1 Sustaining Cooperation in Multi-Hop Wireless Networks Ratul Mahajan, Maya Rodrig, David Wetherall and John Zahorjan University of Washington Presented by: Prasad (Slides in courtesy of: Bin Ni, University of South Carolina)

2 Key words Wireless network - characteristic of broadcasting Multi-Hop - a packet may be forwarded by multi nodes Free-rider and Cooperation - less contribution to the group - consume more than their fair share of a resource

3 Topics Problem Definition The power of Anonymity The Catch Protocol Experimental Evaluation Simulation and Analysis

4 Problem Definition B can avoid these forwarding loads in two distinct ways: - forwarding level : drop packets for forwarding from A - routing level : refuse to send routing messages that acknowledge connectivity with A

5 Assumptions Most nodes are cooperative in that they run the protocol defined; Omni-directional radio transmitters and antennas; Nodes have unforgeable ID.

6 Power of Anonymity The goal is to use cooperative nodes to monitor for the presence of free-riders and to isolate them. Two problems - distinguish them - signal all of the free-rider’s neighbor

7 Anonymous Challenges and Watchdogs A watchdog is used; Anonymous challenge message (ACM) sub-protocol is defined; Gateway regularly but unpredictably sends an anonymous challenge to the testee to rebroadcast.

8 Anonymous Neighbor Verification Once free-rider is detected, other testers must also be informed; Challenge: the only path must be via the testee; Anonymous neighbor verification (ANV) sub-protocol is defined;

9 Anonymous Neighbor Verification ANV open - all the testers become aware of each other via the testee; - each tester sends a cryptographic hash of a randomly generated token; - testee rebroadcasts, other testers take note; ANV close - each tester releases its token to the testee if it behaved well; - the testee broadcasts the token; - testers compare the received token;

10 Catch protocol Epoch-Start - testee broadcasts packet that includes its ID and epoch ID; - nodes that receives the packets participate as testers for this epoch; Packet Forwarding and Accounting - testers run a watchdog to count the number that are correctly relayed; - testers run the ACM protocol to estimate true connectivity; Anonymous Neighbor Verification Open - tester sends an anonymous packet containing a nonce and a hashed token to the testee for rebroadcast

11 Catch protocol Tester Information Exchange - tester compares the fraction of data packets and challenges; - one-bit sign:0 for challenges and 1 for data packets; - send the sign bit and ID to testee for rebroadcasting ; Epoch Evaluation and ANV close - each testers determine whether the testee behaviors correctly - this is done with a pair of statistical test - both tests pass, the tester releases its token; Isolation Decision

12 Protocol flow: packets exchange between a tester and a cooperative (left side) or free-riding (right side) testee.

13 Evaluation 15 PCs equipped with b Operating in the ad-hoc mode Diameter is between 3 and 5 hops Length of one epoch is set to one minute There are 15 anonymous ACM messages per epoch

14 Evaluation Three nodes: the second one acts as free-rider The number of epochs required to detect free- riders in the testbed versus the fraction of packets a free-rider dropped. Each point is the average of 10 experiments.

15 Evaluation Three nodes are free- riders that drop all the packets ; Nodes select only cooperative nodes as file servers; It shows that the cooperative nodes successfully shut up the free-riders;

16 Evaluation We ran two five hour experiments in which all nodes were cooperative. Each node repeatedly downloaded les from randomly chosen servers. We observed no false positives in the first experiment and a single false positive in the second.

17 Evaluation cheater that uses signal strength to differentiate among its neighbors. For example, when 20% (3) of the nodes cheat, that probability is lowered from about 60% to about 10% when using the highest quality links.

18 Simulation and Analysis the impact of high wireless loss rates on Catch is quite small. Catch seems to be robust, working well in both high and low density networks.

19 Future work Collusion of the free riders Taking specific steps in Catch to discourage signal strength based cheats

20 Thanks! Any question?