Post on 31-Aug-2020
A Robust Kolmogorov–Smirnov
Detector for
Misbehavior in IEEE 802.11
DCF
Alberto Lopez Toledo and Xiaodong Wang
Columbia University
ICC 2007 Glasgow, UK
June 27, 2007
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The CSMA/CA Protocol uses random deferment of
packet transmission for content resolution.
Random backoff timer is chosen in the interval [0,v), with
Two modes of transmission: basic and RTS/CTS access.
CSMA/CA Protocol
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CSMA/CA Protocol Misbehavior
The operation of the protocol assumes that all
nodes will abide by the protocol rules.
Nowadays devices are easily programmable,
and hence can easily change their behavior and
affect other users: misbehavior.
Two types of misbehavior
Malicious: disrupting the operation of the network,
even in their own determent.
Selfish: users willing to increase their share of the
network.
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Selfish Misbehavior
Selfish misbehavior is more dangerous:
It is very easy to implement: users can simply change
the value of the contention window.
Every user has an incentive to implement it.
Misbehavior is difficult to detect:
CSMA/CA operation is random.
The medium is random: channel impairments and
interference are different for different users
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Misbehavior Technique: Modify
Backoff
Source: Levente Buttyan and Jean-Pierre Hubaux, Security and Cooperation in Wireless Networks, http://secowinet.epfl.ch.
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Misbehavior Effect is Catastrophic
DoS Bandwidth
theft
A misbehaving node
can starve other
nodes
Current situation:
anyone can steal
bandwidth today.
Targets
Municipal networks.
Wireless Mesh
Networks.
Serious threat to
open networks
revenue model.
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Problem Formulation
Let x1,…,xK be a sequence of observations related
to the operation of a CSMA/CA terminal. Then
we define the problem as
where f0 and f1 are the probability distributions of
the observations when a node is not
misbehaving and misbehaving respectively.
We want to design a decision rule to
discriminate between the two hypotheses.
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Characterizing a Legitimate Terminal
The observations x1,…,xK are the number of idle slots
between successful transmissions of the observed
terminal. Then f0 is given by
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0 50 100 150 2000
0.005
0.01
0.015
0.02
0.025
Number of idle slots between successful transmissions
Analyticalns−2
The strategy f0 of a saturating legitimate terminal
Characterizing a Legitimate Terminal
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Intuitively, a misbehaving terminal would access the
network more than a saturating legitimate terminal.
Characterizing a Misbehaving Terminal
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We use the one-sided Kolmogorov-Smirnov test given by
Where F1 is the empirical cdf of f1, given by
and F0 is the estimated cdf of f0, with is the empirical cdf
of f1, given by
Kolmogorov-Smirnov Test
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The Kolmogorov-Smirnov statistic is given by
The Hypothesis H0 is rejected at significance level if
where
Kolmogorov-Smirnov Test
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Detection Algorithm
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1Mbps IEEE 802.11b in ns-2 2.28.
Legitimate terminal uses CWmin=32, CWmax=1024.
Misbehaving terminals use CWmax=25CWmin, andCWmin {1,2,…,32}.
For comparison: SPRT test with perfect knowledge of f1with = 0.05.
Legitimate terminal
Simulation Setup
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Performance of K-S vs. Optimum SPRT
10 competing terminals
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Number of Samples to Detect with
PD=0.95
10 competing terminals
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Number of Samples to Detect a Shift
in Collision Probability with PD=0.95
10 competing terminals
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The K-S detector is robust, and can be appliedunder any network scenario and any IEEE802.11 DCF flavor, in basic access or RTS/CTSaccess.
The performance is close to that of the optimumdetectors that assume perfect knowledge aboutthe misbehavior strategy.
Can operate as a ‘black box’, without modifyingthe protocol implementation.
The detector is extremely fast, detecting anymisbehavior of CWmin < 29 in less than asecond in IEEE 802.11g.
Conclusions