4,078 research outputs found
Tight Bounds for Black Hole Search with Scattered Agents in Synchronous Rings
We study the problem of locating a particularly dangerous node, the so-called
black hole in a synchronous anonymous ring network with mobile agents. A black
hole is a harmful stationary process residing in a node of the network and
destroying destroys all mobile agents visiting that node without leaving any
trace. We consider the more challenging scenario when the agents are identical
and initially scattered within the network. Moreover, we solve the problem with
agents that have constant-sized memory and carry a constant number of identical
tokens, which can be placed at nodes of the network. In contrast, the only
known solutions for the case of scattered agents searching for a black hole,
use stronger models where the agents have non-constant memory, can write
messages in whiteboards located at nodes or are allowed to mark both the edges
and nodes of the network with tokens. This paper solves the problem for ring
networks containing a single black hole. We are interested in the minimum
resources (number of agents and tokens) necessary for locating all links
incident to the black hole. We present deterministic algorithms for ring
topologies and provide matching lower and upper bounds for the number of agents
and the number of tokens required for deterministic solutions to the black hole
search problem, in oriented or unoriented rings, using movable or unmovable
tokens
Black Hole Search with Finite Automata Scattered in a Synchronous Torus
We consider the problem of locating a black hole in synchronous anonymous
networks using finite state agents. A black hole is a harmful node in the
network that destroys any agent visiting that node without leaving any trace.
The objective is to locate the black hole without destroying too many agents.
This is difficult to achieve when the agents are initially scattered in the
network and are unaware of the location of each other. Previous studies for
black hole search used more powerful models where the agents had non-constant
memory, were labelled with distinct identifiers and could either write messages
on the nodes of the network or mark the edges of the network. In contrast, we
solve the problem using a small team of finite-state agents each carrying a
constant number of identical tokens that could be placed on the nodes of the
network. Thus, all resources used in our algorithms are independent of the
network size. We restrict our attention to oriented torus networks and first
show that no finite team of finite state agents can solve the problem in such
networks, when the tokens are not movable. In case the agents are equipped with
movable tokens, we determine lower bounds on the number of agents and tokens
required for solving the problem in torus networks of arbitrary size. Further,
we present a deterministic solution to the black hole search problem for
oriented torus networks, using the minimum number of agents and tokens
Exploring an unknown graph to locate a black hole using tokens
Consider a team of (one or more) mobile agents operating in a graph G. Unaware of the graph topology and starting from the same node, the team must explore the graph. This problem, known as graph exploration, was initially formulated by Shannon in 1951, and has been extensively studied since under a variety of conditions. The existing investigations have all assumed that the network is safe for the agents, and the solutions presented in the literature succeed in their task only under this assumption.
Recently, the exploration problem has been examined also when the network is unsafe. The danger examined is the presence in the network of a black hole, a node that disposes of any incoming agent without leaving any observable trace of this destruction. The goal is for at least one agent to survive and to have all the surviving agents to construct a map of the network, indicating the edges leading to the black hole.
This variant of the problem is also known as black hole search. This problem has been investigated assuming powerful inter-agent communication mechanisms: whiteboards at all nodes. Indeed, in this model, the black hole search problem can be solved with a minimal team size and performing a polynomial number of moves.
In this paper, we consider a less powerful token model.We constructively prove that the black hole search problem can be solved also in this model; furthermore, this can be done using a minimal team size and performing a polynomial number of moves. Our algorithm works even if the agents are asynchronous and if both the agents and the nodes are anonymous.4th IFIP International Conference on Theoretical Computer ScienceRed de Universidades con Carreras en Informática (RedUNCI
Exploring an unknown graph to locate a black hole using tokens
Consider a team of (one or more) mobile agents operating in a graph G. Unaware of the graph topology and starting from the same node, the team must explore the graph. This problem, known as graph exploration, was initially formulated by Shannon in 1951, and has been extensively studied since under a variety of conditions. The existing investigations have all assumed that the network is safe for the agents, and the solutions presented in the literature succeed in their task only under this assumption.
Recently, the exploration problem has been examined also when the network is unsafe. The danger examined is the presence in the network of a black hole, a node that disposes of any incoming agent without leaving any observable trace of this destruction. The goal is for at least one agent to survive and to have all the surviving agents to construct a map of the network, indicating the edges leading to the black hole.
This variant of the problem is also known as black hole search. This problem has been investigated assuming powerful inter-agent communication mechanisms: whiteboards at all nodes. Indeed, in this model, the black hole search problem can be solved with a minimal team size and performing a polynomial number of moves.
In this paper, we consider a less powerful token model.We constructively prove that the black hole search problem can be solved also in this model; furthermore, this can be done using a minimal team size and performing a polynomial number of moves. Our algorithm works even if the agents are asynchronous and if both the agents and the nodes are anonymous.4th IFIP International Conference on Theoretical Computer ScienceRed de Universidades con Carreras en Informática (RedUNCI
Black Hole Search in Dynamic Tori
We investigate the black hole search problem by a set of mobile agents in a
dynamic torus. Black hole is defined to be a dangerous stationary node which
has the capability to destroy any number of incoming agents without leaving any
trace of its existence. A torus of size () is a
collection of row rings and column rings, and the dynamicity is such
that each ring is considered to be 1-interval connected, i.e., in other words
at most one edge can be missing from each ring at any round. The parameters
which define the efficiency of any black hole search algorithm are: the number
of agents and the number of rounds (or \textit{time}) for termination. We
consider two initial configurations of mobile agents: first, the agents are
co-located and second, the agents are scattered. In each case, we establish
lower and upper bounds on the number of agents and on the amount of time
required to solve the black hole search problem
Faulty node repair and dynamically spawned black hole search
New threats to networks are constantly arising. This justifies protecting network assets and mitigating the risk associated with attacks. In a distributed environment, researchers aim, in particular, at eliminating faulty network entities. More specifically, much research has been conducted on locating a single static black hole, which is defined as a network site whose existence is known a priori and that disposes of any incoming data without leaving any trace of this occurrence. However, the prevalence of faulty nodes requires an algorithm able to (a) identify faulty nodes that can be repaired without human intervention and (b) locate black holes, which are taken to be faulty nodes whose repair does require human intervention. In this paper, we consider a specific attack model that involves multiple faulty nodes that can be repaired by mobile software agents, as well as a virus v that can infect a previously repaired faulty node and turn it into a black hole. We refer to the task of repairing multiple faulty nodes and pointing out the location of the black hole as the Faulty Node Repair and Dynamically Spawned Black Hole Search. Wefirst analyze the attack model we put forth. We then explain (a) how to identify whether a node is either (1) a normal node or (2) a repairable faulty node or (3) the black hole that has been infected by virus v during the search/repair process and, (b) how to perform the correct relevant actions. These two steps constitute a complex task, which, we explain, significantly differs from the traditional Black Hole Search. We continue by proposing an algorithm to solve this problem in an
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