45,050 research outputs found
Time-Varying Graphs and Dynamic Networks
The past few years have seen intensive research efforts carried out in some
apparently unrelated areas of dynamic systems -- delay-tolerant networks,
opportunistic-mobility networks, social networks -- obtaining closely related
insights. Indeed, the concepts discovered in these investigations can be viewed
as parts of the same conceptual universe; and the formal models proposed so far
to express some specific concepts are components of a larger formal description
of this universe. The main contribution of this paper is to integrate the vast
collection of concepts, formalisms, and results found in the literature into a
unified framework, which we call TVG (for time-varying graphs). Using this
framework, it is possible to express directly in the same formalism not only
the concepts common to all those different areas, but also those specific to
each. Based on this definitional work, employing both existing results and
original observations, we present a hierarchical classification of TVGs; each
class corresponds to a significant property examined in the distributed
computing literature. We then examine how TVGs can be used to study the
evolution of network properties, and propose different techniques, depending on
whether the indicators for these properties are a-temporal (as in the majority
of existing studies) or temporal. Finally, we briefly discuss the introduction
of randomness in TVGs.Comment: A short version appeared in ADHOC-NOW'11. This version is to be
published in Internation Journal of Parallel, Emergent and Distributed
System
Temporal Reachability Graphs
While a natural fit for modeling and understanding mobile networks,
time-varying graphs remain poorly understood. Indeed, many of the usual
concepts of static graphs have no obvious counterpart in time-varying ones. In
this paper, we introduce the notion of temporal reachability graphs. A
(tau,delta)-reachability graph} is a time-varying directed graph derived from
an existing connectivity graph. An edge exists from one node to another in the
reachability graph at time t if there exists a journey (i.e., a spatiotemporal
path) in the connectivity graph from the first node to the second, leaving
after t, with a positive edge traversal time tau, and arriving within a maximum
delay delta. We make three contributions. First, we develop the theoretical
framework around temporal reachability graphs. Second, we harness our
theoretical findings to propose an algorithm for their efficient computation.
Finally, we demonstrate the analytic power of the temporal reachability graph
concept by applying it to synthetic and real-life datasets. On top of defining
clear upper bounds on communication capabilities, reachability graphs highlight
asymmetric communication opportunities and offloading potential.Comment: In proceedings ACM Mobicom 201
The Social Climbing Game
The structure of a society depends, to some extent, on the incentives of the
individuals they are composed of. We study a stylized model of this interplay,
that suggests that the more individuals aim at climbing the social hierarchy,
the more society's hierarchy gets strong. Such a dependence is sharp, in the
sense that a persistent hierarchical order emerges abruptly when the preference
for social status gets larger than a threshold. This phase transition has its
origin in the fact that the presence of a well defined hierarchy allows agents
to climb it, thus reinforcing it, whereas in a "disordered" society it is
harder for agents to find out whom they should connect to in order to become
more central. Interestingly, a social order emerges when agents strive harder
to climb society and it results in a state of reduced social mobility, as a
consequence of ergodicity breaking, where climbing is more difficult.Comment: 14 pages, 9 figure
Portunes: analyzing multi-domain insider threats
The insider threat is an important problem in securing information systems. Skilful insiders use attack vectors that yield the greatest chance of success, and thus do not limit themselves to a restricted set of attacks. They may use access rights to the facility where the system of interest resides, as well as existing relationships with employees. To secure a system, security professionals should therefore consider attacks that include non-digital aspects such as key sharing or exploiting trust relationships among employees. In this paper, we present Portunes, a framework for security design and audit, which incorporates three security domains: (1) the security of the computer system itself (the digital domain), (2) the security of the location where the system is deployed (the physical domain) and (3) the security awareness of the employees that use the system (the social domain). The framework consists of a model, a formal language and a logic. It allows security professionals to formally model elements from the three domains in a single framework, and to analyze possible attack scenarios. The logic enables formal specification of the attack scenarios in terms of state and transition properties
Epidemic processes in complex networks
In recent years the research community has accumulated overwhelming evidence
for the emergence of complex and heterogeneous connectivity patterns in a wide
range of biological and sociotechnical systems. The complex properties of
real-world networks have a profound impact on the behavior of equilibrium and
nonequilibrium phenomena occurring in various systems, and the study of
epidemic spreading is central to our understanding of the unfolding of
dynamical processes in complex networks. The theoretical analysis of epidemic
spreading in heterogeneous networks requires the development of novel
analytical frameworks, and it has produced results of conceptual and practical
relevance. A coherent and comprehensive review of the vast research activity
concerning epidemic processes is presented, detailing the successful
theoretical approaches as well as making their limits and assumptions clear.
Physicists, mathematicians, epidemiologists, computer, and social scientists
share a common interest in studying epidemic spreading and rely on similar
models for the description of the diffusion of pathogens, knowledge, and
innovation. For this reason, while focusing on the main results and the
paradigmatic models in infectious disease modeling, the major results
concerning generalized social contagion processes are also presented. Finally,
the research activity at the forefront in the study of epidemic spreading in
coevolving, coupled, and time-varying networks is reported.Comment: 62 pages, 15 figures, final versio
Determining the Solution Space of Vertex-Cover by Interactions and Backbones
To solve the combinatorial optimization problems especially the minimal
Vertex-cover problem with high efficiency, is a significant task in theoretical
computer science and many other subjects. Aiming at detecting the solution
space of Vertex-cover, a new structure named interaction between nodes is
defined and discovered for random graph, which results in the emergence of the
frustration and long-range correlation phenomenon. Based on the backbones and
interactions with a node adding process, we propose an Interaction and Backbone
Evolution Algorithm to achieve the reduced solution graph, which has a direct
correspondence to the solution space of Vertex-cover. By this algorithm, the
whole solution space can be obtained strictly when there is no leaf-removal
core on the graph and the odd cycles of unfrozen nodes bring great obstacles to
its efficiency. Besides, this algorithm possesses favorable exactness and has
good performance on random instances even with high average degrees. The
interaction with the algorithm provides a new viewpoint to solve Vertex-cover,
which will have a wide range of applications to different types of graphs,
better usage of which can lower the computational complexity for solving
Vertex-cover
Mapping Fusion and Synchronized Hyperedge Replacement into Logic Programming
In this paper we compare three different formalisms that can be used in the
area of models for distributed, concurrent and mobile systems. In particular we
analyze the relationships between a process calculus, the Fusion Calculus,
graph transformations in the Synchronized Hyperedge Replacement with Hoare
synchronization (HSHR) approach and logic programming. We present a translation
from Fusion Calculus into HSHR (whereas Fusion Calculus uses Milner
synchronization) and prove a correspondence between the reduction semantics of
Fusion Calculus and HSHR transitions. We also present a mapping from HSHR into
a transactional version of logic programming and prove that there is a full
correspondence between the two formalisms. The resulting mapping from Fusion
Calculus to logic programming is interesting since it shows the tight analogies
between the two formalisms, in particular for handling name generation and
mobility. The intermediate step in terms of HSHR is convenient since graph
transformations allow for multiple, remote synchronizations, as required by
Fusion Calculus semantics.Comment: 44 pages, 8 figures, to appear in a special issue of Theory and
Practice of Logic Programming, minor revisio
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