5705354
BAR fault tolerance for cooperative services
This paper describes a general approach to constructing cooperative services that span multiple administrative domains. In such environments, protocols must tolerate both Byzantine behaviors when broken, misconfigured, or malicious nodes arbitrarily deviate from their specification and rational behaviors when selfish nodes deviate from their specification to increase their local benefit. The paper makes three contributions: (1) It introduces the BAR (Byzantine, Altruistic, Rational) model as a foundation for reasoning about cooperative services; (2) It proposes a general three-level architecture to reduce the complexity of building services under the BAR model; and (3) It describes an implementation of BAR-B the first cooperative backup service to tolerate both Byzantine users and an unbounded number of rational users. At the core of BAR-B is an asynchronous replicated state machine that provides the customary safety and liveness guarantees despite nodes exhibiting both Byzantine and rational behaviors. Our prototype provides acceptable performance for our application: our BAR-tolerant state machine executes 15 requests per second, and our BAR-B backup service can back up 100MB of data in under 4 minutes.
731954
Separating agreement from execution for byzantine fault tolerant services
We describe a new architecture for Byzantine fault tolerant state machine replication that separates agreement that orders requests from execution that processes requests. This separation yields two fundamental and practically significant advantages over previous architectures. First, it reduces replication costs because the new architecture can tolerate faults in up to half of the state machine replicas that execute requests. Previous systems can tolerate faults in at most a third of the combined agreement/state machine replicas. Second, separating agreement from execution allows a general privacy firewall architecture to protect confidentiality through replication. In contrast, replication in previous systems hurts confidentiality because exploiting the weakest replica can be sufficient to compromise the system. We have constructed a prototype and evaluated it running both microbenchmarks and an NFS server. Overall, we find that the architecture adds modest latencies to unreplicated systems and that its performance is competitive with existing Byzantine fault tolerant systems.
5491635
Small byzantine quorum systems
In this paper we present two protocols for asynchronous Byzantine quorum systems (BQS) built on top of reliable channels-one for self-verifying data and the other for any data. Our protocols tolerate f Byzantine failures with f fewer servers than existing solutions by eliminating nonessential work in the write protocol and by using read and write quorums of different sizes. Since engineering a reliable network layer on an unreliable network is difficult, two other possibilities must be explored. The first is to strengthen the model by allowing synchronous networks that use time-outs to identify failed links or machines. We consider running synchronous and asynchronous Byzantine quorum protocols over synchronous networks and conclude that, surprisingly, "self-timing" asynchronous Byzantine protocols may offer significant advantages for many synchronous networks when network time-outs are long. We show how to extend an existing Byzantine quorum protocol to eliminate its dependency on reliable networking and to handle message loss and retransmission explicitly.
7300615
Minimal Byzantine Storage
Byzantine fault-tolerant storage systems can provide high availability in hazardous environments, but the redundant servers they require increase software development and hardware costs. In order to minimize the number of servers required to implement fault-tolerant storage services, we develop a new algorithm that uses a "Listeners" pattern of network communication to detect and resolve ordering ambiguities created by concurrent accesses to the system. Our protocol requires 3f + 1 servers to tolerate up to f Byzantine faults--f fewer than the 4f + 1 required by existing protocols for non-self-verifying data. In addition, SBQ-L provides atomic consistency semantics, which is stronger than the regular or pseudo-atomic semantics provided by these existing protocols. We show that this protocol is optimal in the number of servers-- any protocol that provides safe semantics or stronger requires at least 3f + 1 servers to tolerate f Byzantine faults in an asynchronous system. Finally, we examine a non-confirmable writes variation of the SBQ-L protocol where a client cannot determine when its writes complete. We show that SBQ-L with non-confirmable writes provides regular semantics with 2f + 1 servers and that this number of servers is minimal.
57028
Fast Byzantine Consensus
We present the first protocol that reaches asynchronous Byzantine consensus in two communication steps in the common case. We prove that our protocol is optimal in terms of both number of communication steps and number of processes for two-step consensus. The protocol can be used to build a replicated state machine that requires only three communication steps per request in the common case. Further, we show a parameterized version of the protocol that is safe despite f Byzantine failures and, in the common case, guarantees two-step execution despite some number t of failures (t les f). We show that this parameterized two-step consensus protocol is also optimal in terms of both number of communication steps and number of processes
53305414
Fast Byzantine Consensus
We present the first protocol that reaches asynchronous Byzantine consensus in two communication steps in the common case. We prove that our protocol is optimal in terms of both number of communication steps and number of processes for two-step consensus. The protocol can be used to build a replicated state machine that requires only three communication steps per request in the common case. Further, we show a parameterized version of the protocol that is safe despite f Byzantine failures and, in the common case, guarantees two-step execution despite some number t of failures (t les f). We show that this parameterized two-step consensus protocol is also optimal in terms of both number of communication steps and number of processes
15290012
Shallow Semantic Parsing using Support Vector Machines
In this paper, we propose a machine learning algorithm for shallow semantic parsing, extending the work of Gildea and Jurafsky (2002), Surdeanu et al. (2003) and others. Our algorithm is based on Support Vector Machines which we show give an improvement in performance over earlier classifiers. We show performance improvements through a number of new features and measure their ability to generalize to a new test set drawn from the AQUAINT corpus.
14531338
Building a Foundation System for Producing Short Answers to Factual Questions
In this paper we describe question answering research being pursued as a joint project between Columbia University and the University of Colorado at Boulder as part of ARDA’s AQUAINT program. As a foundation for targeting complex questions involving opinions, events, and paragraph-length answers, we recently built two systems for answering short factual questions. We submitted results from the two systems to TREC’s Q&A track, and the bulk of this paper describes the methods used in building each system and the results obtained. We conclude by discussing current work aiming at combining modules from the two systems in a unified, more accurate system and adding capabilities for producing complex answers in addition to short ones.
27507077
Temporal codes within a typology of cooperation between modalities
The Human-Machine Communication Department of LIMSI (LIMSI Report 1994), a laboratory of the French National Scientific Research Agency (CNRS), conducts research in the field of Pattern Recognition, Artificial Intelligence and Pattern Generation. Several approaches are currently investigated there for studying interactions between modalities: studies about the cognitive processes involved in human intermodal tasks (Denis and Cocude 1992; Gryl 1994; Daniel et al. 1994), image and language interactions for learning (Bordeaux 1993), human factors (Castaing and True 1993), symbolic approaches for designing multimodal human-computer interfaces (Bellik and Teil 1993; Bares et al. 1992), spatial reasoning (Ligozat 1992; Briffault 1992).
46416118
Active hand tracking
This paper describes a system which uses multiple visual processes to detect and track hands for gesture recognition and human-computer interaction. This system is based on an architecture in which a supervisor selects and activates visual processes. Each process provides a confidence factor which makes it possible for the system to dynamically reconfigure itself in response to events in the scene. Visual processes for hand tracking are described using image differencing and normalized histogram matching. The result of hand detection is used by a recursive estimator (Kalman filter) to provide an estimate of the position and size of the hand. The resulting system provides robust and precise tracking which operates continuously at approximately 5 images per second on a 150 megahertz Silicon Graphics Indy.
14471368
A Unified Framework for Constructing Multimodal Experiments and Applications
Inspired by a Wizard of Oz (WOZ) simulation experiment, we developed a working prototype of a system that enables users to interact with a map display through synergistic combinations of pen and voice. To address many of the issues raised by multimodal fusion, our implementation employed a distributed multi-agent framework to coordinate parallel competition and cooperation among processing components. Since then, the agent-based infrastructure has been enhanced with a collaboration technology, creating a framework in which multiple humans and automated agents can naturally interact within the same graphical workspace.Our current endeavour is the leveraging of this architecture to create a unified implementation framework for simultaneously developing both WOZ simulated systems and their fully-automated counterparts. Bootstrapping effects made possible by such an approach are illustrated by an experiment currently under way in our laboratory: as a naive subject draws, writes, and speaks requests to a (simulated) interactive map, a hidden Wizard responds as efficiently as possible using our best fullyautomated system, through either standard graphical interface devices or multimodal combinations of pen and voice. The input choices made by both subject and Wizard are invaluable, and the data collected from each can be applied directly to evaluating and improving the automated part of the system.