6,188 research outputs found
Construction and Applications of CRT Sequences
Protocol sequences are used for channel access in the collision channel
without feedback. Each user accesses the channel according to a deterministic
zero-one pattern, called the protocol sequence. In order to minimize
fluctuation of throughput due to delay offsets, we want to construct protocol
sequences whose pairwise Hamming cross-correlation is as close to a constant as
possible. In this paper, we present a construction of protocol sequences which
is based on the bijective mapping between one-dimensional sequence and
two-dimensional array by the Chinese Remainder Theorem (CRT). In the
application to the collision channel without feedback, a worst-case lower bound
on system throughput is derived.Comment: 16 pages, 5 figures. Some typos in Section V are correcte
Compositional competitiveness for distributed algorithms
We define a measure of competitive performance for distributed algorithms
based on throughput, the number of tasks that an algorithm can carry out in a
fixed amount of work. This new measure complements the latency measure of Ajtai
et al., which measures how quickly an algorithm can finish tasks that start at
specified times. The novel feature of the throughput measure, which
distinguishes it from the latency measure, is that it is compositional: it
supports a notion of algorithms that are competitive relative to a class of
subroutines, with the property that an algorithm that is k-competitive relative
to a class of subroutines, combined with an l-competitive member of that class,
gives a combined algorithm that is kl-competitive.
In particular, we prove the throughput-competitiveness of a class of
algorithms for collect operations, in which each of a group of n processes
obtains all values stored in an array of n registers. Collects are a
fundamental building block of a wide variety of shared-memory distributed
algorithms, and we show that several such algorithms are competitive relative
to collects. Inserting a competitive collect in these algorithms gives the
first examples of competitive distributed algorithms obtained by composition
using a general construction.Comment: 33 pages, 2 figures; full version of STOC 96 paper titled "Modular
competitiveness for distributed algorithms.
ObliviSync: Practical Oblivious File Backup and Synchronization
Oblivious RAM (ORAM) protocols are powerful techniques that hide a client's
data as well as access patterns from untrusted service providers. We present an
oblivious cloud storage system, ObliviSync, that specifically targets one of
the most widely-used personal cloud storage paradigms: synchronization and
backup services, popular examples of which are Dropbox, iCloud Drive, and
Google Drive. This setting provides a unique opportunity because the above
privacy properties can be achieved with a simpler form of ORAM called
write-only ORAM, which allows for dramatically increased efficiency compared to
related work. Our solution is asymptotically optimal and practically efficient,
with a small constant overhead of approximately 4x compared with non-private
file storage, depending only on the total data size and parameters chosen
according to the usage rate, and not on the number or size of individual files.
Our construction also offers protection against timing-channel attacks, which
has not been previously considered in ORAM protocols. We built and evaluated a
full implementation of ObliviSync that supports multiple simultaneous read-only
clients and a single concurrent read/write client whose edits automatically and
seamlessly propagate to the readers. We show that our system functions under
high work loads, with realistic file size distributions, and with small
additional latency (as compared to a baseline encrypted file system) when
paired with Dropbox as the synchronization service.Comment: 15 pages. Accepted to NDSS 201
KeyForge: Mitigating Email Breaches with Forward-Forgeable Signatures
Email breaches are commonplace, and they expose a wealth of personal,
business, and political data that may have devastating consequences. The
current email system allows any attacker who gains access to your email to
prove the authenticity of the stolen messages to third parties -- a property
arising from a necessary anti-spam / anti-spoofing protocol called DKIM. This
exacerbates the problem of email breaches by greatly increasing the potential
for attackers to damage the users' reputation, blackmail them, or sell the
stolen information to third parties.
In this paper, we introduce "non-attributable email", which guarantees that a
wide class of adversaries are unable to convince any third party of the
authenticity of stolen emails. We formally define non-attributability, and
present two practical system proposals -- KeyForge and TimeForge -- that
provably achieve non-attributability while maintaining the important protection
against spam and spoofing that is currently provided by DKIM. Moreover, we
implement KeyForge and demonstrate that that scheme is practical, achieving
competitive verification and signing speed while also requiring 42% less
bandwidth per email than RSA2048
A Dual Digraph Approach for Leaderless Atomic Broadcast (Extended Version)
Many distributed systems work on a common shared state; in such systems,
distributed agreement is necessary for consistency. With an increasing number
of servers, these systems become more susceptible to single-server failures,
increasing the relevance of fault-tolerance. Atomic broadcast enables
fault-tolerant distributed agreement, yet it is costly to solve. Most practical
algorithms entail linear work per broadcast message. AllConcur -- a leaderless
approach -- reduces the work, by connecting the servers via a sparse resilient
overlay network; yet, this resiliency entails redundancy, limiting the
reduction of work. In this paper, we propose AllConcur+, an atomic broadcast
algorithm that lifts this limitation: During intervals with no failures, it
achieves minimal work by using a redundancy-free overlay network. When failures
do occur, it automatically recovers by switching to a resilient overlay
network. In our performance evaluation of non-failure scenarios, AllConcur+
achieves comparable throughput to AllGather -- a non-fault-tolerant distributed
agreement algorithm -- and outperforms AllConcur, LCR and Libpaxos both in
terms of throughput and latency. Furthermore, our evaluation of failure
scenarios shows that AllConcur+'s expected performance is robust with regard to
occasional failures. Thus, for realistic use cases, leveraging redundancy-free
distributed agreement during intervals with no failures improves performance
significantly.Comment: Overview: 24 pages, 6 sections, 3 appendices, 8 figures, 3 tables.
Modifications from previous version: extended the evaluation of AllConcur+
with a simulation of a multiple datacenters deploymen
Merlin: A Language for Provisioning Network Resources
This paper presents Merlin, a new framework for managing resources in
software-defined networks. With Merlin, administrators express high-level
policies using programs in a declarative language. The language includes
logical predicates to identify sets of packets, regular expressions to encode
forwarding paths, and arithmetic formulas to specify bandwidth constraints. The
Merlin compiler uses a combination of advanced techniques to translate these
policies into code that can be executed on network elements including a
constraint solver that allocates bandwidth using parameterizable heuristics. To
facilitate dynamic adaptation, Merlin provides mechanisms for delegating
control of sub-policies and for verifying that modifications made to
sub-policies do not violate global constraints. Experiments demonstrate the
expressiveness and scalability of Merlin on real-world topologies and
applications. Overall, Merlin simplifies network administration by providing
high-level abstractions for specifying network policies and scalable
infrastructure for enforcing them
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