1,090 research outputs found
Broadcasting in Noisy Radio Networks
The widely-studied radio network model [Chlamtac and Kutten, 1985] is a
graph-based description that captures the inherent impact of collisions in
wireless communication. In this model, the strong assumption is made that node
receives a message from a neighbor if and only if exactly one of its
neighbors broadcasts.
We relax this assumption by introducing a new noisy radio network model in
which random faults occur at senders or receivers. Specifically, for a constant
noise parameter , either every sender has probability of
transmitting noise or every receiver of a single transmission in its
neighborhood has probability of receiving noise.
We first study single-message broadcast algorithms in noisy radio networks
and show that the Decay algorithm [Bar-Yehuda et al., 1992] remains robust in
the noisy model while the diameter-linear algorithm of Gasieniec et al., 2007
does not. We give a modified version of the algorithm of Gasieniec et al., 2007
that is robust to sender and receiver faults, and extend both this modified
algorithm and the Decay algorithm to robust multi-message broadcast algorithms.
We next investigate the extent to which (network) coding improves throughput
in noisy radio networks. We address the previously perplexing result of Alon et
al. 2014 that worst case coding throughput is no better than worst case routing
throughput up to constants: we show that the worst case throughput performance
of coding is, in fact, superior to that of routing -- by a
gap -- provided receiver faults are introduced. However, we show that any
coding or routing scheme for the noiseless setting can be transformed to be
robust to sender faults with only a constant throughput overhead. These
transformations imply that the results of Alon et al., 2014 carry over to noisy
radio networks with sender faults.Comment: Principles of Distributed Computing 201
Doing-it-All with Bounded Work and Communication
We consider the Do-All problem, where cooperating processors need to
complete similar and independent tasks in an adversarial setting. Here we
deal with a synchronous message passing system with processors that are subject
to crash failures. Efficiency of algorithms in this setting is measured in
terms of work complexity (also known as total available processor steps) and
communication complexity (total number of point-to-point messages). When work
and communication are considered to be comparable resources, then the overall
efficiency is meaningfully expressed in terms of effort defined as work +
communication. We develop and analyze a constructive algorithm that has work
and a nonconstructive
algorithm that has work . The latter result is close to the
lower bound on work. The effort of each of
these algorithms is proportional to its work when the number of crashes is
bounded above by , for some positive constant . We also present a
nonconstructive algorithm that has effort
Deterministic Computations on a PRAM with Static Processor and Memory Faults.
We consider Parallel Random Access Machine (PRAM) which has some processors
and memory cells faulty. The faults considered are static, i.e., once the
machine starts to operate, the operational/faulty status of PRAM components
does not change. We develop a deterministic simulation of a fully operational
PRAM on a similar faulty machine which has constant fractions of faults among
processors and memory cells. The simulating PRAM has processors and
memory cells, and simulates a PRAM with processors and a constant fraction
of memory cells. The simulation is in two phases: it starts with
preprocessing, which is followed by the simulation proper performed in a
step-by-step fashion. Preprocessing is performed in time . The slowdown of a step-by-step part of the simulation is
Narwhal and Tusk: A DAG-based Mempool and Efficient BFT Consensus
We propose separating the task of reliable transaction dissemination from transaction ordering, to enable high-performance Byzantine fault-tolerant quorum-based consensus. We design and evaluate a mempool protocol, Narwhal, specializing in high-throughput reliable dissemination and storage of causal histories of transactions. Narwhal tolerates an asynchronous network and maintains high performance despite failures. Narwhal is designed to easily scale-out using multiple workers at each validator, and we demonstrate that there is no foreseeable limit to the throughput we can achieve. Composing Narwhal with a partially synchronous consensus protocol (Narwhal-HotStuff) yields significantly better throughput even in the presence of faults or intermittent loss of liveness due to asynchrony. However, loss of liveness can result in higher latency. To achieve overall good performance when faults occur we design Tusk, a zero-message overhead asynchronous consensus protocol, to work with Narwhal. We demonstrate its high performance under a variety of configurations and faults. As a summary of results, on a WAN, Narwhal-Hotstuff achieves over 130,000 tx/sec at less than 2-sec latency compared with 1,800 tx/sec at 1-sec latency for Hotstuff. Additional workers increase throughput linearly to 600,000 tx/sec without any latency increase. Tusk achieves 160,000 tx/sec with about 3 seconds latency. Under faults, both protocols maintain high throughput, but Narwhal-HotStuff suffers from increased latency
Broadcast CONGEST Algorithms against Adversarial Edges
We consider the corner-stone broadcast task with an adaptive adversary that
controls a fixed number of 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 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. We provide the first round-efficient broadcast
algorithms against adaptive edge adversaries. Our two key results for -node
graphs of diameter are as follows:
1. For , there is a deterministic algorithm that solves the problem
within rounds, provided that the graph is 3
edge-connected. This round complexity beats the natural barrier of
rounds, the existential lower bound on the maximal length of edge-disjoint
paths between a given pair of nodes in . This algorithm can be extended to a
-round algorithm against adversarial edges in
edge-connected graphs.
2. For expander graphs with minimum degree of , there is
an improved broadcast algorithm with rounds against
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.Comment: accepted to DISC2
Resilient Network Coding in the Presence of Byzantine Adversaries
Network coding substantially increases network throughput. But since it involves mixing of information inside the network, a single corrupted packet generated by a malicious node can end up contaminating all the information reaching a
destination, preventing decoding.
This paper introduces distributed polynomial-time rate-optimal network codes that work in the presence of Byzantine nodes. We present algorithms that target adversaries with different attacking capabilities. When the adversary can eavesdrop on all links and jam zO links, our first algorithm achieves a rate of C - 2zO, where C is the network capacity. In contrast, when the adversary has limited eavesdropping capabilities, we provide algorithms that achieve the higher rate of C - zO.
Our algorithms attain the optimal rate given the strength of the adversary. They are information-theoretically secure. They operate in a distributed manner, assume no knowledge of the topology, and can be designed and implemented in polynomial time. Furthermore, only the source and destination need to be modified; nonmalicious nodes inside the network are oblivious to the presence of adversaries and implement a classical distributed network code. Finally, our algorithms work over wired and wireless networks
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