1. Trust Based Mechanism Design By Rajdeep Dash, Gopal Ramchurn, Nicholas Jennings

2. Goal Fuse the fields of trust-modelling and mechanism design – Mechanism design concerns itself with the allocation of resources. – Trust measures how good an interaction partner is according to personal experience and its reputation

3. Deciding on a service provider Garage X is better than Y Garage Y is better than X! 1 2 Anna Jo Vanessa 3 pr i ce 3 pr i ce 1 pr i ce 2

4. Motivation Which garage to choose to get the best price AND my car repaired successfully? 1 2 Anna 3

5. Mechanism Design The design of systems of interacting agents – Protocols (allocation + payment schemes) to ensure that certain global properties are achieved at equilibrium (game-theoretic). – Selfish, rational agents possessing private information i.e. each agent maximises its utility function given its type.

6. Task Allocation Agent wants a task to be completed (e.g. repairing a car). Problem – Who to allocate task to ? – How much to pay task performer ? Solution - Vickrey Clarkes Groves (VCG) mechanism - (e.g. second price (Vickrey) auction)

7. Class of mechanisms satisfying – Efficiency – outcome maximising utility for all agents – Individually rationality – incentivise participation – Incentive compatibility – incentivise truthful report under dominant strategy: Works by: – Allocation : choose allocation maximising utility – Payment : charge marginal utility contributed by agent VCG Mechanism

8. Applying VCG Centre Ask(£80,repair_car) (£210,repair_car) Ask(£50,repair_car) Allocate task to 1, 4 pays 1 £50 1 Anna (4) Ask(£40,repair_car) 3 2

9. Cheapest not always best …..

10. Suppose that Anna has formulated a belief about the Probability of Success (POS),,of other agents. Adding uncertainties Agent i 1 400.52570.25 2 801.0130 3 500.85128.5 C i E [ u A ( K ; µ A )] = v A ´ i A ¡ c i E [ u A ( K ; µ A )] ´ i A ´ i A E [ u A ( K ; µ A )] Chose allocation that maximises expected utility,

11. Trust So far : one’s own experience,. Would be better to factor in others’ experiences,, as well. – First time agent – Greater pool of experience for seasoned user. Trust a way of aggregating one’s direct experience with the reputation information provided by others. ´ i A ´ i j

12. Our model of trust Each time a task is performed, record POS Publicly transmit the POS to other agents Trust – Use a simple averaging function over reports from all agents – Give more weight to more trusted referrals, or agents with similar properties Vanessa 2 Anna Transmit POS

13. Incorporate Trust Agent i 1 400.41.00.850.565 2 800.61.00.91.0130 3 500.51.00.90.87132.7 4 0.5251.00.85na ´ 1 i ´ 2 i ´ 3 i 1 t i 4 E [ v 4 ( j ; µ 4 )] ® = [ 0 : 30 : 20 : 10 : 4 ] ; v 4 ( ¿ ) = 210 c i

14. TBMD Centre (£210,task1) Trust model c 1 = 40 ´ i 3 ´ i 2 c 2 = 80 ´ i 1 c 3 = 50 ´ i 4 1 2 3 4 transmits to centre -- value of task, observed POS and trust model1, 2, 3 transmit to centre -- cost of doing task and observed POSCentre calculates optimal allocation b K ¤ Anna

15. TBMD Centre (£210,task1) Trust model c 1 = 40 ´ i 3 ´ i 2 c 2 = 80 ´ i 1 c 3 = 50 ´ i 4 1 2 3 D i = U ( b K ¤ ;: ) ¡ U ( K ¤ ¡ i ;: ) Anna Centre calculates optimal allocation without each agent i’s POS report. K ¤ ¡ i ´ i 2 c 2 = 80 ´ i 1 c 3 = 50 2 3

16. TBMD Centre(£210,task1) Trust model c 1 = 40 ´ i 3 ´ i 2 c 2 = 80 ´ i 1 c 3 = 50 ´ i 4 1 3 Centre calculates transfer to i, r i = mc i ¡ D i Anna r i K ¤ ; ´ i 2 c 2 = 80 ´ i 1 c 3 = 50 2 3

17. Trust Based Mechanism Design (TBMD) An individually rational and efficient ICDR mechanism that can also select those agents that are most successful at their task

18. Experimental Setup Set of buyers and sellers. Biased (but truthful!) report from seller. Buyers attach different levels of importance to seller’s report.

19. Results K*FTM K*TBM 0.5 K*VCG K* K*TBM 0.25 K*TBM

20. Conclusions Task allocation with uncertainty Only incentive-compatible, individually-rational efficient reputation mechanism Affords flexibility of context based trust modeling with efficiency

21. End Any Questions? rkd02r@ecs.soton.ac.uk

22. Our Case Study Task Allocation – Some agents may perform some tasks for which they incur a cost C i – Others may ask for tasks to be performed for which they have a valuation V i – Eg. File-sharing on P2P networks, Selfish Multi-Sensor Nets BUT there exists some uncertainty! – Agents may not perform tasks completely at times i.e. there exists a probability of success (POS) in executing a task

23. Trust-Based Mechanism 1. Find the optimal allocation given all trust reports 2. Find the optimal allocation with the trust report of each agent taken out in turn. K ¤ = argmax K 2 K P i 2 I [ v i ( K ; µ i ) ¡ c i ( K ;; µ i )] t i = P i 2 I n I [ v i ( K ¤ ; µ i ) ¡ c i ( K ¤ ;; µ i )] ¡ max K 2 K P i 2 I n i [ v i ( K ; µ i ) ¡ c i ( K ;; µ i )]