2010 International Conference on Broadband, Wireless Computing, Communication and Applications

Behavior Shaver: An Application Based Layer 3
VPN that Conceals Trafﬁc Patterns Using SCTP
Mamoru Mimura

Hidehiko Tanaka

Graduate School of Information Security
Institute of Information Security
Yokohama, JAPAN
Email: dgs104101@iisec.ac.jp

Graduate School of Information Security
Institute of Information Security
Yokohama, JAPAN
Email: tanaka@iisec.ac.jp

Abstract—In recent years, distributed systems are connected
by VPN (Virtual Private Network) through the Internet, andconstruct complicated information systems. These information
systems bring beneﬁt and security risks to many users. Representative security risks, vulnerabilities are closely related toapplication software installed in information systems. If a malicious adversary identiﬁes the application software, he can seek
the vulnerabilities easily. Thus, to ensure security of information
systems, it is necessary to conceal application software installed in
information systems. On the other hand, some attempts have been
proposed to identify application software or protocol without
scanning the payload. These proposed methods can analyze
encrypted trafﬁc, because the methods scan trafﬁc patterns suchas packet sizes and transmission intervals. While there are some
legitimate uses for encrypted trafﬁc analysis, these methods also
raise problems about the conﬁdentiality of encrypted trafﬁc.
Many researchers proposed countermeasures against trafﬁc analysis to ensure anonymity in a public network. They indicated how
to alter trafﬁc patterns in the main. However, a few researcher
indicated how to implement the method. Indeed, though previous
VPN applications protect payloads against an eavesdropper, do
not conceal side channel information including trafﬁc patterns.
Our work applies these proposed countermeasures and shows
how to implement a secure VPN application that conceals trafﬁcpatterns. To alter trafﬁc patterns, it is necessary to control packet
sizes. Many popular application based VPN encapsulates packets
by TCP or UDP. However, TCP cannot control packet sizes
strictly. Though UDP can control packet sizes without difﬁculty,
does not ensure reliable data transmission. A secure application
based VPN requires a protocol that can control packet sizes
strictly and can ensure reliable data transmission in untrusted
networks. SCTP (Stream Control Transmission Protocol) is asuitable solution for these requirements. This paper proposes the
behavior shaver, an application based layer 3 VPN that conceals
trafﬁc patterns using SCTP. The results of experiments show theperformance.

security is a great problem to be solved. Many information
systems have many security risks such as vulnerability. Today,
vulnerabilities are reported from day to day and anyone canﬁnd security information easily from the Internet. A zero-day
attack, a computer threat that tries to exploit computer application vulnerabilities that are unknown to others, undisclosed tothe software vendor, or for which no security ﬁx is available,
has lost much of its novelty now. Vulnerabilities are closely
related to OS (Operating System) or application software
installed in information systems. If a malicious adversary
identiﬁes the OS or the application software of an information
system, he can seek the vulnerabilities easily. Thus, to ensure
security of information systems, it is necessary to conceal OS
or application software installed in information systems.
On the other hand, some attempts have been proposed to
identify application software or protocol without scanningthe payload. These proposed methods analyze trafﬁc patterns
such as packet sizes and transmission intervals. If a malicious
adversary can analyze encrypted trafﬁc and reveal the application software by these techniques, they can become threats
to ensure security. Though previous VPN applications e.g.
OpenSSH[1], OpenVPN[2] protect payloads against an eavesdropper, do not conceal side channel information including
trafﬁc patterns. Threfore, previous VPN applications cannot
ensure security of information systems.
This paper makes threats of trafﬁc analysis in untrusted networks clear and consider constructing secure communicationroute to ensure security of distributed systems. Our goal isdevelopping a secure VPN application that conceals trafﬁcpatterns.

I. I NTRODUCTION

II. S COPE

In recent years, many information systems are constructedby organizations. Some distributed systems are connected
by VPN (Virtual Private Network) through the Internet, andconstruct complicated information systems. VPN applications
play an important role in distributed systems through untrusted
networks like the Internet. In general, security of VPN depends
on strength of the cryptographic communication. These information systems join the Internet to users and bring beneﬁt
to many users. At the same time, the beneﬁt often bringsecurity risks to many users. To cope with both beneﬁt andTrafﬁc analysis methods can be classiﬁed into payload analysis, header analysis and behavior analysis. A countermeasureagainst payload analysis is cryptographic communication. The
second method is mainly called OS ﬁngerprinting[3], and a
protocol scrubber[4] is one of many countermeasures against
this method. However, there is a few countermeasure against
behavior analysis that infers application software or protocol.
Our work belongs to this category.
Our purpose is not anonymity in a public network but conﬁdentiality in a private network. Many researchers proposed

978-0-7695-4236-2/10 $26.00 © 2010 IEEE
DOI 10.1109/BWCCA.2010.152666

countermeasures against trafﬁc analysis to ensure anonymity
in a public network. They indicated how to alter trafﬁc patterns
in the main. However, a few researcher indicated how to
implement the method. Our work applies these proposed
methods to conceal trafﬁc patterns and enhance conﬁdentiality.
This paper shows how to implement a secure application basedlayer 3 VPN that conceals trafﬁc patterns, and does not study
how to alter trafﬁc patterns thoroughly.
There are some cases where implementations of cryptographic communication reveal vulnerabilities to an eavesdropper. One of the causes is ambiguity and the other is a particular
trafﬁc pattern generated by packet sizes and their inter-arrival
times. We will use the term ”behavior” to refer to trafﬁcpatterns that packet sizes and their inter-arrival times generate.

the information system, he may access conﬁdential information or personal information. Therefore, to ensure security of
information systems, it is necessary to conceal application
software installed in information systems. Moreover, to ensure security of distributed systems connected by VPN, it isnecessary to develop a secure VPN application that conceals
trafﬁc patterns.
IV. R ELATED WORKS
This section brieﬂy reviews previous works about a secure
VPN application that conceals trafﬁc patterns and brings up
problems to be solved.
A malicious eavesdropper can analyze encrypted trafﬁc andidentify the application software, because the implementationof application software generates a particular trafﬁc pattern.
To disturb identifying the application, it should conceal aparticular trafﬁc pattern. In particular, a pattern of packet sizes
should be concealed.
There are many countermeasures for the purpose ofanonymity. The concept of data anonimization throughcryptography and forwarding was originally introduced by
Chaum[24]. He proposed the use of a Mix, a computer proxy.
The Mix collects a number of packets called batch, reorders
packets and transmits ﬁxed size packets. However, the reality
is that a Mix cannot always get sufﬁcient packets efﬁciently
from users. Hence, it is suggested that users send dummy
messages of random and meaningless data. Many anonymous
systems use ﬁxed length messages by padding in order toconceal message size. Some anonymous systems use dummy
messages (cover trafﬁc) to conceal the correlation between
source and destination. A common tactic for mitigating suchthreats is to pad packets to ﬁxed sizes or to send packets
at ﬁxed intervals. Though these methods were proposed foranonymity, some methods can be applied to conﬁdentiality.
They indicated how to alter trafﬁc patterns in the main.
However, a few researcher indicated how to implement
the method. Csaba[25] adopted Trafﬁc Flow Conﬁdentiality
mechanisms, and implemented the functions (padding, fragmentation, dummy packet generation and artiﬁcial alterationof the packet delay) in IPsec[26]. The IPsec supports fragmentation by reusing the IPv6 fragmentation header. There issome possibility that a malicious eavesdropper reassembles
the fragmented packets. IPsec is a network based kernel
space VPN, and does not ensure reliable data transmission
by itself. Hense, a VPN application that ensures reliable datatransmission is also necessary. However, suchlike popularapplication based VPN does not even conceal a pattern of
packet sizes.
Accordingly, a tunnel that conceals trafﬁc patterns was
proposed[27]. Their tactic that padding packets to ﬁxed sizes
and sending packets at ﬁxed intervals is not novel. However,they brought up some problems to implement an application
based user space VPN that conceals trafﬁc patterns. Theirtunnel divides arriving packets with a ﬁxed size and sends
the fragmented packets at ﬁxed transmission intervals. The
fragmented packets are encapsulated by TCP or UDP in theIII. T HREATThis section reviews behavior analysis methods and makes
threats of trafﬁc analysis in untrusted networks clear.
Recently, several methods that analyze behavior such as
packet sizes and transmission intervals were developed. Thesemethods need only packet sizes and their inter-arrival times,
and thus, there is no need to scan payloads. Thus, thesemethods can analyze encrypted trafﬁc[5]. Moore and Zuev[6],[7] provided a robust trafﬁc classiﬁcation scheme based on
Bayesian analysis techniques, which requires access only topacket header data. Shizuno[8] and Kitamura[9] proposed
the application identiﬁcation methods based on analyzing
trafﬁc ﬂow behavior such as packet sizes and transmissionintervals. Still more, some researchers use transition patternsof packet sizes to classify network applications[10], [11],[12]. Bernaille[13], [14] proposed a technique that uses only
the size of the ﬁrst few packets to identify the application.
Recent techniques for the network trafﬁc analysis include
machine learning to classify trafﬁc. Early[15], McGregor[16]and Moore[17] used machine learning to classify trafﬁc by anapplication of the network trafﬁc analysis. Wright[18], [19]developed their own trafﬁc classiﬁcation system with a hidden
Markov model. Kohara[20], [21], [22] and Sena[23] attempted
to classify trafﬁc by using Support Vector. These methods
analyze trafﬁc patterns generated by the implementation of
application software or protocol. The implementation of application software or protocol generates packets in its own
sizes and their inter-arrival times. In particular, a pattern of
packet sizes is large, and many early application identiﬁcation
methods use only packet sizes.
These methods are novel and worthwhile. However, a malicious eavesdropper can also analyze encrypted trafﬁc by thesemethods to identify the application software. As previously
stated, if a malicious adversary identiﬁes the the application
software on an information system, he can seek vulnerabilities
easily. Especially, vulnerabilities of each application softwareare often reported. Such a few vulnerability of each application
software has a great potential for vulnerabilities of the whole
information system. Then, he can attempt an intrusion on theinformation system abusing the vulnerabilities. After intruding

667

(A) Source Behavior Shaversame popular application based VPN. However, in the case of
stream-oriented protocols such as TCP, it is difﬁcult to control
the packet sizes strictly. Datagram-oriented protocols such as
UDP can control the packet sizes without difﬁculty. UDP does
not ensure reliable data transmission in untrusted networks.
Thus, the tunnel cannot ensure enough security for distributed
systems connected by VPN. A secure application based VPN
requires a protocol that can control packet sizes strictly andcan ensure reliable data transmission in untrusted networks.

1.Divide a packet into a fixed size and encrypt the fragmented packets
A3

Packet A
TrustedNetwork

Sending Buffer

A2

Untrusted
Network

A1
A3

A2

A1

2.Send packets at any transmission intervals(B) Destination Behavior Shaver

3.Receive and decrypt the whole fragmented packets

V. B EHAVIOR S HAVER
In this section, we propose the behavior shaver that conceals
trafﬁc patterns using SCTP (Stream Control Transmission
Protocol)[28].

A3
Untrusted
Network

A2

A1

Receiving Buffer

A3
A1

A. Proposal

TrustedNetwork

A2
Packet A

4.Restore the former packet and forward it

The proposed tunnel[27] does not encrypt packets andencapsulates packets by TCP or UDP in the same popularapplication based VPN. For that reason, their tunnel cannot
control packet sizes strictly or cannot ensure reliable datatransmission in untrusted networks. To solve these problems,
we propose an application based layer 3 VPN that encapsulates
packets by SCTP. The SCTP is a transport layer protocol,
serving in a similar role as the popular protocols TCP or UDP.
It provides some of the same service features of TCP, ensuring
reliable, in-sequence transport of messages with congestion
control. Moreover, the SCTP may be characterized as recordoriented, meaning it transports data in terms of messages, in asimilar fashion to the UDP. Therefore, the SCTP can control
packet sizes strictly and can ensure reliable data transmission.

Fig. 1.

A design of the behavior shaver
TABLE IT RANSMISSION INTERVALS

Buffer Space Rate
Less than 50%
Less than 90%
else

Transmission Interval Range
00-1ms
1-10msI. We set the transmission interval by a random generator
within the range. Therefore, original transmission intervals arequantized smoothly.

B. Design

D. Implementation

Figure 1 shows the design of the behavior shaver. The source
behavior shaver divides an arriving packet with a ﬁxed size andadds to a sending buffer. If the arriving packet sizes is lessthan the ﬁxed size, a gap is padded with random numbers.
The fragmented packets are encrypted and encapsulated by
SCTP. It then sends the encapsulated packets at any transmission intervals. The destination behavior shaver receives
the encapsulated packets and adds to a receiving buffer. The
encapsulated packets are decapsulated and decrypted. After
reception of the whole fragmented packets, the destination
behavior shaver restores the former packet and forwards tothe destination.

We implemented the behavior shaver with C programming language on a Fedora 10[29] system. Table II shows
the development environment and speciﬁcations. We adopted
AES[30] algorithm to encrypt payloads, and the Mersenne
Twister[31] with a hush function, a random number generator.
Before starting, we can select parameters, packet sizes andtunneling protocols. In the case of SCTP, the number of
streams is always one. We adopted the lksctp-1.0.9[38], an
implementation of the Stream Control Transmission Protocolin the Linux kernel. The behavior shaver has 2 network
interfaces in a trusted network and an untrusted network. One
interface in a trusted network receives or sends plain packets.
Another interface in an untrusted network receives or sends
encrypted packets. The behavior shaver has 2 buffers, namely
a sending buffer and a receiving buffer. The sending buffer
stores encrypted packets to send into an untrusted network.
The receiving buffer stores decrypted packets to send into a
trusted network.

C. Quantization of Transmission Intervals
Converting packet sizes to a ﬁxed size, variation of trafﬁcpatterns diminish dramatically. Because an eavesdropper can
get only packets in the same size. However, transmissionintervals may generate a particular pattern. The proposed
tunnel[27] sends packets at ﬁxed intervals. This method is
inefﬁcient and cannot ensure enough throughput. Then, we
adopt variable transmission intervals, and quantize transmission intervals to 3 kinds of ranges. Table I shows the empirically derived ranges. Transmission interval ranges are inversely
proportional to the corresponding buffer space rate as in Table

VI. E XPERIMENTIn this section, we discuss the performance of the behavior
shaver. We have conducted experiments and the result shows
performance of the behavior shaver.

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TABLE III
R ESULTS OF A FUNCTIONAL TESTTABLE IID EVELOPMENT ENVIRONMENT AND SPECIFICATIONSOS
Programming Language
Encryption Algorithm
Random Number Generator
Packet Sizes
Transmission Intervals
Tunneling Layer
Tunneling Protocol

Protocol
ICMP
SMTP, POP, IMAPDNS
HTTP, HTTPSNTPLinux-2.6 (Fedora 10)
C (gcc-4.3.2)
AES
Mersenne Twister
128/256/512/1024 bytes
0-10 ms
L3 (Network Layer)
UDP/SCTP

Result
OK
OK
OK
OK
OK

3000

Untrusted Network

2500

Dummynet

GW2(FreeBSD 7.2)

(Fedora 10)

2000(Fedora 10)

RTT[relative]

GW1

no_delaydelay10ms
delay30ms
delay30ms+loss5%

1500

1000

500

0
0

host1

(Fedora 10)

SV(Fedora 10)

Trusted Network

Fig. 2.

host2

host3

200

Fig. 3.

(Fedora 10) (Windows XP)

400
600
Packet Sizes[byte]

800

1000

RTT of the behavior shaver

Trusted Network

3) Throughput: We estimate throughput between host2 andSV for every packet sizes. In order to measure throughput, we
use Iperf-2.0.4[34]. Iperf was developed by NLANR/DAST(The Distributed Applications Support Team) as a modern
alternative for measuring maximum TCP and UDP bandwidth
performance. The delays and packet losses in the untrusted
network are altered by dummynet.

An experimental network

A. Environment
We ﬁrst describe the experimental environment. Figure 2
shows the network topology. Any hosts are connected with
100BASE/T Ethernet. The OS is displayed in the ﬁgure. GW1
and GW2 are routers where the behavior shaver is installedin. A network between GW1 and GW2 is a untrusted networkthat is eavesdropped by a malicious adversary. The behavior
shavers construct a secure VPN tunnel between GW1 and
GW2. The untrusted network is simulated by dummynet[33].
Dummynet simulates queue and bandwidth limitations, delays,
packet losses and so on. The main platform for dummynet is
FreeBSD. Other networks are trusted networks that do notneed encryption. SV provides E-mail, DNS, World Wide Web
or NTP services. Other hosts are clients that use these services.

C. Result1) Function: Table III shows the results of a functional test.
All of protocols or applications in any OS worked normally.
We analyzed trafﬁc patterns and conﬁrmed that all packets
were uniform size.
2) RTT: Figure 3 shows the RTT for every packet sizes.
Let the performance of the previous tunnel[27] equal 100.
The previous tunnel divides packets and encapsulates thefragmented packets by UDP. The parameter of packet sizes
is set 1024 bytes with no delay. The results are based on theperformance. The x-coordinate corresponds to the packet size,
and the y-coordinate corresponds to the relative value of RTT.
In the case of no loss, the RTTs are almost regular values
correlating without packet sizes. There is not much to choose
between the RTT at no delay and the base value. When packets
are lost, the RTTs are regularly long regardless of packet sizes.
This is because packet loss caused retransmission. Therefore,
the implementation using SCTP does not cause excessive
overhead.
3) Throughput: Figure 4 shows the throughput for every
packet sizes. Let the performance of the previous tunnel[27]equal 100. The results based on the performance. The xcoordinate corresponds to the packet size, and the y-coordinate

B. Experiment
1) Function: We conﬁrm that any hosts can use E-mail,
DNS, World Wide Web or NTP services provided by SV
through the secure VPN tunnel. Then, we also analyze trafﬁcpatterns and conﬁrm that all packets are uniform size.
2) RTT: We estimate RTT (Round Trip Time) between
host2 and SV for every packet sizes. In order to measure RTT,
host2 sends ICMP echo requests 100 times and calculate themean. The delays and packet losses in the untrusted network
are altered by dummynet.

669

analysis for a long term. Particularly, to defeat statisticalanalysis for a long term entirely, low-latency VPN applications
probably has to pay compensation. Because most trafﬁc overa general VPN tunnel is interactive and does not permit muchdelay.

no_delaydelay10ms
delay30ms
delay30ms+loss5%

140

120

Throughput[relative]

100

80

VII. C ONCLUSION
60

In this paper, we made the threat that behavior analysis
revealed vulnerabilities of information systems clear, and indicated that previous VPN applications could not ensure securityof information systems. We focused on implementation of
application based layer 3 VPN and proposed the behavior
shaver that concealed trafﬁc patterns using SCTP. Our works
proposed not how to alter trafﬁc patterns but how to implement
an application based user space VPN that controls packet sizes
strictly. Moreover, we implemented the behavior shaver andthe results of experiments showed the performance.
The conﬁdentiality of the cryptographic communication
mostly depends on crypto, and the behavior shaver enhanced
the conﬁdentiality. If an implementation of the cryptographic
communication has fatal vulnerabilities, the behavior shaver
may conceal the vulnerabilities and ensure conﬁdentiality.
The behavior shaver prevents behavior analysis attacks anddisables identifying the application or the protocol. Thus, it
is difﬁcult for a malicious adversary to seek the vulnerabilities of information systems. The behavior shaver forces amalicious adversary to cost a long time. In the meantime,
the administrator of the information system can detect thesign or take countermeasures. Security Requirements for NGN(Next Generation Network)[36] provide three kinds of security
zones, namely Trusted, Trusted but Vulnerable and Untrusted.
Our work prevents information leaks from trusted zones to
untrusted zones or trusted but vulnerable zones, and ensures
security of trusted zones.
However, the evaluation of security is not enough. We stillcannot declare that the behavior shaver entirely defeat trafﬁc
analysis. To follow up this matter further would involve us ina discussion of implementation and would take us beyond thescope of this work. However, behavior analysis methods that
identify the application or the protocol without using packet
sizes are not practical. We also need to conduct experiments inother various environments. Our implementation is based on atactic that padding packets to ﬁxed sizes and sending packets at
random intervals. However, this approach is less efﬁcient than
Wright’s method[37]. How to implement the new methods isa future work. How to realize efﬁciently is considered as a
future research which requires considerable effort.

40

20

0
0

200

Fig. 4.

400
600
Packet Sizes[byte]

800

1000

Throughput of the behavior shavercorresponds to the relative value of throughput. In the caseof no loss, the throughputs are almost regular values correlating without packet sizes over 256 bytes. There is a rapid
drop at 128 bytes. The cause can be our implementation
decreases the goodput. When packets are lost, the throughput
are regularly low regardless of packet sizes. This is because
packet loss caused retransmission. Thus, the cause might not
be our implementation decreases the goodput. Therefore, theimplementation using SCTP does not cause excessive overhead
under the condition that the parameter of packet sizes is set
over 256 bytes.
D. Discussion
Previous VPN applications ensure conﬁdentiality by encryption. However, these VPN applications forward packets
without altering behavior. Hence, these VPN applications may
reveal the application software that generates a particular trafﬁc pattern on the VPN tunnel. Thus, these VPN applications
are vulnerable to trafﬁc analysis. Moreover, some implementations of cryptographic communication are vulnerable to timing
attacks[35].
The behavior shaver transmits only packets in the same size.
Thus, the behavior shaver entirely defeats simple behavior
analysis methods[13], [14] using only packet sizes. Because
their method uses only the size of the ﬁrst few packets to
identify the application. The methods using transition patternsof packet sizes[10], [11], [12] are also defeated entirely.
Moreover, the transmission intervals were altered by a random
number generator in order to defeat timing attacks. Thus,
the behavior shaver is more conﬁdential than previous VPN
applications.
IPsec with Trafﬁc Flow Conﬁdentiality mechanisms[25] isalso more conﬁdential than previous VPN applications. The
IPsec is a network based kernel space VPN. The behavior
shaver is an application based user space VPN, and plays
another role. If you want to ensure reliable data transmissionof unreliable protocols or applications, the behavior shaver will
enable that.
However, there is some doubt to defeat more sophisticatedbehavior analysis methods using machine learning or statistical

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