sustaining cooperation in multi-hop wireless networks ratul mahajan, maya rodrig, david wetherall,...
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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
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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Incentives to free-ride
Personal bandwidth maximization
Power conservation
It’s easy!
You’ll get away with it
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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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
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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Wireless range is not a simple function of distance and can be asymmetric
Challenge in detecting free-riding
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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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
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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A cooperative node does not receive the packet
Challenge in detecting free-ridingFree-riding Lack of connectivity
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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Challenge in detecting free-ridingFree-riding Lack of connectivity
Hard to differentiate the two cases given asymmetric, lossy links
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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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
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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Tradeoffs between approachesProposed solutions
Catch
Protection
Ove
rhea
d
Applicab
ility
Deployed protocols
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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1. Detecting wireless connectivity
Nodes want to connect to at least one neighbor
Send anonymous connectivity probes
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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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
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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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
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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Catch evaluation
Speed and accuracy of detection
Effectiveness of isolation
Overhead
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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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
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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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
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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Overhead
Bandwidth: Only 24Kbps per node in our testbed
CPU: Maintain packet counters, but no crypto operations per data packet
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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Future work: Signal strength attacks
Physical layer hints can undermine anonymity Catch already offers some protection
Maya Rodrig | UW | NSDI ’05Sustaining Cooperation in Multi-Hop Wireless
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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