2,524 research outputs found
A framework for proving the self-organization of dynamic systems
This paper aims at providing a rigorous definition of self- organization, one
of the most desired properties for dynamic systems (e.g., peer-to-peer systems,
sensor networks, cooperative robotics, or ad-hoc networks). We characterize
different classes of self-organization through liveness and safety properties
that both capture information re- garding the system entropy. We illustrate
these classes through study cases. The first ones are two representative P2P
overlays (CAN and Pas- try) and the others are specific implementations of
\Omega (the leader oracle) and one-shot query abstractions for dynamic
settings. Our study aims at understanding the limits and respective power of
existing self-organized protocols and lays the basis of designing robust
algorithm for dynamic systems
A Graph Algorithmic Approach to Separate Direct from Indirect Neural Interactions
Network graphs have become a popular tool to represent complex systems
composed of many interacting subunits; especially in neuroscience, network
graphs are increasingly used to represent and analyze functional interactions
between neural sources. Interactions are often reconstructed using pairwise
bivariate analyses, overlooking their multivariate nature: it is neglected that
investigating the effect of one source on a target necessitates to take all
other sources as potential nuisance variables into account; also combinations
of sources may act jointly on a given target. Bivariate analyses produce
networks that may contain spurious interactions, which reduce the
interpretability of the network and its graph metrics. A truly multivariate
reconstruction, however, is computationally intractable due to combinatorial
explosion in the number of potential interactions. Thus, we have to resort to
approximative methods to handle the intractability of multivariate interaction
reconstruction, and thereby enable the use of networks in neuroscience. Here,
we suggest such an approximative approach in the form of an algorithm that
extends fast bivariate interaction reconstruction by identifying potentially
spurious interactions post-hoc: the algorithm flags potentially spurious edges,
which may then be pruned from the network. This produces a statistically
conservative network approximation that is guaranteed to contain non-spurious
interactions only. We describe the algorithm and present a reference
implementation to test its performance. We discuss the algorithm in relation to
other approximative multivariate methods and highlight suitable application
scenarios. Our approach is a tractable and data-efficient way of reconstructing
approximative networks of multivariate interactions. It is preferable if
available data are limited or if fully multivariate approaches are
computationally infeasible.Comment: 24 pages, 8 figures, published in PLOS On
The Canonical Amoebot Model: Algorithms and Concurrency Control
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
at 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. 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. As a case study, we demonstrate
both approaches using a simple algorithm for hexagon formation. Together, the
canonical amoebot model and these complementary approaches to concurrent
algorithm design open new directions for distributed computing research on
programmable matter.Comment: 48 pages, 7 figures, 2 table
Building a generalized distributed system model
The key elements in the second year (1991-92) of our project are: (1) implementation of the distributed system prototype; (2) successful passing of the candidacy examination and a PhD proposal acceptance by the funded student; (3) design of storage efficient schemes for replicated distributed systems; and (4) modeling of gracefully degrading reliable computing systems. In the third year of the project (1992-93), we propose to: (1) complete the testing of the prototype; (2) enhance the functionality of the modules by enabling the experimentation with more complex protocols; (3) use the prototype to verify the theoretically predicted performance of locking protocols, etc.; and (4) work on issues related to real-time distributed systems. This should result in efficient protocols for these systems
Byzantine fault-tolerant agreement protocols for wireless Ad hoc networks
Tese de doutoramento, Informática (Ciências da Computação), Universidade de Lisboa, Faculdade de Ciências, 2010.The thesis investigates the problem of fault- and intrusion-tolerant consensus
in resource-constrained wireless ad hoc networks. This is a fundamental
problem in distributed computing because it abstracts the need
to coordinate activities among various nodes. It has been shown to be a
building block for several other important distributed computing problems
like state-machine replication and atomic broadcast.
The thesis begins by making a thorough performance assessment of existing
intrusion-tolerant consensus protocols, which shows that the performance
bottlenecks of current solutions are in part related to their system
modeling assumptions. Based on these results, the communication failure
model is identified as a model that simultaneously captures the reality
of wireless ad hoc networks and allows the design of efficient protocols.
Unfortunately, the model is subject to an impossibility result stating that
there is no deterministic algorithm that allows n nodes to reach agreement
if more than n2 omission transmission failures can occur in a communication
step. This result is valid even under strict timing assumptions (i.e.,
a synchronous system).
The thesis applies randomization techniques in increasingly weaker variants
of this model, until an efficient intrusion-tolerant consensus protocol
is achieved. The first variant simplifies the problem by restricting the
number of nodes that may be at the source of a transmission failure at
each communication step. An algorithm is designed that tolerates f dynamic
nodes at the source of faulty transmissions in a system with a total
of n 3f + 1 nodes.
The second variant imposes no restrictions on the pattern of transmission
failures. The proposed algorithm effectively circumvents the Santoro-
Widmayer impossibility result for the first time. It allows k out of n nodes
to decide despite dn
2 e(nk)+k2 omission failures per communication
step. This algorithm also has the interesting property of guaranteeing
safety during arbitrary periods of unrestricted message loss.
The final variant shares the same properties of the previous one, but relaxes
the model in the sense that the system is asynchronous and that a
static subset of nodes may be malicious. The obtained algorithm, called
Turquois, admits f < n
3 malicious nodes, and ensures progress in communication
steps where dnf
2 e(n k f) + k 2. The algorithm is
subject to a comparative performance evaluation against other intrusiontolerant
protocols. The results show that, as the system scales, Turquois
outperforms the other protocols by more than an order of magnitude.Esta tese investiga o problema do consenso tolerante a faltas acidentais
e maliciosas em redes ad hoc sem fios. Trata-se de um problema fundamental
que captura a essência da coordenação em actividades envolvendo
vários nós de um sistema, sendo um bloco construtor de outros importantes
problemas dos sistemas distribuídos como a replicação de máquina
de estados ou a difusão atómica.
A tese começa por efectuar uma avaliação de desempenho a protocolos
tolerantes a intrusões já existentes na literatura. Os resultados mostram
que as limitações de desempenho das soluções existentes estão em parte
relacionadas com o seu modelo de sistema. Baseado nestes resultados, é
identificado o modelo de falhas de comunicação como um modelo que simultaneamente
permite capturar o ambiente das redes ad hoc sem fios e
projectar protocolos eficientes. Todavia, o modelo é restrito por um resultado
de impossibilidade que afirma não existir algoritmo algum que permita
a n nós chegaram a acordo num sistema que admita mais do que n2
transmissões omissas num dado passo de comunicação. Este resultado é
válido mesmo sob fortes hipóteses temporais (i.e., em sistemas síncronos)
A tese aplica técnicas de aleatoriedade em variantes progressivamente
mais fracas do modelo até ser alcançado um protocolo eficiente e tolerante
a intrusões. A primeira variante do modelo, de forma a simplificar
o problema, restringe o número de nós que estão na origem de transmissões
faltosas. É apresentado um algoritmo que tolera f nós dinâmicos na
origem de transmissões faltosas em sistemas com um total de n 3f + 1
nós.
A segunda variante do modelo não impõe quaisquer restrições no padrão
de transmissões faltosas. É apresentado um algoritmo que contorna efectivamente
o resultado de impossibilidade Santoro-Widmayer pela primeira
vez e que permite a k de n nós efectuarem progresso nos passos de comunicação
em que o número de transmissões omissas seja dn
2 e(n
k) + k 2. O algoritmo possui ainda a interessante propriedade de tolerar
períodos arbitrários em que o número de transmissões omissas seja
superior a .
A última variante do modelo partilha das mesmas características da variante
anterior, mas com pressupostos mais fracos sobre o sistema. Em particular,
assume-se que o sistema é assíncrono e que um subconjunto estático
dos nós pode ser malicioso. O algoritmo apresentado, denominado
Turquois, admite f < n
3 nós maliciosos e assegura progresso nos passos
de comunicação em que dnf
2 e(n k f) + k 2. O algoritmo é
sujeito a uma análise de desempenho comparativa com outros protocolos
na literatura. Os resultados demonstram que, à medida que o número de
nós no sistema aumenta, o desempenho do protocolo Turquois ultrapassa
os restantes em mais do que uma ordem de magnitude.FC
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