Snap-Stabilization in Message-Passing Systems

In this paper, we tackle the open problem of snap-stabilization in message-passing systems. Snap-stabilization is a nice approach to design protocols that withstand transient faults. Compared to the well-known self-stabilizing approach, snap-stabilization guarantees that the effect of faults is contained immediately after faults cease to occur. Our contribution is twofold: we show that (1) snap-stabilization is impossible for a wide class of problems if we consider networks with finite yet unbounded channel capacity; (2) snap-stabilization becomes possible in the same setting if we assume bounded-capacity channels. We propose three snap-stabilizing protocols working in fully-connected networks. Our work opens exciting new research perspectives, as it enables the snap-stabilizing paradigm to be implemented in actual networks

Fast and Compact Distributed Verification and Self-Stabilization of a DFS Tree

We present algorithms for distributed verification and silent-stabilization of a DFS(Depth First Search) spanning tree of a connected network. Computing and maintaining such a DFS tree is an important task, e.g., for constructing efficient routing schemes. Our algorithm improves upon previous work in various ways. Comparable previous work has space and time complexities of $O(n\log \Delta)$ bits per node and $O(nD)$ respectively, where $\Delta$ is the highest degree of a node, $n$ is the number of nodes and $D$ is the diameter of the network. In contrast, our algorithm has a space complexity of $O(\log n)$ bits per node, which is optimal for silent-stabilizing spanning trees and runs in $O(n)$ time. In addition, our solution is modular since it utilizes the distributed verification algorithm as an independent subtask of the overall solution. It is possible to use the verification algorithm as a stand alone task or as a subtask in another algorithm. To demonstrate the simplicity of constructing efficient DFS algorithms using the modular approach, We also present a (non-sielnt) self-stabilizing DFS token circulation algorithm for general networks based on our silent-stabilizing DFS tree. The complexities of this token circulation algorithm are comparable to the known ones

Self-stabilizing algorithms for Connected Vertex Cover and Clique decomposition problems

In many wireless networks, there is no fixed physical backbone nor centralized network management. The nodes of such a network have to self-organize in order to maintain a virtual backbone used to route messages. Moreover, any node of the network can be a priori at the origin of a malicious attack. Thus, in one hand the backbone must be fault-tolerant and in other hand it can be useful to monitor all network communications to identify an attack as soon as possible. We are interested in the minimum \emph{Connected Vertex Cover} problem, a generalization of the classical minimum Vertex Cover problem, which allows to obtain a connected backbone. Recently, Delbot et al.~\cite{DelbotLP13} proposed a new centralized algorithm with a constant approximation ratio of $2$ for this problem. In this paper, we propose a distributed and self-stabilizing version of their algorithm with the same approximation guarantee. To the best knowledge of the authors, it is the first distributed and fault-tolerant algorithm for this problem. The approach followed to solve the considered problem is based on the construction of a connected minimal clique partition. Therefore, we also design the first distributed self-stabilizing algorithm for this problem, which is of independent interest

Optimization in a Self-Stabilizing Service Discovery Framework for Large Scale Systems

Ability to find and get services is a key requirement in the development of large-scale distributed sys- tems. We consider dynamic and unstable environments, namely Peer-to-Peer (P2P) systems. In previous work, we designed a service discovery solution called Distributed Lexicographic Placement Table (DLPT), based on a hierar- chical overlay structure. A self-stabilizing version was given using the Propagation of Information with Feedback (PIF) paradigm. In this paper, we introduce the self-stabilizing COPIF (for Collaborative PIF) scheme. An algo- rithm is provided with its correctness proof. We use this approach to improve a distributed P2P framework designed for the services discovery. Significantly efficient experimental results are presented

Bounded Search Tree Algorithms for Parameterized Cograph Deletion: Efficient Branching Rules by Exploiting Structures of Special Graph Classes

Many fixed-parameter tractable algorithms using a bounded search tree have been repeatedly improved, often by describing a larger number of branching rules involving an increasingly complex case analysis. We introduce a novel and general search strategy that branches on the forbidden subgraphs of a graph class relaxation. By using the class of $P_4$-sparse graphs as the relaxed graph class, we obtain efficient bounded search tree algorithms for several parameterized deletion problems. We give the first non-trivial bounded search tree algorithms for the cograph edge-deletion problem and the trivially perfect edge-deletion problems. For the cograph vertex deletion problem, a refined analysis of the runtime of our simple bounded search algorithm gives a faster exponential factor than those algorithms designed with the help of complicated case distinctions and non-trivial running time analysis [21] and computer-aided branching rules [11].Comment: 23 pages. Accepted in Discrete Mathematics, Algorithms and Applications (DMAA

A Simple Linear Time LexBFS Cograph Recognition Algorithm

International audienceThis paper introduces a new simple linear time algorithm to recognize cographs (graphs without an induced P 4). Unlike other cograph recognition algorithms, the new algorithm uses a multisweep Lexicographic Breadth First Search (LexBFS) approach, and introduces a new variant of LexBFS, called LexBFS−, operating on the complement of the given graph G and breaking ties with respect to an initial LexBFS. The algorithm either produces the cotree of G or identifies an induced P 4

Snap-Stabilizing PIF on Non-oriented Trees and Message Passing Model

International audienceStarting from any configuration, a snap-stabilizing protocol guarantees that the system always behaves according to its specification while a self-stabilizing protocol only guarantees that the system will behave according to its specification in a finite time. So, a snap-stabilizing protocol is a time optimal self-stabilizing protocol (because it stabilizes in 0 rounds). That property is very suitable in the case of systems that are prone to transient faults. There exist a lot of approaches of the concept of self-stabilization, but to our knowledge, snap-stabilization is the only variant of self-stabilization which has been proved power equivalent to self-stabilization in the context of the state model (a locally shared memory model) and for non anonymous systems. So the problem of the existence of snap-stabilizing solutions in the message passing model is a very crucial question from a practical point of view. In this paper, we present the first snap-stabilizing propagation of information with feedback (PIF) protocol for non-oriented trees in the message passing model. Moreover using slow and fast timers, the round complexity of our algorithm is in θ(h ×k) and θ((h ×k) + k 2), respectively, where h is the height of the tree and k is the maximal capacity of the channels. We conjecture that our algorithm is optimal

On subsidies and home-ownership: Colombian housing policy during the 1990s

In the early 1990s, the World Bank began to recommend that governments reform their national housing, land and financial sectors in ways that would 'enable the market to work'. Chile and Colombia have both taken this advice seriously. Unfortunately, Colombian experience demonstrates that while some elements in the new approach are eminently sensible, there are certain flaws. Offering subsidies for homes but not for services is highly questionable and the criticism made of sites and services schemes is inappropriate. A further weakness of the Washington approach is the sacrifice of rental accommodation on the altar of owner-occupation