490 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 57, NO. 3, MARCH 2008
Network Intrusion Detection Using CFAR
Abrupt-Change Detectors
Di He, Member, IEEE, and Henry Leung, Member, IEEE
Abstract—In this paper, the constant false alarm rate (CFAR)
detectors are proposed for network intrusion detection. By using
an autoregressive system to model the network traffic, predictor
error is shown to closely follow a Gaussian distribution. CFAR
detector approaches are then developed on the prediction error
distribution. In the present study, we consider the optimal CFAR,
the cell-averaging CFAR, and the order statistics CFAR. The use
of these CFAR techniques can significantly improve the detection
performance. In addition, we propose the use of fusion of these
CFAR detectors by using Dempster–Shafer and Bayesian tech-
niques. Computer simulations based on the DARPA traffic data
show that the proposed approach achieves higher detection prob-
abilities than the conventional detection method. Even under dif-
ferent types of attacks, the intrusion detection performances based
on the proposed CFAR detectors shows consistent improvement.
Index Terms—Cell-averaging (CA-CFAR), constant false
alarm rate (CFAR), Dempster–Shafer, detector fusion, intrusion
detection, network traffic model, optimal CFAR, order statistics
CFAR (OS-CFAR).
I. INTRODUCTION
W
ITH THE fast increase of network connections, the
problem of intrusion detection becomes more and more
important [1]. Although the Internet service can provide useful
information due to its open property, it should also be noticed
that the number of network intrusions increases faster than
before, which introduces a lot of inconvenience to the users
[2]. Nowadays, computer network traffic is influenced by many
kinds of intrusions such as attacks, viruses, worms, etc. Some
of these intrusions might not cause serious damages, but most
of them will bring an unexpected influence to the servers or
personal users [3], [4].
Misuse detection and anomaly detection are two basic cate-
gories of intrusion detection techniques [5]. Misuse detection
is a kind of passive method that is used to avoid known
intrusions [6]. By using misuse detection, patterns of differ-
ent intrusion signals should first be studied [7]. Detection is
then carried out through some pattern recognition techniques
Manuscript received January 10, 2007; revised June 25, 2007. This work was
supported in part by the Natural Science and Engineering Council of Canada,
by the Natural Science Foundation of China under Grant 60272082, and by the
Scientific Research Foundation for the Returned Overseas Chinese Scholars,
State Education Ministry of China.
D. He is with the Department of Electronic Engineering, Shanghai Jiao Tong
University, Shanghai 200240, China (e-mail: dihe@sjtu.edu.cn).
H. Leung is with the Department of Electrical and Computer Engi-
neering, University of Calgary, Calgary, AB T2N 1N4, Canada (e-mail:
leungh@ucalgary.ca).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TIM.2007.910108
[8], [9]. Although misuse detection is widely used in many
current systems, its weakness is that it is invalid to detect new
attacks. Anomaly detection does not suffer from this problem,
but it is usually found to have a lower probability of detection
[10]–[12]. Although various sophisticated techniques such as
neural networks [13], hidden Markov model [14], integrated
access control [15], and sensor fusion [16] have been inves-
tigated, these techniques either cannot fulfill the requirement
for real-time detection or are unable to achieve high detection
probabilities for different types of intrusions. Hence, a robust
anomaly detector with an acceptable detection probability is
still in need.
Most reliable detectors in use to date, such as those for
radar systems, are based on the statistical decision theory. To
develop a robust network intrusion detector, understanding the
distribution model of network traffic is important. It has been
found that the normal traffic obeys some certain distributions
when there is no intrusion [17]–[19]. When attacks occur, the
statistical properties of the process may change. It indicates that
anomaly detection that is based on abrupt-change detection is
a reasonable approach. To capture this property for detection,
prediction-based methods have been considered [20]. By com-
paring the prediction values with the real ones, the prediction
error becomes another vital indication for the appearance of
intrusion signals.
In this paper, we propose the application of a robust radar
detection approach, which is the constant false alarm rate
(CFAR) technique, to the prediction-based abrupt-change de-
tector for network intrusion detection. In particular, three
popular CFAR techniques, namely, the order statistics CFAR
(OS-CFAR) [21], the cell-averaging CFAR (CA-CFAR) [22],
and the optimal CFAR [23], are considered here due to their
good detection performances. Detection probabilities of the var-
ious CFAR detectors are derived. In addition, we propose fusion
of these CFAR detectors to enhance the detection performance.
Bayesian and Dempster–Shafer fusion methods are applied to
achieve a reliable fused CFAR detector. The simulation results
that are based on the standard Defense Advanced Research
Projects Agency (DARPA) data sets confirm the theoretical
results and indicate that the proposed CFAR approach is ef-
fective in detecting network intrusion even under different
attacks.
This paper is organized as follows. Section II briefly dis-
cusses the network traffic models. The error of the abrupt-
change detector that is based on the autoregressive (AR) model
is shown to follow a Gaussian distribution. Section III presents
the CFAR techniques, including OS-CFAR, CA-CFAR, optimal
CFAR, and fused multiple CFAR methods that are based on the
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