sustaining cooperation in multi-hop wireless networks

Sustaining Cooperation in Multi-Hop Wireless

Networks

Ratul Mahajan, Maya Rodrig,

David Wetherall, John Zahorjan

University of Washington

Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless

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Multi-hop networks are real


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Incentives to free-ride

Personal bandwidth maximization

Power conservation

It’s easy!

You’ll get away with it


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The impact of free-riding

Using multi-hop routes improves throughput Per-node delivery rate improvement of 25% on average

Free-riders gain at the expense of cooperative nodes Free-riders’ throughput increased 4X Cooperative nodes’ throughput decreased by 25%

In our testbed, 60% chance of network partition with only 20% free-riding nodes


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Wireless range is not a simple function of distance and can be asymmetric

Challenge in detecting free-riding


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Free-rider pretends to be out of range and drops the packet

Challenge in detecting free-ridingFree-riding Lack of connectivity


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A cooperative node does not receive the packet



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Hard to differentiate the two cases given asymmetric, lossy links


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Catch

Two main challenges: Determining when a node is free-riding Getting its neighbors to agree to punish it

Solution:1. Detecting wireless connectivity

2. Monitoring packet relaying behavior

3. Punishing selfish misbehavior


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Tradeoffs between approachesProposed solutions

Catch

Protection

Ove

rhea

d

Applicab

ility

Deployed protocols


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1. Detecting wireless connectivity

Nodes want to connect to at least one neighbor

Send anonymous connectivity probes


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2. Monitoring packet relaying behavior

Watchdog: relayed packets should be overheard [Marti et al, 2000]

Use statistical tests to compare success rate of anonymous probes and data packets


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Leverage the cooperative majority to collectively deter free-riders Use anonymous probes, one-way hash functions, and signaling

by absence

3. Punishing selfish misbehaviorNx’

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184 ft

Our testbed

In-building experimental testbed 15 802.11b nodes 10 APs on same floor

A real wireless setting Many asymmetric links Frequent packet losses

~20% loss rate


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Catch evaluation

Speed and accuracy of detection

Effectiveness of isolation

Overhead


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Total Drop Rate

Ep

och

s t

o

Dete

cti

on

Speed and accuracy of detection

Rapid detection with few false positives Detection is quicker for more egregious free-riding 1 false positive in 10 hrs across testbed

1 2 3

client

server


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Effectiveness of isolation

Isolation is successful despite asymmetric, lossy links Throughput of free-riders is curbed

Elapsed Time (minutes)

Isolation period


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Overhead

Bandwidth: Only 24Kbps per node in our testbed

CPU: Maintain packet counters, but no crypto operations per data packet


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Future work: Signal strength attacks

Physical layer hints can undermine anonymity Catch already offers some protection


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Conclusion Catch is a lightweight solution to deter free-riding

Modest overheads No restrictive requirements

Key insights: Using anonymous probes to detect connectivity and

communicate via free-riders Leveraging cooperative majority to detect and punish

free-riders

Testbed evaluation shows that Catch is effective in a real wireless environment

sustaining cooperation in multi-hop wireless networks

Documents

freeridinga cooperative

lossy linksthroughput

freeridingwireless range

itthe impact of free

freeriding nodes challenge

packet counters

real wireless

wireless connectivitynodes