Searching for a Black Hole in Interconnected Networks using Mobile Agents and Tokens

1. Searching for a Black Hole in Interconnected Networks using MobileAgents and Tokens Written by Wei Shi, Joaquin Garcia-Alfaro, Jean-Pierre Corriveau MengfeiPeng

2. Assumption • Agents: asynchronous, co-located/dispersed, carry a map of the network. • Not assume FIFO on either nodes or links (first-in first-out) • Network: un-oriented, asynchronous Hypercube, Torus, Complete network. • Model: Token model

4. Black hole Agent

5. Token model Node 3 Node 1 Node 2 Port Shared memory on a node agent

6. Token model This port is dangerous, I don’t go into it. I can stay here waiting or go to another way. A F C B D Node 3 Node 1 Node 2 E I come back and the next node is safe. How can we use a token? Cautious walk 1,when the agent leaves the port, It puts a token at the port----this means “I’m going to the next node” Port 2,when the agent comes back, It picks up the token----that means ”I come back safely” Shared memory on a node agent

7. Maybe A died here, it’s a dangerous port. I won’t go into it. A piece of software

8. Whiteboard model Node 3 Node 1 Node 2 Port agent Shared memory on a node

9. Whiteboard model B C D A F Node 3 Node 1 Node 2 E How can we use a whiteboard? Cautious walk 1,when the agent leaves the port, It writes “I’m going to the next node” on the whiteboard Port A: I’m going to the next node A: I come back safely and the next node is safe 2,when the agent comes back, It writes--”I come back safely” agent Shared memory on a node

10. Synchronous agents 5 kilometers cost 50 minutes. 6 kilometers cost 60 minutes. OK, 11:00 Let’s meet at 11:00 8 kilometers cost 80 minutes. 1 time unit 1 time unit 77 kilometers cost 770 minutes. Asynchronous agents 5 kilometers cost 5 minutes. 6 kilometers cost 150 minutes. 8 kilometers cost 999 minutes. 1 time unit 100 time units 4 time unit When? Let’s meet Whenever you can come Sure, I will try to come early 8 kilometers cost 3 minutes.

11. Co-located agents

12. Dispersed agents

13. Bypass technique 1,use cautious walk to find a safe ring----no black hole in the ring S S S Safe ring Dangerous ring

14. 2,agent A is exploring the dangerous ring, and leave a token at port J2 S S S Safe ring S Dangerous ring

15. 3,agent B comes and sees A’s token, and moves A’s token to a safe port J1. S S S Safe ring S Dangerous ring

16. 3,agent B goes to the safe ring and continues exploration. S S S Safe ring S Dangerous ring

17. 3,agent B leaves its token at port P3, and explores the next node L S S S Safe ring S Dangerous ring

18. Question 1: what happens if there is an agent C comes? S S S Safe ring S Dangerous ring

19. Answer: S S S S Safe ring S Dangerous ring

20. Question: • When can we use the bypass technique? • In a interconnected network, after finding a safe ring. • What’s the purpose of bypass technique? • Allow more than one agent to explore the same ring at the same time.

21. a complete network, each node is linked to every other node by an edge.

22. complexity