In this paper, we design and evaluate algorithms for fast recovery from link failures in a smart grid communication network, addressing all three aspects of link failure recovery: (a) link failure detection, (b) algorithms for computing backup multicast trees, and (c) fast backup tree installation. To address (a), we design link-failure detection and reporting mechanisms that use OpenFlow to detect link failures when and where they occur inside the network. OpenFlow is an open source framework that cleanly separates the control and data planes for use in network management and control. For part (b), we formulate a new problem, Multicast Recycling, that computes backup multicast trees that aim to minimize control plane signaling overhead. We prove Multicast Recycling is at least NP-hard and present a corresponding approximation algorithm. Lastly, two control plane algorithms are proposed that signal data plane switches to install pre-computed backup trees. An optimized version of each installation algorithm is designed that finds a near minimum set of forwarding rules by sharing rules across multicast groups, thereby reducing backup tree install time and associated control state. We implement these algorithms in the POX OpenFlow controller and evaluate them using the Mininet emulator, quantifying control plane signaling and installation time.

MAKING NETWORKS ROBUST TO COMPONENT FAILURES

Significant investments have been made into deploying phasor measurement units (PMUs) on electric power grids worldwide. PMUs allow the state of the power system - the voltage phasor of system buses and current phasors of all incident transmission lines - to be directly measured. In some cases, it is also possible to infer the voltage and current phasors at neighboring buses and lines. Because PMUs are expensive, it is typically not possible to deploy enough PMUs to observe all phasors in a grid network [3, 6]. In this paper, we prove the NP-Completeness of four problems relating to PMU placements at a subset of system buses to achieve different goals: FullObserve, MaxObserve, FullObserve-XV, and MaxObserve-XV. FullObserve considers the minimum number of PMUs needed to observe all nodes, while MaxObserve considers the maximum number of buses that can be observed with a given number of PMUs. While the first of these two has been considered in the past, our formulation here generalizes the systems being considered. Next, FullObserve-XV and MaxObserve-XV consider these two problems under the constraints that PMUs must be placed “close” to each other so their measurements can be cross-validated. FullObserve-XV considers observing the entire network, while MaxObserve-XV considers maximizing the number of observed buses under this new constraint. Motivated by their high complexity, for each problem we investigate the performance of a suitable greedy approximation algorithm for PMU placement. Through simulations, we compare the performance of these algorithms with the optimal placement of PMUs over several IEEE bus systems as well as over synthetic graphs. In our simulations these algorithms yield results that are close to optimal - for all four placement problems, the greedy algorithms yield, on average, a PMU placement that is within 97% of optimal.

Malicious and misconfigured nodes can inject incorrect state into a distributed system, which can then be propagated system-wide as a result of normal network operation. Such false state can degrade the performance of a distributed system or render it unusable. For example, in the case of network routing algorithms, false state corresponding to a node incorrectly declaring a cost of 0 to all destinations (maliciously or due to misconfiguration) can quickly spread through the network. This causes other nodes to (incorrectly) route via the misconfigured node, resulting in suboptimal routing and network congestion. We propose three algorithms for efficient recovery in such scenarios and prove the correctness of each of these algorithms. Through simulation, we evaluate our algorithms – in terms of message and time overhead – when applied to removing false state in distance vector routing. Our analysis shows that over topologies where link costs remain fixed and for the same topologies where link costs change, a recovery algorithm based on system-wide checkpoints and a rollback mechanism yields superior performance when using the poison reverse optimization.

Pattern matching over event streams is increasingly being employed in many areas including financial services, RFIDbased inventory management, click stream analysis, and electronic health systems. While regular expression matching is well studied, pattern matching over streams presents two new challenges: Languages for pattern matching over streams are significantly richer than languages for regular expression matching. Furthermore, efficient evaluation of these pattern queries over streams requires new algorithms and optimizations: the conventional wisdom for stream query processing (i.e., using selection-join-aggregation) is inadequate. In this paper, we present a formal evaluation model that offers precise semantics for this new class of queries and a query evaluation framework permitting optimizations in a principled way. We further analyze the runtime complexity of query evaluation using this model and develop a suite of techniques that improve runtime efficiency by exploiting sharing in storage and processing. Our experimental results provide insights into the various factors on runtime performance and demonstrate the significant performance gains of our sharing techniques.

Complex event patterns involving Kleene closure are finding application in a variety of stream environments for tracking and monitoring purposes. In this paper, we propose a compact language, SASE+, that can be used to define a wide variety of Kleene closure patterns, analyze the expressive power of the language, and outline an automata-based implementation for efficient Kleene closure evaluation over event streams.

In this paper, we present SASE+, a complex event language that supports Kleene closure over event streams, and provide a formal analysis of the expressibility of this language. Complex event patterns involving Kleene closure are finding application in a growing number of stream applications including financial services, RFIDbased inventory management, monitoring in healthcare, etc. While Kleene closure has been well studied for regular expression matching, Kleene closure patterns over streams have unique features regarding the event definition, event selection, and termination criteria, which fundamentally distinguish them from patterns studied in conventional problems. This paper addresses Kleene closure in this new context. In particular, we propose a compact language that can be used to define a wide variety of Kleene closure patterns, develop a formal model that precisely describes the semantics and expressive power of the language, and characterize its relationships to standard languages in the literature as well as recent proposals for stream-based event languages.

RFID technology is gaining adoption on an increasing scale for tracking and monitoring purposes. Wide deployments of RFID devices will soon generate an unprecedented volume of data. Emerging applications require the RFID data to be filtered and correlated for complex pattern detection and transformed to events that provide meaningful, actionable information to end applications. In this work, we design and develop SASE, a com-plex event processing system that performs such data-information transformation over real-time streams. We design a complex event language for specifying application logic for such transformation, devise new query processing techniques to effi-ciently implement the language, and develop a comprehensive system that collects, cleans, and processes RFID data for deliv-ery of relevant, timely information as well as storing necessary data for future querying. We demonstrate an initial prototype of SASE through a real-world retail management scenario.

This demonstration shows a prototype tool that projects an impression of how execution of a formally defined process will facilitate dispute resolution. Tool flexibility supports projecting the look and feel of a range of different processes, facilitating user evaluation of alternatives.

In this system demonstration, we show a formal process model of a manual Alternative Dispute Resolution process and an Online Dispute Resolution system based on this model.