam and Maus, Yannic and Olivetti, Dennis and Uitto, Jara},
  title =	{{Exponential Speedup over Locality in MPC with Optimal Memory}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{9:1--9:21},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.9},
  URN =		{urn:nbn:de:0030-drops-172003},
  doi =		{10.4230/LIPIcs.DISC.2022.9},
  annote =	{Keywords: Distributed computing, Locally checkable labeling problems, Trees, Massively Parallel Computation, Sublinear memory}
}
Document
Holistic Verification of Blockchain Consensus

Authors: Nathalie Bertrand, Vincent Gramoli, Igor Konnov, Marijana Lazić, Pierre Tholoniat, and Josef Widder

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


Abstract
Blockchain has recently attracted the attention of the industry due, in part, to its ability to automate asset transfers. It requires distributed participants to reach a consensus on a block despite the presence of malicious (a.k.a. Byzantine) participants. Malicious participants exploit regularly weaknesses of these blockchain consensus algorithms, with sometimes devastating consequences. In fact, these weaknesses are quite common and are well illustrated by the flaws in various blockchain consensus algorithms [Pierre Tholoniat and Vincent Gramoli, 2019]. Paradoxically, until now, no blockchain consensus has been holistically verified. In this paper, we remedy this paradox by model checking for the first time a blockchain consensus used in industry. We propose a holistic approach to verify the consensus algorithm of the Red Belly Blockchain [Tyler Crain et al., 2021], for any number n of processes and any number f < n/3 of Byzantine processes. We decompose directly the algorithm pseudocode in two parts - an inner broadcast algorithm and an outer decision algorithm - each modelled as a threshold automaton [Igor Konnov et al., 2017], and we formalize their expected properties in linear-time temporal logic. We then automatically check the inner broadcasting algorithm, under a carefully identified fairness assumption. For the verification of the outer algorithm, we simplify the model of the inner algorithm by relying on its proven properties. Doing so, we formally verify, for any parameter, not only the safety properties of the Red Belly Blockchain consensus but also its liveness in less than 70 seconds.

Cite as

Nathalie Bertrand, Vincent Gramoli, Igor Konnov, Marijana Lazić, Pierre Tholoniat, and Josef Widder. Holistic Verification of Blockchain Consensus. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 10:1-10:24, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@InProceedings{bertrand_et_al:LIPIcs.DISC.2022.10,
  author =	{Bertrand, Nathalie and Gramoli, Vincent and Konnov, Igor and Lazi\'{c}, Marijana and Tholoniat, Pierre and Widder, Josef},
  title =	{{Holistic Verification of Blockchain Consensus}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{10:1--10:24},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.10},
  URN =		{urn:nbn:de:0030-drops-172019},
  doi =		{10.4230/LIPIcs.DISC.2022.10},
  annote =	{Keywords: Model checking, automata, logic, byzantine failure}
}
Document
How to Meet at a Node of Any Connected Graph

Authors: Subhash Bhagat and Andrzej Pelc

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


Abstract
Two mobile agents have to meet at the same node of a connected graph with unlabeled nodes. This intensely researched task is known as rendezvous. The adversary assigns the agents different starting nodes in the graph and different integer labels from a set {1,… ,L}. Time is slotted in synchronous rounds. The adversary wakes up the agents in possibly different rounds. After wakeup, the agents move as follows. In each round, an agent can either stay idle or move to an adjacent node. Each agent knows its label but not the label of the other agent, and agents have no a priori information about the graph. They do not know L. They execute the same deterministic algorithm whose parameter is the agent’s label. The time of a rendezvous algorithm is the worst-case number of rounds since the wakeup of the earlier agent till the meeting. In most of the results concerning rendezvous in graphs, the graph is finite and rendezvous relies on the exploration of the entire graph. Thus the time of rendezvous depends on the size of the graph. This approach is inefficient for very large graphs, and cannot be used for infinite graphs. For such graphs it is natural to seek rendezvous algorithms whose time depends on the initial distance D between the agents. In this paper we adopt this approach and consider rendezvous in arbitrary connected graphs with nodes of finite degrees, and whose set of nodes is finite or countably infinite. Our main result is the first deterministic rendezvous algorithm working under this general scenario. For any node v and any positive integer r, let P(v,r) be the number of paths of length r in the graph, starting at node v. For any instance of the rendezvous problem where agents start at nodes v₁ and v₂ at distance D, let P(v₁,v₂,D) = max(P(v₁,D),P(v₂,D)). It is well known that, for example in trees, Ω(D+P(v₁,v₂,D) +log L) is a lower bound on rendezvous time for such an instance. The time of our algorithm, working for arbitrary connected graphs of finite degrees, is polynomial in this lower bound. As an application we solve the problem of approach for synchronous agents in terrains in the plane, in time polynomial in log L and in the initial distance between the agents in the terrain.

Cite as

Subhash Bhagat and Andrzej Pelc. How to Meet at a Node of Any Connected Graph. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 11:1-11:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@InProceedings{bhagat_et_al:LIPIcs.DISC.2022.11,
  author =	{Bhagat, Subhash and Pelc, Andrzej},
  title =	{{How to Meet at a Node of Any Connected Graph}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{11:1--11:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.11},
  URN =		{urn:nbn:de:0030-drops-172024},
  doi =		{10.4230/LIPIcs.DISC.2022.11},
  annote =	{Keywords: Algorithm, graph, rendezvous, mobile agent, terrain}
}
Document
Liveness and Latency of Byzantine State-Machine Replication

Authors: Manuel Bravo, Gregory Chockler, and Alexey Gotsman

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


Abstract
Byzantine state-machine replication (SMR) ensures the consistency of replicated state in the presence of malicious replicas and lies at the heart of the modern blockchain technology. Byzantine SMR protocols often guarantee safety under all circumstances and liveness only under synchrony. However, guaranteeing liveness even under this assumption is nontrivial. So far we have lacked systematic ways of incorporating liveness mechanisms into Byzantine SMR protocols, which often led to subtle bugs. To close this gap, we introduce a modular framework to facilitate the design of provably live and efficient Byzantine SMR protocols. Our framework relies on a view abstraction generated by a special SMR synchronizer primitive to drive the agreement on command ordering. We present a simple formal specification of an SMR synchronizer and its bounded-space implementation under partial synchrony. We also apply our specification to prove liveness and analyze the latency of three Byzantine SMR protocols via a uniform methodology. In particular, one of these results yields what we believe is the first rigorous liveness proof for the algorithmic core of the seminal PBFT protocol.

Cite as

Manuel Bravo, Gregory Chockler, and Alexey Gotsman. Liveness and Latency of Byzantine State-Machine Replication. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 12:1-12:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@InProceedings{bravo_et_al:LIPIcs.DISC.2022.12,
  author =	{Bravo, Manuel and Chockler, Gregory and Gotsman, Alexey},
  title =	{{Liveness and Latency of Byzantine State-Machine Replication}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{12:1--12:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.12},
  URN =		{urn:nbn:de:0030-drops-172037},
  doi =		{10.4230/LIPIcs.DISC.2022.12},
  annote =	{Keywords: Replication, blockchain, partial synchrony, liveness}
}
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