1. Secure Protocols for Behavior Enforcement Slides elaborated by Julien Freudiger and adapted by Jean-Pierre Hubaux http://secowinet.epfl.ch Note: this chapter (and therefore this slide show) is derived from the paper by S. Zhong, L. Erran Li, Y. Liu, and Y. R. Yang, “On Designing Incentive-Compatible Routing and Forwarding Protocols in Wireless Ad Hoc Networks”, Mobicom 2005 Security and Cooperation in Wireless Networks

2. 2 Motivation Packet forwarding consumes resources –Nodes are rational => Maximize their payoff –Nodes avoid forwarding Provide incentive to cooperate within Routing and Forwarding protocols using a Game Theoretic approach

3. 3 Outline 1.Introduction –Incentives –System Model 2.Formal Model –Dominant action/subaction –Cooperation optimal protocol 3.The Corsac Protocol –VCG payments with correct link cost establishment –Forwarding protocol with block confirmation 4.Evaluation 5.Conclusion

4. 4 1. 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. 5 Incentive strategy: –Punish: Reputation, Jamming, Isolation –Reward: Virtual currency Incentive is achieved: –Internally: With 802.11 primitives –Externally: Dedicated protocols Incentives Incentive Punish Reward InternalExternalInternal External

6. 6 Ad-hoc networks as uncooperative strategic games Called Ad Hoc Games Channel model: Packet successfully transmitted if P transmission >= P min –P min = minimum power to reach destination No errors (BER = 0) Nodes can withhold, replace or send a message Node can transmit at any power level We define the payoff of a node as: –bi = benefice (reward) –ci = cost of forwarding System Model

7. 7 2. 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 dominant action is F SP1P2D p1\p2FD F (1-c,1-c) (-c,0) D(0,0)(0,0)(0,0)(0,0)

8. 8 Forwarding Dominant A forwarding protocol is said forwarding dominant protocol if following the protocol is a dominant action We need incentives to enforce cooperation Theorem 1: There does not exist a forwarding-dominant protocol for ad-hoc games.

9. 9 Formal Model for Divided Solution Each node actions is divided into two parts: –Routing subaction: A routing decision specifies what node is supposed to do in the forwarding stage –Forwarding subaction: Specifies what the node actually does The total payoff comprises both subactions

10. 10 Routing stage Routing payoff of a node is the payoff that it will achieve under the routing decision 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

11. 11 Forwarding stage Consider an extensive game model with imperfect information 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)

12. 12 A protocol is a cooperation-optimal protocol to an ad-hoc game if 1.Its routing protocol is a routing-dominant protocol to the routing stage 2.For a routing decision R, its forwarding protocol is a forwarding optimal protocol to the forwarding stage Cooperation-Optimal Protocol

13. 13 3. The Corsac Protocol Corsac is a cooperation optimal protocol –Routing: VCG –Forwarding: Reverse Hash chains

14. 14 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 VCG for routing protocols

15. 15 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: p2 = 14 − 10 + 2 = 6 p3 = 12 − 10 + 3 = 5 These values represent the unit payment (the payment for one forwarded data packet) to nodes v2 and v3, respectively.

16. 16 VCG flaw Assume mutual computation of link cost Consider a node i and its neighbor j 1.Node i cheats by making Pi,j greater: –Node j is less likely to be on LCP –Node j payment will decrease. 2.Node j responds by cheating and making Pi,j smaller: –Node j more likely to be on LCP –Node j increases its payment VCG is not truthful in this case –Possible to cheat in determining link cost ij Pi,j

17. 17 Truthful VCG Assume private computation of link cost Protocol for VCG 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 –O(N^3) ij [cost3]K¦HMAC D [cost2]K¦HMAC [cost1]K¦HMAC [cost4]K¦HMAC [cost3]K¦HMAC [cost4]K¦HMAC

18. 18 VCG conclusion Theorem 2: If the destination is able to collect all involved link costs as described above, then the VCG protocol is a routing dominant protocol to the routing stage.

19. 19 r1 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)=r3 –Nodes verify m1m2m3m4m5m6m7m8m9 b1b2b3b4b5 H r0 HH r2r=r5 H

20. 20 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 do not get confirmation Eliminates incentive to cheat –Disregarding the protocol blocks the protocol

21. 21 Cooperation Optimal Theorem 3: 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 4: The Corsac protocol is a cooperation-optimal protocol to ad-hoc games.

22. 22 Nodes that accumulate more credits spend more energy in forwarding others’ traffic => The protocol is fair 4. Evaluation (1)

23. 23 Evaluation (2) Consider the following topology:

24. 24 + = payment X = cost Node 19 as session source: Evaluation (3) Reach destination directly

25. 25 Evaluation (4) Node 28 as session source: + = payment X = cost Node 3 is critical point Mainly the topology that determines payment

26. 26 Future challenges Modeling –Interference and mobility unreliable link harden use of incentive 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

27. 27 5. Conclusions 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

28. 28 References [1]« 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. Mobicom 2005 [2]« Security and Cooperation in Wireless Networks ». Levente Buttyan and Jean-Pierre Hubaux. Book Cambridge University Press, Chapter 12 [3] « Punishement in Selfish Wireless Networks: A Game Theoretic Analysis ». Dave Levin. NetEcon 2006 [4]« On Selfish Behavior in CSMA/CA Networks ». Mario Cagalj, Saurabh Ganeriwal, Imad Aad and Jean-Pierre Hubaux. Infocom 2005 [5]« 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