1. Encouraging Cooperation in Multi-Hop Wireless Networks Ratul Mahajan, Maya Rodrig, David Wetherall and John Zahorjan University of Washington, June 2004.

2. djw // UW-CSE, 6-23-04 Wireless for the masses Today: High-quality connectivity, but limited areas, high cost Via APs: Carefully planned Separately provisioned Single hop, client to AP All-or-nothing access Single administration Our goal: Inexpensively extend reach Via clients, PCs: Use what you find Shared infrastructure Multi-hop Ubiquitous connectivity Multiple parties What is the appropriate system architecture?

3. djw // UW-CSE, 6-23-04 This talk is about cheating Experience shows some people will cheat given motive and opportunity. This is a risk for meshes. –Users must relay each others packets to form a network –Yet an individual gains bandwidth if they don’t relay … –It’s easy to do, hard to detect, and harmful. Can we mitigate this risk by design? –Our philosophy is to assume a backdrop of cooperation and look for low cost ways to make cheating harder (“90/10”) Goal: Provide a deterrent to cheating that is lightweight and widely applicable.

4. djw // UW-CSE, 6-23-04 Outline 1.Our testbed 2.How to cheat, and its impact 3.CATCH, our solution 4.Some results

5. djw // UW-CSE, 6-23-04 1. In-building 802.11 Testbed 15 nodes on one floor –Also covered by ~10 APs Atheros, Prism II.5 based cards –Currently one radio per PC Wired for manageability 184 feet

6. djw // UW-CSE, 6-23-04 Testbed characterization – links Link quality varies with pairs of nodes, some asymmetry

7. djw // UW-CSE, 6-23-04 2. How to cheat and get away with it Just discard unwanted packets Watchdog detects but doesn’t punish; not a solution. Simpler, better: just don’t acknowledge connectivity Routes via cheater can’t be inferred due to asymmetry 6 4 1 3 2 5 7 8 6 4 1 3 2 5 7 8

8. djw // UW-CSE, 6-23-04 The adverse impact of cheating (I) Cheaters gain significantly by cheating, good nodes lose out collectively

9. djw // UW-CSE, 6-23-04 The adverse impact of cheating (II) Even a couple of cheaters can partition high-quality links, and rampant cheating ruins connectivity

10. djw // UW-CSE, 6-23-04 3. CATCH, our solution Goal is to detect cheaters and isolate them for a period –A credible threat to encourage cooperative forwarding Two difficult problems: –Determine when a node discards packets, even though only the node knows which packets it received –Get neighbors to agree to punish it, even though they must coordinate their actions via the cheating node Approach/Insight: –Leverage anonymous challenges, where receiver doesn’t know the identity of the sender. Can do this with current hardware.

11. djw // UW-CSE, 6-23-04 Key Idea: anonymous challenges Each node tests it neighbor with anonymous challenges; neighbor must respond or lose the link Even a cheater requires some connectivity, and so must respond to preserve it, thus revealing true connectivity to all nodes 6 4 1 3 2 5 7 8

12. djw // UW-CSE, 6-23-04 How to detect that a node is cheating Combine anonymous challenges (which tell you true connectivity) with watchdog (below, which tells you behavior). These are statistical tests in CATCH. 6 4 1 3 2 5 7 8

13. djw // UW-CSE, 6-23-04 How all nodes can isolate a cheater Use anonymous challenges and signaling by absence H1 6 H1 8 H1 7 H1 3 H1 2 {H1 3, H1 6, H1 7, H1 8 } {H1 2, H1 3, H1 6, H1 8 } {H1 2, H1 3, H1 6, H1 7 } {H1 2, H1 3, H1 7, H1 8 } {H1 2, H1 6, H1 7, H1 8 } H0 2 H0 3 H0 7 H0 8 {H0 3, ?, H0 7, H0 8 } {H0 2, ?, H0 7, H0 8 } {H0 2, H0 3, H0 7, H0 8 } {H0 2, H0 3, ?, H0 7 } {H0 2, H0 3, ?, H0 8 }

14. djw // UW-CSE, 6-23-04 Details I’ve omitted The rest of the protocol –Statistical tests to handle the impact of real wireless losses –Adding reliability to the control packet exchanges –Case-by-case analysis (e.g.,“What if I drop half the challenges?”) Implementation –User-level via netfilter, unoptimized –Traffic on testbed is HTTP downloads More information: –Mail me if you’d like a draft paper.

15. djw // UW-CSE, 6-23-04 4. Results under wireless conditions The more you cheat, the more quickly you are caught.

16. djw // UW-CSE, 6-23-04 Overheads, costs Reasonable bandwidth overhead –Only control packets (1 per link per second per neighbor) –Roughly 24Kbps per node in our testbed Minimal processing –Some counters etc. but no crypto operations per data packet –We used to run this on IPAQs Must be able to send anonymous packets and watchdog No real restrictions on traffic workloads, choice of routing protocol, etc.

17. djw // UW-CSE, 6-23-04 More sophisticated cheats Drop a fraction of packets Target only some neighbors Cheat only some of the time Cheat different neighbors at different times Combinations of the above … Or, physical layer hints to undermine anonymity: –Using per packet received signal strength

18. djw // UW-CSE, 6-23-04 CATCH with signal-strength cheats Signal strength helps about half the time; CATCH still offers some protection

19. djw // UW-CSE, 6-23-04 Conclusion CATCH provides a lightweight deterrent to cheating –Built on a backdrop of cooperation –Assumptions aren’t too restrictive: watchdogs, omni-directional –Modest overheads, no limits on workloads, currencies, etc. –Raises the bar rather than makes cheating impossible Future work –Signal-strength cheats, improve robustness even further –Move on to system architecture and protocols …

20. djw // UW-CSE, 6-23-04 Questions?

21. djw // UW-CSE, 6-23-04 Testbed performance – multi-hop Time for 6MB transfer from node 8 to nodes 4, 6, 14, & 9

22. djw // UW-CSE, 6-23-04 Rapid detection with few false positives A simulation that expands on the prior result. (An ideal result would hug the x-y axis. Note the log scale.)