Thwart selfish behavior in wireless networks

1. Thwart selfish behavior in wireless networks • selfishness at MAC layer (CSMA/CA) • selfishness in packet routing & forwarding • selfishness in shared spectrum

2. On Designing Incentive-Compatible Routing and Forwarding Protocols in Wireless Ad-Hoc NetworksAn Integrated Approach Using Game Theoretical and Cryptographic Techniques Sheng Zhong, Li (Erran) Li, Yanbin Grace Liu, Yang Richard Yang (presentation by Pietro Cassarà University of Palermo, based on Y. R. Yang’s slides)

3. Ad-hoc networks with Selfish Nodes In the wireless networks each node is active member in the routing and it must be able to forwarding the traffic. With the increasing number of wireless devices, it is inevitable that certain ad-hoc networks will consist of selfish nodes. • nodes have their own individual goals • nodes may not be obedient

4. Selfish nodes actions • Selfish nodes could cheat about link costs • Selfish nodes could drop packets they should forward

5. Ad-hoc networks with Selfish Nodes A problem is to find a protocol that by means of an incentive, “persuade” the nodes to route and forward the other’s traffic. How can we resolve this problem? A natural tool is the Game theory and the study of rational behavior in competitive, collaborative setting.

6. Game Theory as approach Using the game theory we can model our problem as a game, but there are some problems: • The game could have no solution • The game could have many Nash equilibriums • The game could not converge to the equilibrium that we want (To resolve these problems we must choose our model protocol carefully)

7. A Model of Ad-Hoc Games Ad-hoc network model: • N nodes, each with a discrete set of transmission power levels • Minimal power for i to reach j: Pij • Lowest cost (power) path routing Ad-hoc games: • Each player participates in routing and forwarding • Action space: an action ai can be send/withhold/replace a message required by a protocol • Cheat -> replace a routing message • Drop -> withold a packet • a-i is the set of actions taken by all nodes except i • a is the set of all the actions taken by nodes

8. A Model of Ad-Hoc Games • The utility of a node i is ui= -ci + pi • ci is the cost of transmission • pi is the payment the node receives from the system when it forwards the traffic • Ignore cost of control packets • If i transmits with power level l the cost is l αi , αi is the cost-of-energy parameter (If the node does not make any action ci=pi=0 otherwise ci≠pi ≠ 0)

9. Dominance • i.e. ai dominates all the other actions • Stronger concept than best response (“for a-i“) • It would lead to a single equilibrium (dominance principle in GT) • A dominant action of a player maximizes its utility no matter what actions other players choose • ui(ai, a-i)  ui(a’i, a-i) for any a’i aiand any a-i • An ideal protocol would be routing&forward dominant protocol, i.e. • “Say the truth” about link costs is a dominant action (routing dominant protocol) • “Forward” is a dominant action (forwarding dominant protocol)

10. Results • There is a routing dominant protocol • There is no forwarding dominant protocol • but we can fall back to a good Nash Equilibrium where all the nodes forward the packets • i.e. forwarding is the best response to other nodes forwarding

11. Assume no one can overhear i and j’s transmission All nodes except i and j follow the protocol j always follows the protocol except dropping the first packet of a session Actions of i ai: follow the protocol a’i: follow the protocol except dropping the first packet of a session i j D S C Why forwarding-dominant protocol does not exist? • Proof by contradiction: • System can not distinguish ai from a’i , pi(ai , a-i) = pi(a’i , a-i) • ai has greater cost than a’i, ci(ai , a-i) > ci(a’i , a-i) • Therefore, pi(ai , a-i)- ci(ai , a-i)< pi(a’i , a-i)- ci(a’i , a-i) which is ui(ai , a-i) < ui(a’I , a-i) Contradicts the definition of dominant action!

12. A New Solution Concept: Cooperation- Optimal Protocol • Divide the ad-hoc game into two stages: routing stage and forwarding stage • Each node’s action consists of routing subaction and forwarding subaction • Routing stage: routing-dominant protocol • following the protocol is a dominant subaction • Forwarding stage: forwarding-optimal protocol under routing decision, • all packets are forwarded • following the protocol is a subgame perfect Nash equilibrium • Cooperation-optimal protocol: a routing-dominant protocol and a forwarding optimal protocol under routing decision

13. Routing Stage Prevent cheating in link costs using cryptographic techniques Routing Protocol

14. Payment pi=cost(LCP(S,D;-i))-cost(LCP(S,D)-{i}) LCP(S,D;-i): min cost path without node i LCP((S,D)-{i}): min cost path with link starting at node i removed VCG Payment: Private Type Under the VCG payment mechanism, a player has no incentive to lie about its true cost Private type = Each node knows the cost of downstream link

15. Link transmission cost is determined by two players Sender i sends with Pemit Receiver j feeds back R R=Precv/Precvmin =Pemit/Pmin_emit Estimated min power Pij=Pemit/R Cost of transmission: cost(ij)=Pij ? 4 4 B D S A 6 6 C VCG Payment: Mutually Dependent Types Pemit=5, R = 5 True cost of AB: 1 Nobody lies: cost(LCP(S,D;-B))=6+6=12 cost(LCP(S,D)-{B})=4+1=5 pB=12-5=7, uB=7-4=3 A cheats only (6x cost): cost(AB)=(5*6)/5=6 LCP is through C, so pB=0 B also cheats (3x feedback): cost(AB)=(5*6)/15=2 cost(LCP(S,D))-{B}=4+2=6 pB=12-6=6,uB=6-4=2 > 0! Truthfully helping A to report the cost is not a dominant routing subaction of B!

16. Prevent Downstream node from Cheating Cheating is not beneficial by increasing Pij • Less likely to be on LCP • Even on LCP, payment received decreases Prevent cheating that decreases Pij • Intuition: j cannot forge an encrypted signal i sent with Pemit <Pij • Mechanism: • Node i sends TESTSIGNAL [S,D,r,] at increasing power Pemit • r is a pseudo-random that distinguish different sessions • Pemit is protected by a MAC computed using a share key Ki,D between i and destination D.

17. Routing Protocol Corsac routing protocol: • Source generates TESTSIGNALs at different levels of power • Each intermediate node • forwards TESTSIGNALs from upstream nodes at the maximum power • generates its own TESTSIGNALs at different levels of power • Destination computes the LCP and payment for each node Corsac routing protocol is a routing-dominant protocol

18. Forwarding Stage Block confirmation using reverse hash chain • M data packets divided into n=M/b blocks • Source sends the confirmation to destination at the end of each block • Destination confirms each block by releasing the confirmation to intermediate nodes • Intermediate nodes forward next block after getting confirmation Mutual decision to eliminate incentives to cheat • Source decides whether intermediate nodes should be paid for the next block • Intermediate nodes decide whether source should be charged

19. Forwarding protocol • Routing decision transmission • destination sends out w/ its digital signature • specifies path, power level, unit payment for each node on path • Data transmission • source sends out blocks, last packet contains block confirmation code, waits for confirmation from destination • Intermediate nodes forward, also wait for confirmation • destination sends confirmation • each intermediate node verifies confirmation • source starts sending next block after receiving confirmation • intermediate node j receives payment pj for each unit that is forwarded and confirmed

20. Forwarding-Optimal Protocol • The forwarding protocol as tree of a game

21. Forwarding-Optimal Protocol • Each sub-tree is a subgame • Each path from root down to a leaf is a set of decisions • Remember: a Forwarding protocol is optimal if following the protocol (i.e. forwarding) is a subgame perfect Nash equilibrium • From the Game Theory we know that a sub-game is a sub-game perfect equilibrium if it is a Nash equilibrium for every sub-game

22. Forwarding-Optimal Protocol (Cont’d) Our forwarding protocol is forwarding-optimal given the routing decision By backward induction: at each step the node chooses to forward because the payment is higher than the cost (VCG mechanism)

23. Evaluation • 30 nodes deployed in 2000X2000 area • Two transmission power level: 7 and 14dBm • Random traffic • Poisson session arrival • Destination of each session picked randomly

24. Credit Balance and Energy Consumption • Credits accrued depends on location and connectivity

25. Impact of Cheating • The benefit of virtue

26. Conclusion and Future Work • Forwarding-dominant protocol does not exist • We present a new solution concept---cooperation-optimal routing and forwarding: • Routing-dominant • Forwarding is a subgame perfect Nash equilibrium • We use crypto mechanism to enforce routing-dominant subaction