Louise E. Moser

An Automata-Theoretic Decision Procedure for Future Interval Logic

Graphical Interval Logic (GIL) is a temporal logic in which all reasoning is done by means of diagrammatic formulae. It is a discrete linear-time modal logic in which the basic temporal modality is the interval. Future Interval Logic (FIL) provides the logical foundation for GIL. In this paper we present an automata-theoretic decision procedure for FIL with complexity DTIME\((2^{O(n^k )} )\), where n is the size of the formula and k is the depth of interval nesting. For formulae with bounded depth but length unbounded, the satisfiability problem for FIL is shown to be PSPACE-complete. We believe that this is the first result giving a direct decision procedure of elementary complexity for an interval logic. We also show that the logic is transparent to finite stuttering over the class of ω-sequences, a feature that is useful for composition and refinement.

A Real-Time Interval Logic and Its Decision Procedure

Real-Time Future Interval Logic is a visual logic in which formulae have a natural graphical representation, resembling timing diagrams. It is a dense real-time temporal logic that is based on two simple temporal primitives: interval modalities for the purely qualitative part and duration predicates for the quantitative part. We give a decision procedure for the logic by reduction to the emptiness problem for Timed Buchi Automata. The decision procedure forms the core of a proof checker for the logic that we have recently implemented. The logic does not admit instantaneous states, and is invariant under realtime stuttering. These properties facilitate proof methods based on abstraction and refinement. Two natural extensions of the logic lead to nonelementariness and undecidability.

A Graphical Interval Logic Toolset for Verifying Concurrent Systems

Graphical Interval Logic is the foundation of a toolset we have developed to support formal specification and verification of concurrent systems. The logic is a discrete linear-time temporal logic with the distinguishing feature that formulas in the logic have an intuitive graphical representation. The toolset includes a graphical editor that allows the user to compose and edit graphical formulas on a workstation display and a theorem prover that mechanically checks the validity of proofs in the logic. This paper describes the toolset and illustrates its use.

Total Ordering Algorithms for Asynchronous Byzantine Systems

The Total algorithms are used within asynchronous faulttolerant distributed systems to derive a total order on messages from a causal order provided by an underlying multicast communication protocol. We present several Total algorithms that represent varying compromises between latency to message ordering and resilience to crash and Byzantine faults. The algorithms use a multi-stage voting strategy to achieve agreement on the total order, and depend on the random structure of the causal order to achieve probabilistic termination.

The Real-Time Graphical Interval Logic Toolset

Our experience in using the RTGIL tools has shown that these tools and the graphical representation of the logic are very helpful for specifying and verifying properties of concurrent real-time systems. In addition to the aircraft example, we have used these tools to specify and verify properties of a railroad crossing system, a robot, an alarm system, and a four-phase handshaking protocol.

Reliable Ticket Routing in Expert Networks

Problem ticket resolution is an important aspect of the delivery of IT services. A large service provider needs to handle, on a daily basis, thousands of tickets that report various types of problems. Many of those tickets bounce among multiple expert groups before being transferred to the group with the expertise to solve the problem. Finding a methodology that can automatically make reliable ticket routing decisions and that reduces such bouncing and, hence, shortens ticket resolution time is a long-standing challenge. Reliable ticket routing forwards the ticket to an expert who either can solve the problem reported in the ticket, or can reach an expert who can resolve the ticket. In this chapter, we present a unified generative model, the Optimized Network Model (ONM), that characterizes the lifecycle of a ticket, using both the content and the routing sequence of the ticket. ONM uses maximum likelihood estimation to capture reliable ticket transfer profiles on each edge of an expert network. These transfer profiles reflect how the information contained in a ticket is used by human experts to make ticket routing decisions. Based on ONM, we develop a probabilistic algorithm to generate reliable ticket routing recommendations for new tickets in a network of expert groups. Our algorithm calculates all possible routes to potential resolvers and makes globally optimal recommendations, in contrast to existing classification methods that make static and locally optimal.

Live Upgrade Techniques for CORBA Applications

The ability to perform live software upgrades is essential for long-running applications that provide critical services. Program modifications are necessary as programmer errors and new user requirements are uncovered. If software is to remain relevant, it must be upgradable. The Eternal Evolution Manager allows distributed CORBA applications to be upgraded while they continue to provide service. In addition to avoiding planned downtime, the Evolution Manager accomplishes the difficult tasks inherent to software evolution with minimal help from the application programmer. With our live upgrade techniques, and the underlying fault tolerance of the Eternal System, we can allow applications to run forever.

First-Order Future Interval Logic

Future Interval Logic (FIL) is a linear-time temporal logic that is intended for specification and verification of reactive and concurrent systems. To make FIL useful for specifying and reasoning about practical systems, we present a first-order extension of FIL, including equality and n-ary function and predicate symbols, and a set of sound proof rules for reasoning in the logic. We illustrate the use of the logic by specifying a sliding window protocol and proving that the specifications satisfy a set of correctness requirements.

Maintaining censorship resistance in the iTrust network for publication, search and retrieval

This paper presents the architecture of the iTrust system together with algorithms for maintaining censorship resistance. In iTrust, metadata describing documents, and requests containing keywords, are distributed to randomly chosen nodes in the iTrust network. If a node receives a request containing keywords that match metadata it holds, it sends the URL of the matching document to the requesting node, which then retrieves the document from the source node. A novel detection algorithm estimates the proportion of operational nodes in the iTrust network, by comparing the empirical probabilities of the number of responses received for a node’s request with the analytical probabilities for a match, for various proportions of operational nodes. A novel defensive adaptation algorithm increases the number of nodes to which the requests are distributed, in order to maintain the same high probability of a match when some of the nodes are non-operational or malicious as when all of the nodes are operational. Extensive experimental evaluations demonstrate the effectiveness of the architecture and the algorithms for maintaining censorship resistance in the iTrust network.

Probabilistic Duration Automata for Analyzing Real-Time Systems

We present a novel methodology and tools for analyzing real-time systems that use probability density functions (pdfs) to represent the durations of operations within the system. We introduce the concept of a probabilistic duration automaton in which clocks are defined by pdfs rather than by explicit times. A state of a probabilistic duration automaton is a set of active clocks, and a transition is triggered by the expiration of one or more of these clocks. We present an algorithm for determining the probability that a clock in a state expires, the residual pdfs for the unexpired clocks, the probability of each transition, the probability of each state, and the duration of each state represented as a pdf. The algorithm also calculates the pdfs for durations of intervals between pairs of states within the automaton. These pdfs are used to determine whether a real-time system can meet its probabilistic timing constraints. An example application illustrates the use of this methodology in analyzing the real-time behavior of a four-phase handshaking protocol used in input/output systems.