160,769 research outputs found
Distributed Consensus of Linear Multi-Agent Systems with Adaptive Dynamic Protocols
This paper considers the distributed consensus problem of multi-agent systems
with general continuous-time linear dynamics. Two distributed adaptive dynamic
consensus protocols are proposed, based on the relative output information of
neighboring agents. One protocol assigns an adaptive coupling weight to each
edge in the communication graph while the other uses an adaptive coupling
weight for each node. These two adaptive protocols are designed to ensure that
consensus is reached in a fully distributed fashion for any undirected
connected communication graphs without using any global information. A
sufficient condition for the existence of these adaptive protocols is that each
agent is stabilizable and detectable. The cases with leader-follower and
switching communication graphs are also studied.Comment: 17 pages, 5 figue
Designing Fully Distributed Consensus Protocols for Linear Multi-agent Systems with Directed Graphs
This paper addresses the distributed consensus protocol design problem for
multi-agent systems with general linear dynamics and directed communication
graphs. Existing works usually design consensus protocols using the smallest
real part of the nonzero eigenvalues of the Laplacian matrix associated with
the communication graph, which however is global information. In this paper,
based on only the agent dynamics and the relative states of neighboring agents,
a distributed adaptive consensus protocol is designed to achieve
leader-follower consensus for any communication graph containing a directed
spanning tree with the leader as the root node. The proposed adaptive protocol
is independent of any global information of the communication graph and thereby
is fully distributed. Extensions to the case with multiple leaders are further
studied.Comment: 16 page, 3 figures. To appear in IEEE Transactions on Automatic
Contro
Consensus of Multi-Agent Systems with General Linear and Lipschitz Nonlinear Dynamics Using Distributed Adaptive Protocols
This paper considers the distributed consensus problems for multi-agent
systems with general linear and Lipschitz nonlinear dynamics. Distributed
relative-state consensus protocols with an adaptive law for adjusting the
coupling weights between neighboring agents are designed for both the linear
and nonlinear cases, under which consensus is reached for all undirected
connected communication graphs. Extensions to the case with a leader-follower
communication graph are further studied. In contrast to the existing results in
the literature, the adaptive consensus protocols here can be implemented by
each agent in a fully distributed fashion without using any global information.Comment: 15 pages, 6 figures, submitted to IEEE TA
Distributed Protocols with Threshold and General Trust Assumptions
Distributed systems today power almost all online applications. Consequently, a wide range of distributed protocols, such as consensus, and distributed cryptographic primitives are being researched and deployed in practice. This thesis addresses multiple aspects of distributed protocols and cryptographic schemes, enhancing their resilience, efficiency, and scalability.
Fundamental to every secure distributed protocols are its trust assumptions. These assumptions not only measure a protocol's resilience but also determine its scope of application, as well as, in some sense, the expressiveness and freedom of the participating parties. Dominant in practice is so far the threshold setting, where at most some f out of the n parties may fail in any execution. However, in this setting, all parties are viewed as identical, making correlations indescribable. These constraints can be surpassed with general trust assumptions, which allow arbitrary sets of parties to fail in an execution. Despite significant theoretical efforts, relevant practical aspects of this setting are yet to be addressed. Our work fills this gap. We show how general trust assumptions can be efficiently specified, encoded, and used in distributed protocols and cryptographic schemes. Additionally, we investigate a consensus protocol and distributed cryptographic schemes with general trust assumptions. Moreover, we show how the general trust assumptions of different systems, with intersecting or disjoint sets of participants, can be composed into a unified system.
When it comes to decentralized systems, such as blockchains, efficiency and scalability are often compromised due to the total ordering of all user transactions. Guerraoui (Distributed Computing, 2022) have contradicted the common design of major blockchains, proving that consensus is not required to prevent double-spending in a cryptocurrency. Modern blockchains support a variety of distributed applications beyond cryptocurrencies, which let users execute arbitrary code in a distributed and decentralized fashion. In this work we explore the synchronization requirements of a family of Ethereum smart contracts and formally establish the subsets of participants that need to synchronize their transactions.
Moreover, a common requirement of all asynchronous consensus protocols is randomness. A simple and efficient approach is to employ threshold cryptography for this. However, this necessitates in practice a distributed setup protocol, often leading to performance bottlenecks. Blum (TCC 2020) propose a solution bypassing this requirement, which is, however, practically inefficient, due to the employment of fully homomorphic encryption. Recognizing that randomness for consensus does not need to be perfect (that is, always unpredictable and agreed-upon) we propose a practical and concretely-efficient protocol for randomness generation.
Lastly, this thesis addresses the issue of deniability in distributed systems. The problem arises from the fact that a digital signature authenticates a message for an indefinite period. We introduce a scheme that allows the recipients to verify signatures, while allowing plausible deniability for signers. This scheme transforms a polynomial commitment scheme into a digital signature scheme
Byzantine Generals in the Permissionless Setting
Consensus protocols have traditionally been studied in a setting where all
participants are known to each other from the start of the protocol execution.
In the parlance of the 'blockchain' literature, this is referred to as the
permissioned setting. What differentiates Bitcoin from these previously studied
protocols is that it operates in a permissionless setting, i.e. it is a
protocol for establishing consensus over an unknown network of participants
that anybody can join, with as many identities as they like in any role. The
arrival of this new form of protocol brings with it many questions. Beyond
Bitcoin, what can we prove about permissionless protocols in a general sense?
How does recent work on permissionless protocols in the blockchain literature
relate to the well-developed history of research on permissioned protocols in
distributed computing?
To answer these questions, we describe a formal framework for the analysis of
both permissioned and permissionless systems. Our framework allows for
"apples-to-apples" comparisons between different categories of protocols and,
in turn, the development of theory to formally discuss their relative merits. A
major benefit of the framework is that it facilitates the application of a rich
history of proofs and techniques in distributed computing to problems in
blockchain and the study of permissionless systems. Within our framework, we
then address the questions above. We consider the Byzantine Generals Problem as
a formalisation of the problem of reaching consensus, and address a programme
of research that asks, "Under what adversarial conditions, and for what types
of permissionless protocol, is consensus possible?" We prove a number of
results for this programme, our main result being that deterministic consensus
is not possible for decentralised permissionless protocols. To close, we give a
list of eight open questions
Parameterized Verification of Systems with Global Synchronization and Guards
Inspired by distributed applications that use consensus or other agreement
protocols for global coordination, we define a new computational model for
parameterized systems that is based on a general global synchronization
primitive and allows for global transition guards. Our model generalizes many
existing models in the literature, including broadcast protocols and guarded
protocols. We show that reachability properties are decidable for systems
without guards, and give sufficient conditions under which they remain
decidable in the presence of guards. Furthermore, we investigate cutoffs for
reachability properties and provide sufficient conditions for small cutoffs in
a number of cases that are inspired by our target applications.Comment: Accepted at CAV 202
Towards Scaling Blockchain Systems via Sharding
Existing blockchain systems scale poorly because of their distributed
consensus protocols. Current attempts at improving blockchain scalability are
limited to cryptocurrency. Scaling blockchain systems under general workloads
(i.e., non-cryptocurrency applications) remains an open question. In this work,
we take a principled approach to apply sharding, which is a well-studied and
proven technique to scale out databases, to blockchain systems in order to
improve their transaction throughput at scale. This is challenging, however,
due to the fundamental difference in failure models between databases and
blockchain. To achieve our goal, we first enhance the performance of Byzantine
consensus protocols, by doing so we improve individual shards' throughput.
Next, we design an efficient shard formation protocol that leverages a trusted
random beacon to securely assign nodes into shards. We rely on trusted
hardware, namely Intel SGX, to achieve high performance for both consensus and
shard formation protocol. Third, we design a general distributed transaction
protocol that ensures safety and liveness even when transaction coordinators
are malicious. Finally, we conduct an extensive evaluation of our design both
on a local cluster and on Google Cloud Platform. The results show that our
consensus and shard formation protocols outperform state-of-the-art solutions
at scale. More importantly, our sharded blockchain reaches a high throughput
that can handle Visa-level workloads, and is the largest ever reported in a
realistic environment.Comment: This is an updated version of the Chain of Trust: Can Trusted
Hardware Help Scaling Blockchains? paper. This version is to be appeared in
SIGMOD 201
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