1. Sustaining Cooperation in Multi-Hop Wireless Networks Ratul Mahajan, Maya Rodrig, David Wetherall, John Zahorjan University of Washington

2. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 2 Multi-hop networks are real

3. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 3 Incentives to free-ride Personal bandwidth maximization Power conservation It’s easy! You’ll get away with it

4. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 4 The impact of free-riding Using multi-hop routes improves throughput  Per-node delivery rate improvement of 25% on average Free-riders gain at the expense of cooperative nodes  Free-riders’ throughput increased 4X  Cooperative nodes’ throughput decreased by 25% In our testbed, 60% chance of network partition with only 20% free-riding nodes

5. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 5 Wireless range is not a simple function of distance and can be asymmetric Challenge in detecting free-riding

6. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 6 Free-rider pretends to be out of range and drops the packet Challenge in detecting free-riding Free-ridingLack of connectivity

7. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 7 A cooperative node does not receive the packet Challenge in detecting free-riding Free-ridingLack of connectivity

8. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 8 Challenge in detecting free-riding Free-ridingLack of connectivity Hard to differentiate the two cases given asymmetric, lossy links

9. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 9 Catch Two main challenges:  Determining when a node is free-riding  Getting its neighbors to agree to punish it Solution: 1. Detecting wireless connectivity 2. Monitoring packet relaying behavior 3. Punishing selfish misbehavior

10. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 10 Tradeoffs between approaches Proposed solutions Catch Protection Overhead Applicability Deployed protocols

11. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 11 1. Detecting wireless connectivity Nodes want to connect to at least one neighbor Send anonymous connectivity probes

12. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 12 2. Monitoring packet relaying behavior Watchdog: relayed packets should be overheard [Marti et al, 2000] Use statistical tests to compare success rate of anonymous probes and data packets

13. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 13 Leverage the cooperative majority to collectively deter free-riders Use anonymous probes, one-way hash functions, and signaling by absence 3. Punishing selfish misbehavior Nx’ Ny’ Nz’ Nx’ Ny’ Nz’ Nx’ Ny’ Nz’ Nx Ny’Ny Nz’Nz Nx Ny’Ny Nz’Nz Nx Ny’Ny Nz’Nz Nx’ Ny’Ny Nz’Nz Nx’ Ny’Ny Nz’Nz Nx’ Ny’Ny Nz’Nz

14. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 14 184 ft Our testbed In-building experimental testbed  15 802.11b nodes  10 APs on same floor A real wireless setting  Many asymmetric links  Frequent packet losses ~20% loss rate

15. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 15 Catch evaluation Speed and accuracy of detection Effectiveness of isolation Overhead

16. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 16 Total Drop Rate Epochs to Detection Speed and accuracy of detection Rapid detection with few false positives  Detection is quicker for more egregious free-riding  1 false positive in 10 hrs across testbed 123 client server

17. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 17 Effectiveness of isolation Isolation is successful despite asymmetric, lossy links Throughput of free-riders is curbed Elapsed Time (minutes) Isolation period

18. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 18 Overhead Bandwidth: Only 24Kbps per node in our testbed CPU: Maintain packet counters, but no crypto operations per data packet

19. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 19 Future work: Signal strength attacks Physical layer hints can undermine anonymity Catch already offers some protection

20. Maya Rodrig | UW | NSDI ’05 Sustaining Cooperation in Multi-Hop Wireless Networks 20 Conclusion Catch is a lightweight solution to deter free-riding  Modest overheads  No restrictive requirements Key insights:  Using anonymous probes to detect connectivity and communicate via free-riders  Leveraging cooperative majority to detect and punish free-riders Testbed evaluation shows that Catch is effective in a real wireless environment