36 research outputs found
Using redundancy to cope with failures in a delay tolerant network
We consider the problem of routing in a delay tolerant net-work (DTN) in the presence of path failures. Previous work on DTN routing has focused on using precisely known network dy-namics, which does not account for message losses due to link failures, buffer overruns, path selection errors, unscheduled de-lays, or other problems. We show how to split, replicate, and erasure code message fragments over multiple delivery paths to optimize the probability of successful message delivery. We provide a formulation of this problem and solve it for two cases: a 0/1 (Bernoulli) path delivery model where messages are ei-ther fully lost or delivered, and a Gaussian path delivery model where only a fraction of a message may be delivered. Ideas from the modern portfolio theory literature are borrowed to solve the underlying optimization problem. Our approach is directly relevant to solving similar problems that arise in replica place-ment in distributed file systems and virtual node placement in DHTs. In three different simulated DTN scenarios covering a wide range of applications, we show the effectiveness of our ap-proach in handling failures
Symmetric Allocations for Distributed Storage
We consider the problem of optimally allocating a given total storage budget
in a distributed storage system. A source has a data object which it can code
and store over a set of storage nodes; it is allowed to store any amount of
coded data in each node, as long as the total amount of storage used does not
exceed the given budget. A data collector subsequently attempts to recover the
original data object by accessing each of the nodes independently with some
constant probability. By using an appropriate code, successful recovery occurs
when the total amount of data in the accessed nodes is at least the size of the
original data object. The goal is to find an optimal storage allocation that
maximizes the probability of successful recovery. This optimization problem is
challenging because of its discrete nature and nonconvexity, despite its simple
formulation. Symmetric allocations (in which all nonempty nodes store the same
amount of data), though intuitive, may be suboptimal; the problem is nontrivial
even if we optimize over only symmetric allocations. Our main result shows that
the symmetric allocation that spreads the budget maximally over all nodes is
asymptotically optimal in a regime of interest. Specifically, we derive an
upper bound for the suboptimality of this allocation and show that the
performance gap vanishes asymptotically in the specified regime. Further, we
explicitly find the optimal symmetric allocation for a variety of cases. Our
results can be applied to distributed storage systems and other problems
dealing with reliability under uncertainty, including delay tolerant networks
(DTNs) and content delivery networks (CDNs).Comment: 7 pages, 3 figures, extended version of an IEEE GLOBECOM 2010 pape
On file-based content distribution over wireless networks via multiple paths: Coding and delay trade-off
With the emergence of the adaptive bit rate (ABR) streaming technology, the video/content streaming technology is shifting toward a file-based content distribution. That is, video content is encoded into a set of smaller media files containing video of 2-10 seconds before transmission. This file-based content distribution, coupled with increasingly rapid adoption of smartphones, requires an efficient file-based distribution algorithm to satisfy the QoS demand in wireless networks. In this paper, we study the transmission of a finite-sized file over wireless networks using multipath routing, with the objective to minimize file transmission delay instead of average packet delay. The file transmission delay is defined as the time interval from the instant that a file is first transmitted to the time at which the file can be reconstructed in the destination node. We observe that file transmission delay depends not only on the mean of the packet delay but also on its distribution, especially the tail. This observation leads to a better understanding of the file transfer delay in wireless networks and a minimum delay file transmission strategy. In a wireless multipath communication scenario, we propose to use packet level erasure code (e.g., digital fountain code) to transmit data file with redundancy. Given that a file with k packets is encoded into n packets for transmission, the use of digital fountain code allows the file to be received when only k out of n packets are received. By adding redundant packets, the destination node does not have to wait for the packet to arrive late, hence reducing the delay of the file transmission. We characterize the tradeoff between the code rate (i.e., the ratio of the number of transmitted packets to the number of the original packets) and the file delay reduction. As a rule of thumb, we provide practical guidelines in determining an appropriate code rate for a fixed file to achieve a reasonable transmission delay. We show that only- - a few redundant packets are needed to achieve a significant reduction in file transmission delay
Reality-Check for DTN Routing Algorithms
Many applications of ad-hoc networks include in-termittent connectivity. Anyone wishing to implement routing into her delay-tolerant network can select from a wide variation of options, but the choice is hard, as there is no strong comparative evidence to the relative performance of the algorithms. Every paper uses a dif-ferent setting, mostly far from realistic. In our desire to improve the basis for decisions, we simulated a promis-ing selection of DTN routing algorithms in three vastly different scenarios, all based on publicly available real-world traces. Using our open-source DTN simulator, we compare and analyse 11 routing techniques, then provide explanations for the behaviour and give advice for choosing a suitable mechanism. To our own sur-prise, the results challenge the conventional wisdom gained from synthetic simulations and poses the ques-tion whether the world is ready for DTNs.
Low latency via redundancy
Low latency is critical for interactive networked applications. But while we
know how to scale systems to increase capacity, reducing latency --- especially
the tail of the latency distribution --- can be much more difficult. In this
paper, we argue that the use of redundancy is an effective way to convert extra
capacity into reduced latency. By initiating redundant operations across
diverse resources and using the first result which completes, redundancy
improves a system's latency even under exceptional conditions. We study the
tradeoff with added system utilization, characterizing the situations in which
replicating all tasks reduces mean latency. We then demonstrate empirically
that replicating all operations can result in significant mean and tail latency
reduction in real-world systems including DNS queries, database servers, and
packet forwarding within networks
Dynamic control of Coding in Delay Tolerant Networks
Delay tolerant Networks (DTNs) leverage the mobility of relay nodes to
compensate for lack of permanent connectivity and thus enable communication
between nodes that are out of range of each other. To decrease message delivery
delay, the information to be transmitted is replicated in the network. We study
replication mechanisms that include Reed-Solomon type codes as well as network
coding in order to improve the probability of successful delivery within a
given time limit. We propose an analytical approach that allows us to compute
the probability of successful delivery. We study the effect of coding on the
performance of the network while optimizing parameters that govern routing
Survivability in Time-varying Networks
Time-varying graphs are a useful model for networks with dynamic connectivity
such as vehicular networks, yet, despite their great modeling power, many
important features of time-varying graphs are still poorly understood. In this
paper, we study the survivability properties of time-varying networks against
unpredictable interruptions. We first show that the traditional definition of
survivability is not effective in time-varying networks, and propose a new
survivability framework. To evaluate the survivability of time-varying networks
under the new framework, we propose two metrics that are analogous to MaxFlow
and MinCut in static networks. We show that some fundamental
survivability-related results such as Menger's Theorem only conditionally hold
in time-varying networks. Then we analyze the complexity of computing the
proposed metrics and develop several approximation algorithms. Finally, we
conduct trace-driven simulations to demonstrate the application of our
survivability framework to the robust design of a real-world bus communication
network
Wireless mobile ad-hoc sensor networks for very large scale cattle monitoring
This paper investigates the use of wireless mobile ad hoc sensor networks in the nationwide cattle monitoring systems. This problem is essential for monitoring general animal health and detecting outbreaks of animal diseases that can be a serious threat for the national cattle industry and human health.
We begin by describing a number of related approaches for supporting animal monitoring applications and identify a comprehensive set of requirements that guides our approach. We then propose a novel infrastructure-less, self -organized peer to peer architecture that fulfills these requirements. The core of our work is the novel data storage and routing protocol for large scale, highly mobile ad hoc sensor networks that is based on the Distributed Hash Table (DHT) substrate that we optimize for disconnections. We show over a range of extensive simulations that by exploiting nodes’ mobility, packet overhearing and proactive caching we significantly improve availability of sensor data in these extreme conditions
Wireless mobile ad-hoc sensor networks for very large scale cattle monitoring
This paper investigates the use of wireless mobile ad hoc sensor networks in the nationwide cattle monitoring systems. This problem is essential for monitoring general animal health and detecting outbreaks of animal diseases that can be a serious threat for the national cattle industry and human health.
We begin by describing a number of related approaches for supporting animal monitoring applications and identify a comprehensive set of requirements that guides our approach. We then propose a novel infrastructure-less, self -organized peer to peer architecture that fulfills these requirements. The core of our work is the novel data storage and routing protocol for large scale, highly mobile ad hoc sensor networks that is based on the Distributed Hash Table (DHT) substrate that we optimize for disconnections. We show over a range of extensive simulations that by exploiting nodes’ mobility, packet overhearing and proactive caching we significantly improve availability of sensor data in these extreme conditions