Incentive-Compatible Opportunistic Incentive-Compatible Opportunistic Routing for Wireless NetworksRouting for Wireless Networks

Fan Wu, Tingting Chen, Sheng ZhongFan Wu, Tingting Chen, Sheng Zhong (SUNY Buffalo)

Li Erran LiLi Erran Li (Bell Labs)Yang Richard YangYang Richard Yang (Yale University)

Speaker: Fan WuFan Wu

Motivation

• User-contributed wireless mesh networks– Low cost– Unpredictable and lossy wireless links

From http://an.kaist.ac.kr/~tdinhtoan/

Motivation (cont’)

• Opportunistic routing emerged to improve throughput, e.g.,– ExOR (Biswas and Morris [2005])– MORE (Chachulski et. al. [2007])

S R DLLP = 0.4

LLP: Link Loss Probability

LLP = 0.8

ENT: Expected Number of Transmissions

ENT = 1.18LLP = 0.4

ENT = 1.47

p1 p2p1 p2

p1+p2

p1 p2

Motivation (cont’)

• Selfish behavior may reduce performance– Free-rider problem– Adverse selection problem

S R DLLP = 0.1 ↓

LLP: Link Loss Probability

LLP = 0.8

ENT: Expected Number of Transmissions

ENT = 0.78 ↓LLP = 0.4

ENT = 1.47

Motivation (cont’)

• Existing incentive mechanisms are mainly based on shortest path routing

• Need to design incentive-compatible routing protocols so that each user node participates in opportunistic routing honestly

Objective

• Develop incentive compatible techniques that can be integrated with a wide class of opportunistic routing protocols

• A basic opportunistic routing protocol:– collects link states and then– computes a forwarding behavior profile for

user nodes

Basic Opportunistic Routing Protocol

• Source Node, S– Divides traffic into batches of packets– Keeps sending coded packets in current batch– Moves to next batch if acknowledged

• Intermediate Node, i– Broadcasts a coded packet if needed– Targets expected number of transmissions zi:

• εi,j: loss probability on link (i, j)

• Destination Node, D– Decodes received packets– Sends acknowledgments

Issues

Issue 1: Motivating Honest Reporting

Issue 2: Motivating Honest Measuring

Issue: Motivating Honest Reporting

• Ideal scenario: Each node i reports the loss probabilities of its outgoing links

• Reality: Without proper incentive, node i may not report its link loss probabilities honestly

Techniques to Motivate Honest Reporting

• We design a routing protocol, such that reporting loss probabilities truthfully is the best strategy of each node

• Techniques: We influence the strategies of the players by introducing – an auxiliary transmission and

– a carefully designed payment formula

Motivating Honest Reporting: Auxiliary Transmission

For each packet that a node i should forward, it is required to send an auxiliary traffic of size z*i,j to each node j Vp

• α is a very small constant

• ε’i,j is the reported loss probability on link (i,j)

Motivating Honest Reporting: Payment Formula

• L is the packet length

– covers the cost of packet transmissions – covers the cost of auxiliary transmissions

(We assume that transmitting a packet of size 1 has one unit of cost.)

1

1

2

2

Why Auxiliary Transmission and the Payment Formula?

• Utility:

Get maximized when

ACK

How does the protocol work?

S

A

D

B

ε’ A,D, ε’ A,B

ε’B,D , ε’

B,A

z’ A, z* A,D

, z* A,B

z’B , z*

B,D , z*B,A

ACKp A

pB

AUX

AUX

AU

X

AU

X

Motivating Honest Reporting: Analysis

– Theorem: It is a strictly dominant strategy equilibrium for all player nodes to truthfully report loss probabilities.

Strictly Dominant Strategy Equilibrium: The equilibrium strategy is strictly better than any other strategy for each node regardless of other nodes’ behaviors.

Two Steps

Step 1: Motivating Honest Reporting

Step 2: Motivating Honest Measuring

Issue: Cheating in Measurements

• Practical scenario: A node needs the cooperation (feedback) of its neighbors to measure link loss probabilities

• Dishonest feedback may allow one node to cheat its neighbors in order to raise its own utility

Techniques to Achieve Truthful Measurements

• We design an enhanced routing protocol, such that truthfully measuring the loss probabilities is to the best interest of each node

• Techniques: We influence the strategies of the players by carefully designing– measurement (test) signals and– a payment formula(Auxiliary transmission is the same as before.)

Measurement Signaling

– Each node i sends nt measurement signals– Format of measurement signal:

• kS,i is a secret key shared by S and i• MAC is a cryptographic Message Authentication

Code function

– Each node forwards measurement signals using traditional routing protocol

– If ni,j measurement signals are received, then

Payment Covering Measurements

• Payment Formula:

– covers the cost of packet transmissions– covers the cost of auxiliary transmissions– prevents dropping measurement signals

1

1

2

2

3

3

TEST_1

TEST_2

TEST_nBA

…

How does the enhanced protocol work?

S

A

D

B

z’ A, z* A,D

, z* A,B

z’B , z*

B,D , z*B,A

ACK

ACK

p A

pB

AUX

AUX

TEST_1

TEST_2

TEST_nDA

…TEST_1

TEST_2

TEST_nDB

…

AU

X

AU

X

TEST_1

TEST_2

TEST_nAB

…

TEST_1

TEST_2

TEST_nAD

…

TEST_1

TEST_2

TEST_nBD

…

Motivating Truthful Measurements: Analysis

– Theorem: There is a strict Nash equilibrium for all player nodes to behave honestly in sending test signals and forwarding the received test signals.

– Theorem: The above equilibrium is the only strict Nash equilibrium in the system.

Strict Nash Equilibrium: Unilaterally deviating from the equilibrium strategy will hurt a player’s utility.

Evaluation Setup

• Real implementation and tests on the ORBIT testbed

• 25 nodes• 802.11b ad hoc mode• Trans. power 20 dBm• Bit-rate 11Mbps• MORE batch size 32• Packet size 1500 bytes• Loss prob. 24%~100%• Session length 30 s• α=0.1• β=0.05

Evaluation Setup

• Node Behavior:– Honest behavior:

• Each node follows our protocol faithfully

– Cheating behavior: • Misreporting link loss probabilities in the simple

extension;• Sending incorrect number of measurement signals

and • Dropping others’ measurement signals in the

enhanced extension

Cheating Behavior and Node Utility

Simple extensionUtilities obtained by node 18

Enhanced extensionUtilities obtained by node 11

Utilities obtained by honest reporting and cheating randomly

Cheating Behavior and Node Utility

Utilities obtained by applying various strategies

Simple extension Enhanced extension

Impacts on End-to-End Throughput

Average throughput as a function of the number of hops on the path

Simple extensionUp to 33.2% (58.0%) gain when 20% (40%) cheating

Enhanced extensionUp to 13.7% (23.4%) gain when 20% (40%) cheating

Miscellaneous

• Overhead:– Average auxiliary transmissions: 26.73 KB– Average data transmitted: 3.93 MB– Ratio: 0.66%

• Auxiliary payment:– Ratio between auxiliary payment and total

payment– Simple extension: 0.23%– Enhanced extension: 1.20%

Conclusion

• We study incentives in opportunistic routing and provide first solutions.

• We present a simple and practical protocol to guarantees that it is a strict dominant strategy for each user node to behave honestly.

• We also design an enhanced protocol to prevent cheating in measuring loss probabilities.

• We implement and evaluate our protocols on the ORBIT lab. The experimental results show that our protocols can bring the system throughput achieved by opportunistic routing protocols back to the high level.

Thank you!Thank you!