Catch Packet Droppers in
Wireless Ad-hoc Networks
Wei-Cheng Xiao, Lei Tang
Mobile Ad-hoc Network
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No access point, self-configured
Every node is a router.
Packet transmission relies on node cooperation.
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Motivation
Catch packet droppers in wireless ad-hoc
networks that:
1) Maliciously drop packets
2) Drop packets due to network partition or
poor link quality
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Challenges and Assumptions
1. Challenges
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No global monitoring
No trust relationship among nodes
2. Assumptions
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Source-routing based packet transmission
Power of dropper
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Random drop only
Cannot modify packet content
Pre-distributed pairwise shared secret key.
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The Proof-based Scheme
Packet
Packet
S
A
Proofhop
D
B
Proofe2e
Proofhop = {src, dest, seq, I, HMACKS,B(src, dest, seq, I )}
Proofe2e = {src, dest, seq, HMACKS,D(src, dest, seq)}
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Node X is unable to forge proofs between node Y and Z.
The source is able to verify the proofs.
Only symmetric cryptography (HMAC-SHA1) is used.
Intermediate nodes cache proofs for later investigation.
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Investigation
The source requests each node on the route to
present its cached proofs.
Investigation
Proof
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Aggregated Investigation
To reduce overhead, multiple packets are investigated
using only one investigation and one reply.
Investigation:
source
destination
start seq.
Reply:
end seq.
Hash(proofs)



Bitmap of packets having a proof
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Dropper detection and Dropping Metric
packet
1
2
refuse proofhop
packet
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• Narrow down the scope of possible droppers via proof
analysis
• Dropping metrics of nodes
 Maintained by each source, not shared with other sources.
• Metrics computation
M  (1   ) M    Inc.
 Possible dropper: metrics increased
 Other nodes: metric decreased (has minimum metric)
• Periodical metric decreasing for nodes not on the routes
 Give chance to nodes that are no longer bad
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Evaluation
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We implemented CATCH in ns2 network
simulator.
Parameter Inc= -1 when a packet is delivered to
the destination. Inc=3, when a node is unable to
present a proof. α is set as 0.2.
In the experiments, DSR was used as the routing
protocol.
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10-node-chain, node 5 drop rate 1.0
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10-node-chain, node 5 drop rate 0.2
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49-node grid network—false negative rate
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49-node grid network--false positive rate
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Conclusion
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With CATCH, a dropper ends up having a higher
dropping metric than non-droppers. But false
positives are inevitable.
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In most experiments, CATCH scheme
successfully caught the droppers.
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