## Kannan, Sampath

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Publication Randomized Pursuit-Evasion With Limited Visibility(2003-01-01) Kannan, Sampath; Isler, Volkan; Khanna, SanjeevWe study the following pursuit-evasion game: One or more hunters are seeking to capture an evading rabbit on a graph. At each round, the rabbit tries to gather information about the location of the hunters but it can see them only if they are located on adjacent nodes. We show that two hunters suffice for catching rabbits with such local visibility with high probability. We distinguish between reactive rabbits who move only when a hunter is visible and general rabbits who can employ more sophisticated strategies. We present polynomial time algorithms that decide whether a graph G is hunter-win, that is, if a single hunter can capture a rabbit of either kind on G.Publication AS-TRUST: A Trust Characterization Scheme for Autonomous Systems in BGP(2010-01-01) Kannan, Sampath; Chang, Jian; Loo, Boon Thau; Venkatasubramanian, Krishna K.; Sokolsky, Oleg; West, Andrew G.; Lee, InsupBorder Gateway Protocol (BGP) works by frequently exchanging updates which, disseminate reachability information (RI) about IP prefixes (i.e., address blocks) between Autonomous Systems (ASes) on the Internet. The current operation of BGP implicitly trusts the ASes to disseminate validâ€”accurate, stable and routing policy compliant â€” RI. This assumption is problematic as demonstrated by the recent documented instances of invalid RI dissemination. This paper presents AS-TRUST, a scheme which comprehensively characterizes the trustworthiness of ASes, with respect to disseminating valid RI. AS-TRUST quantifies trust using the notion of reputation. To compute reputation, AS-TRUST evaluates the past RI received for validity, based on a set of well-defined properties. It then classifies the resulting observations into multiple types of feedback. The feedback is used by a reputation function to compute a probabilistic view of AS trustworthiness. The contributions of the paper are: (1) a comprehensive trust characterization of ASes; (2) a set of well-defined properties for evaluating the validity of RI provided by ASes; and (3) a novel and theoretically sound reputation computation mechanism. Our implementation of AS-TRUST scheme using publicly available BGP traces demonstrates: the number of ASes involved in violating the BGP operational trust assumption is significant, dissemination of invalid RI is consistently present, and the proposed reputation mechanism is sensitive enough to capture even rare instances of an ASâ€™ deviation from trustworthy behavior.Publication VC-Dimension of Exterior Visibility of Polyhedra(2001-01-01) Kannan, Sampath; Isler, Volkan; Daniilidis, KostasIn this paper, we address the problem of finding the minimal number of viewpoints outside a polyhedron in two or three dimensions such that every point on the exterior of the polyhedron is visible from at least one of the chosen viewpoints. This problem which we call the minimum fortress guard problem (MFGP) is the optimization version of a variant of the art-gallery problem (sometimes called the fortress problem with point guards) and has practical importance in surveillance and image-based rendering. Solutions in the vision and graphics literature are based on image quality constraints and are not concerned with the number of viewpoints needed. The corresponding question for art galleries (minimum number of viewpoints in the interior of a polygon to see the interior of the polygon) which we call the minimum art-gallery guard problem (MAGP) has been shown to be NP-complete. A simple reduction from this problem shows the NP-completeness of MFGP. Instead of relying on heuristic searches, we address the approximability of the camera placement problem. It is well known (and easy to see) that this problem can be cast as a hitting set problem. While the approximability of generic instances of the hitting set problem is well understood, BrÃ¶nnimann and Goodrich[3] presented improved approximation algorithms for the problem in the case that the input instances have bounded Vapnik-Chervonenkis (VC) dimension. In this paper we explore the VC-dimension of set systems associated with the camera placement problem described above. We show a constant bound for the VC dimension in the two dimensional case but a tight logarithmic bound in the three dimensional case. In the two dimensional case we are also able to present an algorithm that uses at most one more viewpoint than the optimal in the case that the viewpoints are restricted to be on a circumscribing circle - a restriction that is justified in practice.Publication Graph Distances in the Data-Stream Model(2008-12-19) Feigenbaum, Joan; Kannan, Sampth; Mcgregor, Andrew; Suri, Siddarth; Zhang, JianWe explore problems related to computing graph distances in the data-stream model. The goal is to design algorithms that can process the edges of a graph in an arbitrary order given only a limited amount of working memory. We are motivated by both the practical challenge of processing massive graphs such as the web graph and the desire for a better theoretical understanding of the data-stream model. In particular, we are interested in the trade-offs between model parameters such as per-data-item processing time, total space, and the number of passes that may be taken over the stream. These trade-offs are more apparent when considering graph problems than they were in previous streaming work that solved problems of a statistical nature. Our results include the following: (1) Spanner construction: There exists a single-pass, (O) over tilde (tn(1+1/t))-space, (O) over tilde (t(2)n(1/t))-time-per-edge algorithm that constructs a (2t + 1)-spanner. For t = Omega(log n/log log n), the algorithm satisfies the semistreaming space restriction of O(n polylog n) and has per-edge processing time O(polylog n). This resolves an open question from [J. Feigenbaum et al., Theoret. Comput. Sci., 348 (2005), pp. 207-216]. (2) Breadth-first-search (BFS) trees: For any even constant k, we show that any algorithm that computes the first k layers of a BFS tree from a prescribed node with probability at least 2/3 requires either greater than k/2 passes or Omega(n(1+1/k)) space. Since constructing BFS trees is an important subroutine in many traditional graph algorithms, this demonstrates the need for new algorithmic techniques when processing graphs in the data-stream model. (3) Graph-distance lower bounds: Any t-approximation of the distance between two nodes requires Omega(n(1+1/t)) space. We also prove lower bounds for determining the length of the shortest cycle and other graph properties. (4) Techniques for decreasing per-edge processing: We discuss two general techniques for speeding up the per-edge computation time of streaming algorithms while increasing the space by only a small factor.Publication Security Protocols With Isotropic Channels(2006-01-01) Anand, Madhukar; Cronin, Eric; Sherr, Micah; Blaze, Matthew A; Kannan, SampathWe investigate the security properties of isotropic channels, broadcast media in which a receiver cannot reliably determine whether a message originated from any particular sender and a sender cannot reliably direct a message away from any particular receiver. We show that perfect isotropism implies perfect (information-theoretic) secrecy, and that asymptotically close to perfect secrecy can be achieved on any channel that provides some (bounded) uncertainty as to sender identity. We give isotropic security protocols under both passive and active adversary models, and discuss the practicality of realizing isotropic channels over various media.Publication Selection with Monotone Comparison Costs(2003-01-12) Kannan, Sampath; Khanna, SanjeevWe consider the problem of selecting the rth -smallest element from a list of nelements under a model where the comparisons may have different costs depending on the elements being compared. This model was introduced by [3] and is realistic in the context of comparisons between complex objects. An important special case of this general cost model is one where the comparison costs are monotone in the sizes of the elements being compared. This monotone cost model covers most "natural" cost models that arise and the selection problem turns out to be the most challenging one among the usual problems for comparison-based algorithms. We present an O(log2 n)-competitive algorithm for selection under the monotone cost model. This is in contrast to an Î© (n)lower bound that is known for arbitrary comparison costs. We also consider selection under a special case of monotone costsâ€”-the min model where the cost of comparing two elements is the minimum of the sizes. We give a randomized O(1)-competitive algorithm for the min model.Publication AS-CRED: Reputation Service for Trustworthy Inter-Domain Routing(2010-01-01) Kannan, Sampath; Chang, Jian; Lee, Insup; Venkatasubramanian, Krishna K.; Loo, Boon Thau; West, Andrew G.; Sokolsky, OlegThe current design of BGP implicitly assumes the existence of trust between ASes with respect to exchanging valid BGP updates. This assumption of complete trust is problematic given the frequent announcement of invalid -- inaccurate or unnecessary -- updates. This paper presents AS-CRED, a reputation service for ASes which quantifies the level of trust one can have with respect to its announcing valid updates. To compute the reputation, AS-CRED analyzes the past updates announced by each observable AS in the Internet, over a time-window, based on well-defined properties. It then classifies the resulting observations into multiple types of feedback. The feedback values are input into a mathematical function for computing AS reputation. The reputation is then used to track the instances of invalid updates announced in the Internet and trigger alerts. The contributions of the paper are: (1) a reputation service for ASes, characterizing their trustworthiness; (2) a set of well-defined properties for analyzing AS behavior; (3) a simple reputation function and feedback mechanism; (4) a reputation portal which regularly publishes AS reputation; and (5) a reputation-based alert service which tracks potentially invalid updates in the Internet. Detailed analysis of AS-CRED demonstrates: (a) AS behavior is repetitive making reputation an effective trust metric, and (b) AS-CRED's alerts for invalid updates show an eight fold improvement over existing alert systems.Publication MaC: A Framework for Run-Time Correctness Assurance of Real-Time Systems(1998) Kim, Moonjoo; Kannan, Sampath; Viswanathan, Mahesh; Lee, Insup; Ben-Abdallah, Hanene; Sokolsky, OlegWe describe the Monitoring and Checking (MaC) framework which provides assurance on the correctness of program execution at run-time. Our approach complements the two traditional approaches for ensuring that a system is correct, namely static analysis and testing. Unlike these approaches, which try to ensure that all possible executions of the system are correct, our approach concentrates on the correctness of the current execution of the system. The MaC architecture consists of three components: a filter, an event recognizer, and a run-time checker. The filter extracts low-level information, e.g,, values of program variables and function calls, from the system code, and sends it to the event recognizer. From this low-level information, the event recognizer detects the occurrence of "abstract" requirementsâ€“level events, and informs the run-time checker about them. The run-time checker uses these events to check that the current system execution conforms to the formal requirements specification of the system. This paper overviews our current prototype implementation, which uses JAVA as the implementation language and our Monitoring Script language as the requirements language.Publication Locating and Capturing an Evader in a Polygonal Environment(2003-01-01) Kannan, Sampath; Isler, Volkan; Khanna, SanjeevThis paper contains two main results: First, we revisit the well-known visibility based pursuit-evasion problem and show that, in contrast to deterministic strategies, a single pursuer can locate an unpredictable evader in any simply-connected polygonal environment using a randomized strategy. The evader can be arbitrarily faster than the pursuer and it may know the position of the pursuer at all times but it does not have prior knowledge of the random decisions made by the pursuer. Second, using the randomized algorithm together with the solution of a known lion and man problem [1] as subroutines, we present a strategy for two pursuers (one of which is at least as fast as the evader) to quickly capture an evader in a simply-connected polygonal environment. We show how this strategy can be extended to obtain a strategy for (i) capturing the evader in a polygonal room with a door, (ii) two pursuers who have only line-of-sight communication, and (iii) a single pursuer (at the expense of increased capture time).Publication Detecting Wikipedia Vandalism via Spatio-Temporal Analysis of Revision Metadata(2010-01-01) Kannan, Sampath; West, Andrew G.; Lee, InsupBlatantly unproductive edits undermine the quality of the collaboratively-edited encyclopedia, Wikipedia. They not only disseminate dishonest and offensive content, but force editors to waste time undoing such acts of vandalism. Language- processing has been applied to combat these malicious edits, but as with email spam, these filters are evadable and computationally complex. Meanwhile, recent research has shown spatial and temporal features effective in mitigating email spam, while being lightweight and robust. In this paper, we leverage the spatio-temporal properties of revision metadata to detect vandalism on Wikipedia. An administrative form of reversion called rollback enables the tagging of malicious edits, which are contrasted with nonoffending edits in numerous dimensions. Crucially, none of these features require inspection of the article or revision text. Ultimately, a classifier is produced which flags vandalism at performance comparable to the natural-language efforts we intend to complement (85% accuracy at 50% recall). The classifier is scalable (processing 100+ edits a second) and has been used to locate over 5,000 manually-confirmed incidents of vandalism outside our labeled set.