Ramesh Viswanathan

A conceptual framework for network management event correlation and filtering systems

Event correlation is a key functionality of a network management system that is used to determine the root cause of faults in a network, and to filter out redundant and spurious events. A number of event correlation systems have been proposed. The event correlation systems generally combine causal and temporal correlation models with the topology of a network. The power and robustness of the models used and the algorithms developed vary from system to system. However, in the absence of a simple, uniform, and precise presentation of the event-correlation problem, it is impossible to compare their relative power or even analyze them for their properties. In general, causal and temporal-based correlation models have not been rigorously presented or thoroughly investigated. In this paper we formalize the concepts of causal and temporal correlation using a single conceptual framework. We characterize various properties of the framework. We can characterize existing systems based on the formal properties of our framework, and we consider one system as an illustrative example.

Topology inference in the presence of anonymous routers

Many topology discovery systems rely on traceroute to discover path information in public networks. However, for some routers, traceroute detects their existence but not their address; we term such routers anonymous routers. This paper considers the problem of inferring the network topology in the presence of anonymous routers. We illustrate how obvious approaches to handle anonymous routers lead to incomplete, inflated, or inaccurate topologies. We formalize the topology inference problem and show that producing both exact and approximate solutions is intractable. Two heuristics are proposed and evaluated through simulation. These heuristics have been used to infer the topology of the 6Bone, and could be incorporated into existing tools to infer more comprehensive and accurate topologies.

Optimal Resource Allocation in Clouds

Cloud platforms enable enterprises to lease computing power in the form of virtual machines. An important problem for such enterprise users is to understand how many and what kinds of virtual machines will be needed from clouds. We formulate demand for computing power and other resources as a resource allocation problem with multiplicity, where computations that have to be performed concurrently are represented as tasks and a later task can reuse resources released by an earlier task. We show that finding a minimized allocation is NP-complete. This paper presents an approximation algorithm with a proof of its approximation bound that can yield close to optimum solutions in polynomial time. Enterprise users can exploit the solution to reduce the leasing cost and amortize the administration overhead (e.g., setting up VPNs or configuring a cluster). Cloud providers may utilize the solution to share their resources among a larger number of users.

Topology discovery for public IPv6 networks

In just three decades the Internet has grown from a small experimental research network into a complex network of routers, switches, and hosts. Understanding the topology of such large scale networks is essential to the procurement of good architectural design decisions, particularly with respect to address allocation and distribution schemes.A number of techniques for IPv4 network topology already exist. Of these ICMP-based probing has shown to be most useful in determining router-level topologies of public networks. However, many of these techniques cannot be readily applied to IPv6 because of changes in the addressing scheme and ICMP behaviour. Furthermore, increases in the proliferation of equal-cost multi-path routing, and other forms of transient routing, indicate that traditional traceroute-based topology discovery approaches are becoming less effective in the Internet.This paper presents Atlas, a system that faciliates the automated capture of IPv6 network topology information from a single probing host. It describes the Atlas infrastructure and its data collection processes and discusses IPv6 network phenomena that must to be taken into account by the probing scheme. We also present some initial results from our probing of the 6Bone, currently the largest public IPv6 network. The results illustrate the effectiveness of the probing algorithm and also identify some trends in prefix allocation and routing policy.

A Higher Order Modal Fixed Point Logic

We present a higher order modal fixed point logic (HFL) that extends the modal μ-calculus to allow predicates on states (sets of states) to be specified using recursively defined higher order functions on predicates. The logic HFL includes negation as a first-class construct and uses a simple type system to identify the monotonic functions on which the application of fixed point operators is semantically meaningful. The model checking problem for HFL over finite transition systems remains decidable, but its expressiveness is rich. We construct a property of finite transition systems that is not expressible in the Fixed Point Logic with Chop [1] but which can be expressed in HFL. Over infinite transition systems, HFL can express bisimulation and simulation of push down automata, and any recursively enumerable property of a class of transition systems representing the natural numbers.

Full abstraction for first-order objects with recursive types and subtyping

We present a new interpretation of typed object-oriented concepts in terms of well-understood, purely procedural concepts, that preserves observational equivalence. More precisely, we give compositional translations of (a) Ob/sub 1/spl mu//, an object calculus supporting method invocation and functional method update with first-order object types and recursive types, and (b) Ob/sub 1<:/spl mu//, an extension of Ob/sub 1/spl mu// with subtyping, that are fully abstract on closed terms. The target of the translations are a first-order /spl lambda/-calculus with records and recursive types, with and without subtyping. The translation of the calculus with subtyping is subtype-preserving as well.

Isolating side effects in sequential languages

It is well known that adding side effects to functional languages changes the operational equivalences of the language. We develop a new language construct, encap, that forces imperative pieces of code to behave purely functionally, i.e.,without any visible side effects. The coercion operator encap provides a means of extending the simple reasoning principles for equivalences of code in a functional language to a language with side effects. In earlier work, similar coercion operators were developed, but their correctness required the underlying functional language to include parallel operations. The coercion operators developed here are simpler and are proven correct for purely sequential languages. The sequential setting requires the construction of fully abstract models for sequential call-by-value languages and the formulation of a weak form of “monad” suitable for expressing the semantics of call-by-value languages with side effects.

Placement in Clouds for Application-Level Latency Requirements

CPU and device virtualization technology allows applications to be hosted on cloud platforms; some of the resulting benefits are lower cost and greater elasticity. In such cloud hosted applications, some components reside on the cloud while others, such as end users and components tied to physical devices, are located outside the cloud. Many applications, e.g., telecom services, have stringent latency requirements in terms of within how much time certain procedures must be completed. The application latency is strongly determined by the locations of all the interacting components that are both within and outside the cloud. In this paper, we study the problem of determining the optimal placement of the application components in the cloud so that the latency requirements of the application can be met. We present a precise formulation of the placement problem which includes a specification of the cloud platform, and collective latency expressions for application-level latency requirements. We show that Message Sequence Charts (MSCs), a widely-used mechanism for describing the execution of application procedures, can be naturally translated into our formalism of collective latency expressions. We present placement algorithms that exploit the Euclidean triangular inequality property of network topologies: (a) an exact algorithm for determining the most optimal placement but which has a worst-case exponential running time, and (b) an algorithm for determining a close to-optimal placement that has a fast polynomial running time. Additionally, we present an exact technique for partitioning a placement problem into smaller sub problems so that greater efficiency and accuracy can be achieved. We evaluate the performance of the algorithms on a representative telecom application --- a distributed deployment of the LTE Mobility Management Entity (MME). Our evaluation results show that our approximate algorithm can outperform a random placement by up to 49% for finding a successful placement.

Correct Passive Testing Algorithms and Complete Fault Coverage

The aim of passive testing is to detect faults in a system while observing the system during normal operation, that is, without forcing the system to specialized inputs explicitly for the purposes of testing. We formulate some general correctness requirements on any passive-testing algorithm which we term soundness and completeness. With respect to these definitions, we show that the homing algorithm, first proposed in [4], and subsequently used in [6–8], is sound and complete for passively testing the conformance of an implementation for several distinct conformance notions ranging from trace-containment to observational equivalence to even exact identity. This implies that, for some notions of conformance, there are faulty implementations that would not be detectable by any sound passive testing algorithm. We define a property to be passively testable as one admitting complete fault coverage under passive testing, i.e., one for which any faulty execution can be detected through passive testing. We provide an exact characterization of passively testable properties as being a natural subclass of safety properties, namely, those that are trace-contained in sets that are prefix- and suffix-closed. For such properties, we derive efficient complete passive testing algorithms that take constant time. We demonstrate the applicability of these results to networks and network devices by considering the problem of passively testing an implementation for conformance to the TCP protocol.

Least Upper Bounds for Probability Measures and Their Applications to Abstractions

Abstraction is a key technique to combat the state space explosion problem in model checking probabilistic systems. In this paper we present new ways to abstract Discrete Time Markov Chains (DTMCs), Markov Decision Processes (MDPs), and Continuous Time Markov Chains (CTMCs). The main advantage of our abstractions is that they result in abstract models that are purely probabilistic, which maybe more amenable to automatic analysis than models with both nondeterministic and probabilistic steps that typically arise from previously known abstraction techniques. A key technical tool, developed in this paper, is the construction of least upper bounds for any collection of probability measures. This upper bound construction may be of independent interest that could be useful in the abstract interpretation and static analysis of probabilistic programs.