33218482
Some Algorithms for Extension Computation of Nonmonotonic Rule Systems
In this paper, we will place our emphasis on the problem of existence of extensions and on finding more efficient algorithms for computing extensions of nonmonotonic rule systems. First, in order to dedicate further research into nonmonotonic rule systems a new characterization and some new results are given. This characterization and these results lead to new algorithms for computing extensions. Second, a class of nonmonotonic rule systems, called regular nonmonotonic rule systems, is developed. The existence of extensions and semi-monotonicity of regular nonmonotonic rule systems are proved. Third, we prove that each regular nonmonotonic rule system has at most 1.47^n extensions, where n is the number of elements occurring in the system.
31571428
Polynomial time algorithms for computing a representation for minimal unsatisfiable formulas with fixed deficiency
We say a propositional formula F in conjunctive normal form is represented by a formula H and a homomorphism φ, if φ(H) = F. A homomorphism is a mapping consisting of a renaming and an identification of literals. The deficiency of a formula is the difference between the number of clauses and the number of variables. We show that for fixed k ≥ 1 and t ≥ 1 each minimal unsatisfiable formula with deficiency k can be represented by a formula H with deficiency t and a homomorphism and such a representation can be computed in polynomial time.
13301251
Fixed-parameter tractability of disjunction-free default reasoning
In this paper, the parameter which is the source of the complexity of disjunction-free default reasoning is determined. It is shown that when the value of this parameter is fixed, the disjunction-free default reasoning can be solved in time bounded by a polynomial whose degree does not depend on the parameter. Consequently, disjunction-free default reasoning is fixed parameter tractable.
8778857
On Models for Quantified Boolean Formulas
A quantified Boolean formula is true, if for any existentially quantified variable there exists a Boolean function depending on the preceding universal variables, such that substituting the existential variables by the Boolean functions results in a true formula. We call a satisfying set of Boolean functions a model. In this paper, we investigate for various classes of quantified Boolean formulas and various classes of Boolean functions the problem whether a model exists. Furthermore, for these classes the complexity of the model checking problem – whether a set of Boolean functions is a model for a formula – will be shown. Finally, for classes of Boolean functions we establish some characterizations in terms of quantified Boolean formulas which have such a model. For example, roughly speaking any satisfiable quantified Boolean Horn formula can be satisfied by monomials and vice versa.
32094146
On Boolean Models for Quantified Boolean Horn Formulas
For a Quantified Boolean Formula \(({\it QBF })\) Φ=Qφ, an assignment is a function \(\cal M\) that maps each existentially quantified variable of Φ to a Boolean function, where φ is a propositional formula and Q is a linear ordering of quantifiers on the variables of Φ. An assignment \(\cal M\) is said to be proper, if for each existentially quantified variable y i , the associated Boolean function f i does not depend upon the universally quantified variables whose quantifiers in Q succeed the quantifier of y i . An assignment \(\cal M\) is said to be a model for Φ, if it is proper and the formula \(\phi^{\cal M}\) is a tautology, where \(\phi^{\cal M}\) is the formula obtained from φ by substituting f i for each existentially quantified variable y i . We show that any true quantified Horn formula has a Boolean model consisting of monotone monomials and constant functions only; conversely, if a QBF has such a model then it contains a clause–subformula in \({\it QHORN }\cap {\it SAT }\).
5268865
On the structure of some classes of minimal unsatisfiable formulas
We investigate classes of minimal unsatisfiable formulas which are closed under splitting. For marginal formulas the equivalence to some natural classes of formulas is proved. Further, we show that maximal formulas are closely related to the so-called hitting formulas. That are formulas for which any two clauses contain a pair of complementary literals.
2194617
Model Checking Dynamic UML Consistency
UML is widely accepted and extensively used in software modeling. However, using different diagrams to model different aspects of a system brings the risk of inconsistency among diagrams. In this paper, we investigate an approach to check the consistency between the sequence diagrams and statechart diagrams using the SPIN model checker. To deal with the hierarchy structure of statechart diagrams, we propose a formalism called Split Automata, a variant of automata, which is helpful to bridge the statechart diagrams to SPIN efficiently. Compared with the existing work on model checking UML which do not have formal verification for their translation from UML to the model checker, we formally define the semantics and prove that the automatically translated model (i.e. Split Automata) does simulate the UML model. In this way, we can guarantee that the translated model does represent the original model.
13480741
Source authentication in group communication systems
Many group communication systems need to enforce a restriction that limits members are authorized to send messages to the group. Receivers therefore need to authenticate message sources before the received messages are accepted. Source authentication in peer-to-peer systems is trivial: the two communication parties can agree on one pair key and use this key to authenticate each other. However, because the group key is shared by all members in a group system, it is quite challenging to identify the sender and determine its authorization. Furthermore, if the authorization can be changed at run-time, source authentication problem can be even harder. This paper presents a source authentication technique called TTA scheme (Transitive Trust Authentication). TTA supports source authentication as well as dynamic authorization change. In addition, its computation and communication overhead is low.
1639134
Towards the theoretical foundation of choreography
With the growth of interest on the web services, people pay increasinglyattention to the choreography, that is, to describe collaborations ofparticipants in accomplishing a common business goal from a globalviewpoint. In this paper, based on a simple choreography language and arole-oriented process language, we study some fundamental issues relatedto choreography, especially those related to implementation, includingsemantics, projection and natural projection, dominant role in choices anditerations, etc. We propose the concept of dominant role and somenovel languages structures related to it. The study reveals some cluesabout the language, the semantics, the specification and theimplementation of choreography.
1661187
Siren: catching evasive malware
With the growing popularity of anomaly detection systems, which is due partly to the rise in zero-day attacks, a new class of threats have evolved where the attacker mimics legitimate activity to blend in and avoid detection. We propose a new system called Siren that injects crafted human input alongside legitimate user activity to thwart these mimicry attacks. The crafted input is specially designed to trigger a known sequence of network requests, which Siren compares to the actual traffic. It then flags unexpected messages as malicious. Using this method, we were able to detect ten spyware programs that we tested, many of which attempt to blend in with user activity. This paper presents the design, implementation, and evaluation of the Siren activity injection system, as well as a discussion of its potential limitations
18186547
Experiences with a Mobile Testbed
This paper presents results from an eight-day network packet-trace of MosquitoNet. MosquitoNet allows users of laptop computers to switch seamlessly between a metropolitan-area wireless network and a wired network (10 Mbit/s Ethernet) available in offices and on-campus residences. Results include the amount of user mobility between the wired and wireless networks, the amount of mobility within the wireless network, an examination of application end-to-end delays, and an examination of overall packet loss and reordering in the wireless network. We find that the average mobile host switches between the wired and wireless networks 14 times during the trace and moves within the wireless network five times. Round trip latencies in the wireless network are very high, with a minimum of 0.2 seconds. Even higher end-to-end delays, of up to hundreds of seconds, are due to packet loss and reordering. These delays cause users to change their usage patterns when connected to the wireless network. We conclude that latency is a critical problem in the wireless network.