1. © 2007 Levente Buttyán and Jean-Pierre Hubaux Security and Cooperation in Wireless Networks http://secowinet.epfl.ch/ Chapter 12: Secure protocols for behavior enforcement

2. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 2/22 Motivation Provide incentive to cooperate within Routing and Forwarding protocols using a game theoretic approach Packet forwarding consumes resources –Nodes are rational => Maximize their own payoff –Nodes avoid forwarding

3. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 3/22 Outline Introduction –Incentives –System Model Model –Dominant action/subaction –Cooperation optimal protocol Protocols –VCG payments with correct link cost establishment –Forwarding protocol with block confirmation Conclusion

4. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 4/22 Introduction  Routing protocol –Discover efficient routing paths: global welfare –Deal with selfish nodes: local welfare  Packet forwarding protocol –address the fair exchange problem => Joint Incentive

5. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 5/22 Possible incentives Incentive Punish Reward InternalExternalInternal External  Possible incentive strategies: –Punish: Reputation, Jamming, Isolation –Reward: Virtual currency  Possible incentives: –Internally: With intrinsic mechanisms (e.g., deny communication, jam) –Externally: by dedicated protocols

6. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 6/22 System Model  Ad-hoc networks as non-cooperative strategic games Called “Ad Hoc Games”  Channel model: Packet successfully transmitted if P transmission >= P min –P min = minimum power to reach receiver No errors (BER = 0)  Nodes can withhold, replace or send a message  Nodes can transmit at any power level  We define the payoff of a node as: –b i = benefit (reward, by micro-payment) –c i = cost of forwarding (energy, overhead,…)

7. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 7/22 Formal Model  Dominant Action: –A dominant action is one that maximizes player i payoff, no matter what actions other players choose Example: Joint packet forwarding game –Imperfect information –Message from S to D –Two players: p1 and p2 p1 has no dominant action p2’s dominant action is F SP1P2D p1\p2FD F(1-c,1-c)(-c,0) D(0,0)(0,0)(0,0)(0,0)

8. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 8/22 Formal Model  Each node action is comprised of two parts: is node i’s subaction in the routing stage (what it is supposed to do in the routing stage) is node i’s subaction in the forwarding stage (what it really does in the forwarding stage) Routing decision R: determined by the routing subactions of all nodes Prospective routing payoff:

9. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 9/22 Routing stage  Dominant subaction: –In a routing stage, a dominant subaction is one that maximizes its routing payoff no matter what subactions other players choose.  A routing protocol is a routing-dominant protocol to the routing stage if following the protocol is a dominant subaction of each potential forwarding node in the routing stage

10. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 10/22 Forwarding stage A forwarding protocol is a forwarding-optimal protocol to the forwarding stage under routing decision R if –All packets are forwarded to their destinations –Following the protocol is a subgame perfect equilibrium  A path is said to be a subgame perfect equilibrium if it is a Nash equilibrium for every subgame Node 1 Node 2 Last node forward drop p1\p2FD F(1-c,1-c)(-c,0) D(0,0)

11. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 11/22 Cooperation-Optimal Protocol  A protocol is a cooperation-optimal protocol to an ad-hoc game if –Its routing protocol is a routing-dominant protocol to the routing stage –For a routing decision R, its forwarding protocol is a forwarding optimal protocol to the forwarding stage

12. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 12/22 VCG for routing protocols  VCG: Vickrey, Clarke, and Groves – second-best sealed auction  Nodes independently compute and declare their packet transmission cost to destination  Destination computes Lowest Cost Path (LCP)  Source rewards the nodes –declared cost + added value  The added value is the difference between LCP with the node and without it –Incentive to declare the true price => Truthful

13. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 13/22 Example of VCG Least cost path from S to D: LCP(S,D) = S, v2, v3, D with cost(LCP(S,D)) = 5 + 2 + 3 = 10 Least cost path without node v2: LCP(S,D;−v2) = S, v1, v4, D with cost(LCP(S,D);−v2) = 7 + 3 + 4 = 14 Least cost path without node v3: LCP(S,D;−v3) = S, v2, v4, D with cost(LCP(S,D);−v3) = 5 + 3 + 4 = 12. VCG payments: b2 = 14 − 10 + 2 = 6 b3 = 12 − 10 + 3 = 5 These values represent the unit payment (the payment for one forwarded data packet) to nodes v2 and v3, respectively.

14. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 14/22 Cheating about the power level  Assume mutual computation of link cost  Consider a node i and its neighbor j 1.Node i cheats by making Pi,j larger: –Node j is less likely to be on LCP –Node j’ s payment will decrease. 2.Node j can respond by cheating and making Pi,j smaller: –Node j more likely to be on LCP –Node j increases its payment  VCG is thus not truthful in this case ij Pi,j

15. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 15/22 Cryptographic protection  Assume private computation of link cost (the details of the security mechanisms are in the book)  Protocol for link cost establishment: –Nodes share a symmetric key with D –Nodes send an encrypted and signed test signal at increasing power levels containing cost information –Messages are protected from forging with HMAC –Complexity (computation at the destination): O(N^3) i j [cost3]K¦HMAC D [cost2]K¦HMAC [cost1]K¦HMAC [cost4]K¦HMAC [cost3]K¦HMAC [cost4]K¦HMAC

16. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 16/22 Conclusion on the routing stage Theorem 12.1: If the destination is able to collect all involved link costs as described above, then the described protocol is a routing dominant protocol to the routing stage.

17. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 17/22 r1r1 Forwarding Protocol  Messages bundled in blocks  Block confirmation with a Reverse Hash Chain –r is made public by source in an authenticated way –Confirmation of block 2 is done by sending r 5-2 =r 3 –Nodes verify m1m2m3m4m5m6m7m8m9 b1b2b3b4b5 H r0r0 HH r2r2 r=r 5 H

18. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 18/22 Fair Exchange Problem  Source and intermediate nodes can disagree about successful transmission of a block  Mutual decision = contract between source an intermediate nodes –Confirmation is sent with the last packet of each block to destination –Destination forwards confirmation to intermediate nodes if block correctly received –Intermediate nodes stop forwarding if they do not get confirmation  Eliminates incentive to cheat –Not respecting the protocol blocks the protocol

19. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 19/22 Theorems Theorem 12.2: Given a routing decision R, assuming that the computed payment is greater than the cost, the reverse hash chain based forwarding protocol is a forwarding optimal protocol. Theorem 12.3: The complete protocol (routing protocol and packet forwarding protocol) is a cooperation-optimal protocol to AdHocGames.

20. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 20/22 Discussion  Modeling –Interference and mobility unreliable links make use of incentives more difficult  Game theoretic model assumes –Tamper proof hardware to compute best path at destination –Payment center to resolve payment issues  Performance vs. incentive compatibility –Control channel overhead –Throughput –Complexity

21. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 21/22 Summary  Cooperation optimal protocol –Routing dominant + Forwarding optimal –Routing based on VCG –Forwarding based on Reverse Hash Chain  Corsac provides incentives for cooperation –Protocol is fair –The topology determines payment –The incentive protocol reduces the network traffic

22. Security and Cooperation in Wireless Networks Chapter 12:Secure protocols for behavior enforcement 22/22 References  On Designing Incentive-Compatible Routing and Forwarding Protocols in Wireless Ad-Hoc Networks Sheng Zhong, Li Erran Li, Yanbin Grace Liu and Yang Richard Yang. ACM Springer Wireless Networks (WINET), Special Issue of Selected Papers of Mobicom 2005  Punishement in Selfish Wireless Networks: A Game Theoretic Analysis Dave Levin. NetEcon 2006  On Selfish Behavior in CSMA/CA Networks Mario Cagalj, Saurabh Ganeriwal, Imad Aad and Jean-Pierre Hubaux. Infocom 2005  Ad hoc-VCG: A Truthful and Cost-Efficient Routing Protocol for Mobile Ad hoc Networks with Selfish Agents Luzi Anderegg and Stephan Eidenbenz. Mobicom 2003