The IEEE 802.11 defines an optional Wired Equivalent Privacy (WEP) mechanism to implement the confidentiality and integrity of the traffic in the network. WEP is used at the station-to-station level and does not offer any end-to-end security. WEP uses the RC4 PRNG [] algorithm based on a 40 bit secret key and a 24 bit initialization vector (IV) send with the data. WEP includes an integrity check vector (ICV) to allow integrity check. One MPDU frame contains the clear text IV and ICV and the cipher text data block, so receiver is always able to decrypt the cipher text block and to check the integrity. The IV can always be new or reused for a limited time. The scheme is illustrated in .
Figure 2: WEP mechanism []
The PRNG algorithm used in IEEE 802.11 is RC4 [] from RSA inc. The actual algorithm is not public, but has been studyed in independent research laboratories under nondisclossure agreements and no weaknesses has not yet been reported, which does not guarantee that these does not exist. Anyway the secret key used is only 40 bits long, which can be solved by brute-force attack in 2 seconds with $100 000 hardware and 0.2 seconds with $1 000 000 hardware according the 1995 figures []; today the hardware prices are significantly lower. And even with some additional strength gained with variable IV the protection level of WEP may not be considered strength enough for the most sensitive applications. The Shared Key Authentication scheme could be easily fooled using for example the play-back attack. So anyway an additional authentication mechanism is needed.
4 Threats and Vulnerabilities Compared to Wired LANs
In this section we will concentrate on the wireless LANs using the radio path as a transmission medium.
In the wireless LAN environment we have to deal with all the same security problems, which we have in the conventional wired LAN environment. But then we have some security issues, which are stressed when we are using the radio path. The currently know active attacks can be divided in the following categories []:
- Social engineering
- Impersonation
- Exploits
- Data driven
- Transitive trust
- Infrastructure
- Denial of Service
The four first of these are similar in wired and wireless environment, so these are not discussed in this paper. Despite of the active attacks there exists the passive eavesdropping which is discussed at first.
4.1 Eavesdropping
Eavesdropping is very easy in the radio environment, when one sends a message over the radio path, everyone equipped with a suitable transceiver in the range of the transmission can eavesdrop the message. This kind of transceiver equipment, for example standard wireless LAN mobile, maybe with special antenna, are very reasonable priced. The sender or intended receiver has no means to know if the transmission has been eavesdrop or not, so this kind of eavesdropping is absolutely undetectable.
The frequency band and transceiver power used has a great effect on the range where the transmission can be heard. When we are using 2 or 5 MHz radio band and transceiver power up to 1 W, as in the case of the current wireless LAN standards, the traffic of wireless LAN can be eavesdropped from outside the building which the network is operating if there is no special electromagnetic shielding. So we can not truly trust that our network stays inside our office building.
In the wireless LAN environment the ease of eavesdropping justifies quite costly procedures to guarantee the confidentiality of the network traffic. In all wireless LAN standards this is taken care by some kind of link level ciphering done by MAC-entities, but the safety gained with these algorithms may not be good enough for the most demanding applications.
4.2 Transitive Trust
When we have a wireless LAN as a part of our enterprise network, it offers one interface to the attacker, requiring no physical arrangements, to intrude on our network. In wired networks we can always track the wire from our computer to the next network node, but when we are working in the wireless environment there is no such way to find out with whom we are talking to. That makes the efficient authentication mechanisms crucial for the security of the wireless LANs. In all cases the both parties of the transmission should be able to authenticate each others.
The wireless LAN could be used as a launch pad to the transitive trust attack. If the attacker can fool our wireless LAN to trust the mobile he controls, then there is one hostile network node inside all firewalls of our enterprise network and it is very difficult to prevent any hostile actions after that. This kind of attack can be done from outside of our site with standard wireless LAN hardware compatible with our equipment. The only real protection against this kind of attacks is the strong authentication mechanism of the mobiles accessing the wireless LAN. The discovery of the unsuccessful attacks must rely on the logging of unsuccessful logging attempts, but it might be very hard to find out if there has been a real attack attempt, because in the normal operation there comes unsuccessful logon attempts due the high BER in radio path and from mobiles that belongs to some other wireless LAN.
The other kind of transitive trust attack, special for wireless networks, is fooling the mobile to trust the base controlled by attacker as our base. When mobile is switched on it usually tries first to logon the network with strongest signal and if that fails then the rest ones in the order of the signal power. Now, if attacker has a base with high transmission power, he may be able to fool our mobiles to try first to logon the attackers network. Now there is basically two possibilities: the attacker may let as to logon his network and make it pretend our network and find out the passwords secret keys, etc. or the attacker may just reject our logon attempts but record all the messages during the logon process and find out the secret keys or passwords used in authentication in our network by analyzing these messages. The former case is very difficult to implement without very detailed information about our network services and is probably detected very soon, but the later one requires just standard base hardware, maybe with a special antenna, compatible with our equipment, and is very difficult to detect, because the mobiles do not usually report unsuccessful logon tries to the upper layers and the are a lot of unsuccessful logon attempts even in the normal circumstances. The only protection against these attacks is an efficient authentication mechanism which allows the mobile authenticate the base without any disclosure of the secret keys or passwords it uses to logon our network.
4.3 Infrastructure
The Infrastructure attacks are based on some weakness in the system: the software bug, configuration mistake, hardware failure, etc. This kind of situations will certainly occur in wireless LANs, too. But protection against this kind of attacks are almost impossible - You do not know about the bug until something happens. So the only thing to do is to keep the possible damages as small as possible.
4.4 Denial of Service
Due the nature of the radio transmission the wireless LANs are very vulnerable against denial of service attacks. If attacker has powerful enough transceiver, he can easily generate such a radio interference that our wireless LAN is unable to communicate using radio path. This kind of attack can be done from outside of our site, for example from a van parked on the street or from an apartment in the next block. Equipment needed to commit this kind of attack can be bought from any electronic store with reasonable price and any short-wave radio enthusiast has the knowledge needed to construct the equipment.
The protection against this kind of attacks is very difficult and expensive. The only total solution is to have our wireless network inside of the faraday cage, but this is applicable only in the very rare cases. But it is easy for authorities to locate the transceiver used to generate interference, so the attacker has limited time before the transceiver is found.
In the other hand the wireless LANs are not so vulnerable than the wired LANs to the other kind of denial of service attacks. For example the fixed LAN node can be isolated from the network by simple cutting the wire, which is not possible in wireless environment. If attacker cuts down the power of the whole site, then all wired networks are usually useless, but the wireless LANs can be used in the ad-hoc configuration with laptops or other battery powered computers.
5 Secure Solution
One can easily see that the standards described in chapter 3 does not fulfill the security requirements against the attacks described in chapter 4. This section will present some mechanisms and protocols that makes the wireless LANs safer.
5.1 Design Goals
The major requirement for this kind of solution is the seamless integration into existing wired networks. It is very probable that we have plenty of fixed network nodes already installed in our enterprise network, so we should avoid any modifications needs to the existing nodes.
There are different alternatives for securing a connection: end-to-end security at the application level, end-to-end security at the transport layer and link security at the link layer. In current data networks are only few commonly used end-to-end security schemes (like SSL and SSH), so the link security is the only applicable approach, if we want to leave our existing network alone.
Dropping end-to-end mechanisms out rules the user authentication out. We have only station-to-station (or machine-to-machine) authentication left, since those are the entities primry communicating over the wireless link. Machine-to-machine authentication is in fact conceptually correct for a security protocol at the link layer [].
Another design goal is the two-way authentication, for the reasons discussed in 4.2 it is vital that both the base and the mobile are able to authenticate each others. Authentication mechanism should enable the identification of the mobiles and allow distinct keys used in different bases and mobiles.
The final goal is to have some flexibility to utilize the future advances in the cryptography. The should also be some interoperability between all versions of the wireless products, even if there exist different regulatory limitations for the use of the cryptography.
5.2 Design Overview
The solution discussed here needs several modifications for current wireless LAN products and standards, so the implementation of this solution is not currently feasible. But the aim is more to show the direction to which the evolution should go.
This is a hybrid solution: the authentication is done using public key cryptography and the ciphering of the transmission uses shared key cryptography. Shared keys are created during the authentication and may be changed during the transmission. The actual cryptography algorithms are not defined, because of the rapid development in this area.
5.3 Authorization []
defines nomenlactures used in this chapter.
The authorization mechanism uses certificates formatted according to CCITT X.509 [] used in X.500 and PEM. A certificate contains the following information: {Serial Number, Validity Period, Machine Name, Machine Public Key, CA name}. Each certificate is signed by CA which might in our case be the enterprise's own CA.
The first message send from the mobile to the base contains following information: {Cert_Mobile, CH1, List of SKCSs}. CH1 is randomly generated number. The List of SKCSs is transmitted to allow negotiation of the used algorithm, the algorithm identifier and the key size are sent in the list.
When the base has received the first message, it will attempt to verify the signature on Cert_Mobile. A valid signature proofs the public key in the certificate belongs to a certified mobile host but it is not sure if the certificate actually belongs to the mobile that submitted it. If the certificate is invalid, the base rejects the connection attempt.
Now the base will reply to the mobile by sending the message containing {Cert_Base, E(Pub_Mobile, RN1), Chosen SKCS, Sig(Priv_Base, {E(Pub_Mobile, RN1), Chosen SKCS, CH1, List of SKCSs}}}. Random Number RN1 is saved internally for later use. Chosen SKCS is one from the list sent by mobile and includes the algorithm identifier and the key size, the Chosen SKCS is the most secure from those supported by both the base and the mobile.
The mobile validates Cert_Base, if certificate is valid, the Mobile will verify using the public key of the Base the signature off the message. The signature is valid and the base authenticated if the CH1 and the List of SKCSs matches with those sent by mobile to the base. Since the list of SKCSs is included in the signature, the attacker can not send the weakened list of SKCSs by jamming original message and sending his own, and we need not to sign the first message.
Now the mobile sends to the base message containing: {E(Pub_Base, RN2), Sig{Priv_Mobile, {E(Pub_Base, RN2), E(Pub_Mobile, RN1)}}}. The RN2 is a random number generated by the mobile. The mobile will use the RN1 XOR RN2 as a session key for now on.
The Base verifies the signature of the message using Pub_Mobile obtained from Cert_Mobile in the first message. If the signature is valid, the mobile is authenticated. Next the base will decrypt E(Pub_Base, RN2) with it's own private key. Now the base can form the session key RN1 XOR RN2.
The session key is formed from two parts sent in different messages to gain better protection. Now the compromising of the mobile's private key does not compromise the whole traffic between the base and the mobile. Since the both halves of the session key are random and equal length, knowing either RN1 or RN2 tells nothing about the session key.
If all these steps has succeeded the mutual authentication has been done and the session is established. summarizes the authentication protocol. The correctness of this protocol is proofed in [].
This authentication should be done in the MAC layer, before any network access is granted to the mobile. If we give to the mobile IP address before the authentication, it may be used as a launch pad even if it's authentication request is rejected.
Figure 3: Authentication Protocol []
5.4 Integrity and Confidentiality
The confidentiality can be archived by using some existing symmetric cryptography algorithm, like IDEA or DES. Once the session key is agreed, using mechanism described in 5.3, available algorithms are strong enough for our purposes. Anyhow the high BER on the radio link may set some limitations for the selected algorithm.
The integrity is archieved by a fingerprint generated by some one-way hash function, like MD5 or SHA. There should be a fingerprint in each MPDU message, because of the high pakect loss rate in the wireless environment.
There should be some link level ciphering in any case. If we are using some ciphering in our fixed network (e.g. IPSEC), then we can select weaker ciphering for the wireless LANs in the link level. But there should in anyway be some ciphering: To defend against traffic analysis we have to cipher also the network layer headers.
5.5 Key Change Protocol []
The nomelactures defined in the are used here. The key exchange may be initialized from both ends of the communication, the base initialized case is handled first.
First the base sends to the mobile a message: Signed(Priv_Base, { E(Pub_Mobile, New_RN1), E(Pub_Mobile, RN1) }) and the mobile responses with message: Signed(Priv_Mobile, { E(Pub_Base, New_RN2), E(Pub_Base, RN2) }).
If the mobile initializes the key exchange procedure, then it send to the base message: Signed(Priv_Mobile, { E(Pub_Base, New_RN2), E(Pub_Base, RN2) }) and the base responses with: Signed(Priv_Base, { E(Pub_Mobile, New_RN1), E(Pub_Mobile, RN1) }).
Again the value new_RN1 XOR new_RN2 is used as the new session key. The values RN1 and RN2 are always the last ones used. In both cases the RN1 always refers to the random number generated by the base and RN2 the random number generated by the mobile. The values of RN1 and RN2 are verified against the internally saved values and if those does not match, the key exchange is ignored. Now the key exchanges can not be played back and we do not need to save any sequence numbers.
5.6 Key Management
The key management is one of the stuffest part implement convenient way. One possible procedure using the smart card technology is described below:
- CA creates the private and public keys inside the smard card by the way that the private key is never readable from the smart card.
- CA signs the public key with his private key and stored the signed public key to the smart card.
- The smart card is given to the end user, which may now use the smart card in any wireless LAN mobile.
In order to avoid reading the private key from the smart card the public key cryptography system must be run inside the smart card and the calculation power of the smart cards sets some limitations for the efficiency of this approach. Of course the smart card reader is needed for each mobile used in the wireless LAN. But it is not very wild guess that the smart card technology will become more efficient and cheaper in the near future.
The concept described here is not the only one: it is also possible to use the Wep of Trust scheme for the key management (like in PGP) or the user may generate the key par by himself and then give the public key to the CA for the certificate signing, but the user identification must be somehow done also in this case.
5.7 Solution Analysis
The solution described above fulfills are goals stated in 5.1: The authentication mechanism implements the mutual authentication. The negotiation of the symmetric cryptography algorithm gives some flexibility between different versions and allows future enhancements. The concept does not need any modifications to the existing networks.
This solution is designed for maximum security, which may limit the performance of the network. One may consider using faster ciphering for example the insensitive video clips, but a much better (and therefore slower) ciphering for sensitive traffic. There is no end-to-end security offered, that must be taken care in upper layers.
Key management using the smart cards has been found quite functional even in mass products, like GSM. The major challenge is the limited computing power in the smart card, which leads to the longer authentication time. The time used for authentication may become critical if mobile moves from one base station to another and the hand over procedure must be performed. The authentication procedure during hand over could be speed up by using different authentication scheme described in [], but this kind of optimization is out of the scope of this paper. The longer computing time leads also to the greater power consuption, which is always one critical aspect in the mobile environment.
This concept does not support multiple CAs and in large networks that may become a problem, anyhow the multiple CA support could be archived with just minor modifications described in []. Another problem for this kind of concept is multicast support, this solution has no support for ciphered multicast.
6 Conclusions
The current wireless LAN standards offer very unsatisfactory level of security and one could not truly trust them. When using products based on these standards must the security issues been taken care in the upper layers. The authentication mechanism described in 5.3 may be used over IP to perform end-to-end authentication, as described in [], but this approach gives a potential launch pad for the attacker.
Some commonly used attacks are more stressed in wireless environment and some additional effort should be used to prevent those. The nature of the radio communication makes it practically impossible to prevent some attacks, like denial of service using radio interference. When the wireless networks are used in strategic applications, like manufacturing or hospitals, the possibility of this kind of attack should be taken into account with a great care.
As showed in chapter 5 the quite secure wireless LAN is possible to implement with current technology. The current hardware could be used with only some modifications in the MAC layer protocols and over that new MAC the current IP may be used without any problems. Anyway it is not probable that products supporting this level of security comes to the markets soon, mostly due the USA regulations; almost all manufactures are American.
7 References
[1]
K. Pahlavan, A. Zahedi, P. Krishnamurthy. Evolving Wireless LAN Industry - Products and Standards. Invited paper PIMRC'97, Worcester Polytechnic Institute, 1997
[2]
A. Zahedi, P. Krishnamurthy, S. Bagchi, K. Pahlavan. An Update on the Evolution of the Wireless LAN Services. Worcester Polytechnic Institute, 1997
[3]
ETS 300 652. High PErformance Radio Local Area Network (HIPERLAN) Type 1; Functional specification. ETSI, 1996
[4]
ETS 300 652. High PErformance Radio Local Area Network (HIPERLAN) Type 1; Functional specification - URGENT TECHNICAL CORRECTION. ETSI, 1997
[5]
TR 101 054. Rules for the management of the HIPERLAN Standard Encryption Algorithm (HSEA). ETSI, 1997
[6]
draft standard IEEE 802.11. Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. IEEE, 1996
[7]
W. Dipstraten, P. Belanger. 802.11 Mac Entity: MAC Basic Access Mechanism, Privacy and Access Control. <> 802.11 Tutorial, IEEE, 1996
[8]
Anon. RC4 FAQ. <> , RSA FAQ, RSA Inc. 1996
[9]
M. J. Ranum. Internet Attacks <> 1996
[10]
A. Aziz, W. Diffie. Privacy and Authentication for Wireless Local Area Networks. <> Sun Microsystems Inc., 1993
[11]
CCITT X.509 The Directory - Authentication Framework. CCITT, 1988
[12]
Vaduvur Bharghavan. Secure Wireless LANs. <> ACM Conference on Computers and Communications Security '94, University Of California at Berkley, 1994
[13]
T. Raivisto. Applying Cryptography to GSM Short Message Services. Master Thesis, Helsinki University of Technology, Espoo 1997
Last modified: Fri Dec 26 17:42:28 EET 1997
Chapter 1 – Introduction
1
Chapter 1
Introduction
Over the past several years, wireless technology has revolutionised the way people
communicate. In particular, the Wireless Local Area Network (WLAN) technology has become
so popular that it has already been accepted as a convenient alternative to conventional wired
LANs. Some of the key advantages of the WLAN technology can be identified as ubiquitous
network access without wires, relatively high data rates, rapidly improving quality of service,
and competitive pricing in contrast to its wired counterparts. These became major contributing
factors for WLANs in gaining their market momentum.
With this rapidly growing adoption of the wireless networking technology, for many
implementers, security and Quality of Service (QoS) remain as issues of the highest priority.
The main reason for growing concerns in security is the susceptibility of the wireless media to
a number of possible security threats. On the other hand, QoS is increasingly becoming
important for the next generation of wireless multimedia applications.
The aim of this chapter is to establish the objective and scope of this research. Therefore,
the first half begins by introducing the importance of security in wireless networks. This is
followed by a general overview of security solutions for wireless LANs. Next, a brief
introduction to wireless Virtual Private Networks (VPNs) is provided, with the importance of
Quality of Service (QoS) in a wireless environment given due importance. The chapter
concludes with a clear indication of how the entire research has been carried out and how it has
been presented in this thesis.
1.1 Wireless LAN Security: an Overview
While wireless networks have many advantages over conventional wired networks, they
expose the user to great risks, because when the physical media is replaced by Radio
Frequency (RF) communication channels, the communication links do not have the basic
physical security anymore. These high frequency RF waves cannot be confined to any
controlled physical space. Therefore, the wireless technology is inherently open to interception
Chapter 1 – Introduction
2
by a potential attacker who is even beyond the confines of a physically controlled area. Hence,
a robust wireless security solution is a must for every WLAN deployment.
1.1.1 General Security Threats and Attacks on WLANs
Having established the need for wireless security and its importance, in this section, the
possible types of threats and attacks on wireless LANs are discussed. General security threats
and attacks can be categorised into two major types. The first category consists of active
attacks. In the case of an active attack, the potential intruder gains access to the wireless
network and may destroy or alter the data. In contrast, in the second category, which is known
as passive attacks, the potential intruder gains access to the wireless network but only
eavesdrops on the transmitted data.
1.1.1.1 Active Attacks:
a. Invasion and Resource Stealing or Spoofing: This is one of the basic types of active
attacks launched to gain unauthorised access to network resources and services. The
intruder will first try to determine the access parameters of a potentially vulnerable wireless
network. Such access parameters may include a MAC address and an IP address of a
particular client. When the client is not transmitting, the intruder will first reconfigure
his/her terminal with the known information. Once this is done, the intruder’s terminal will
appear as the authorised terminal and will be able to access most of the resources. This
technique is known as MAC spoofing [1].
b. Denial of Service (DoS): A denial of service attack can take many forms. The most
fundamental type of attack could be flooding of the network bandwidth with meaningless
data. This will eventually bring the network to a halt. To initiate such an attack, an intruder
identifies a potential wireless device on the network and will continue to bombard it with
excessive amounts of data. This process will soon overwhelm the wireless device, causing
it to become unusable. There may be more complex and sophisticated denial of service
attacks, such as spoofing disassociation management frames to the wireless terminals or
causing excessive RF interference to jam the communication channel [1].
Chapter 1 – Introduction
3
c. Replay Attacks: Initially, the intruder may use a third-party wireless packet
sniffing/monitoring utility to capture packets exchanged between two wireless entities.
Once such packets are captured, the intruder can analyse the packet content and launch a
number of attacks. One such example could be to initiate a denial of service attack. On the
other hand, the intruder could collect sufficient data to crack an encryption key.
1.1.1.2 Passive Attacks:
a. War-driving: This is the most common form of passive attack. As we know, an RF signal
may sometimes extend beyond the physical confines of a building. In such cases, any
potential intruder with a portable computing device may be able to easily detect such RF
signals. With appropriate war-driving tools, in no time the intruder will be able to penetrate
into the network.
b. Man-in-the-Middle Attacks: This is an attack that requires some sophisticated hacking
software and may cause significant levels of disruption and chaos. The potential intruder
captures packets in transmission between two communicating entities. The two
communicating entities see the intruder as its corresponding authenticated peer entity. The
intruder is in a position to capture the legitimate information or even initiate a DoS attack.
Looking at these security issues relating to wireless networks, two major root causes can be
identified. The first is the lack of understanding and proper management skills of wireless
networks. Most network managers apply the conventional network management techniques and
principles to WLANs. Such approaches may sometimes make the WLAN potentially more
vulnerable to an attack. The second cause is the inherent security vulnerabilities in the IEEE
802.11-1997 standard itself [2]. The next section briefly introduces the security solutions for
wireless LANs and introduces the importance of QoS for a wireless VPN.
Chapter 1 – Introduction
4
1.2 Security Solutions for Wireless LANs
The fundamental security services addressed in the IEEE 802.11 standard are authentication
and the Wired Equivalent Privacy (WEP) mechanism [2]. The authentication service is used in
the process of association of stations and the Access Points (APs) in an infrastructure Basic
Service Set (BSS) or for the association of two stations in an Independent Basic Service Set
(IBSS). In these situations, a so called Service Set Identifier (SSID) is used to distinguish each
BSS (or IBSS) from others. It is a common practice to use the SSID as a shared password,
providing wireless stations and Wireless LANs (WLANs) with some form of access control
mechanism. Furthermore, IEEE 802.11 standard specifies WEP as an optional mechanism for
protecting authorised users of a wireless LAN from casual eavesdropping. The security
services provided by the WEP are confidentiality, authentication and access control, in
conjunction with layer management [2]. The shortcomings and deficiencies of these security
services have been well identified and documented by several researchers [3], [4].
As a quick response to address the inadequacies of WEP, an umbrella standard referred
to as the IEEE 802.1x standard was introduced. The IEEE 802.1x standard adopts an enhanced
“port based network access control” framework for authentication and key exchange in
wireless LANs [5]. The IEEE 802.11 Task Group i proposed yet another new-generation
security standard. This solution initially proposed the Temporal Key Integrity Protocol (TKIP).
At the time, it appeared as the best short-term solution within the deployed hardware base. A
further improvement to TKIP is the incorporation of the Advance Encryption Standard (AES).
AES is a crypto brick, which is capable of providing an enhanced form of encryption. It is also
a vital part of the IEEE 802.11i specifications, which recently received its final approval
process [6]. However, its implementation may require the use of a much more powerful
processor than what is currently available in most of the existing products.
An alternative and generally well-received solution is based on implementing a VPN
tunnel over the wireless infrastructure. A VPN is a solution that aims to provide secure private
communications over a public network infrastructure such as the Internet. VPNs can be
deployed over wireless networks to secure communication between wireless clients and their
conventional enterprise networks through secure virtual tunnels.
Chapter 1 – Introduction
5
1.3 The Quality of Service of Wireless VPNs
Amongst many open research questions on wireless VPNs the issue of Quality of Service
(QoS) remains, and how it is supported in these networks. Since the existing IEEE 802.11
standard does not address the above issue, wireless networks are susceptible to errors owing to
the nature of their link layer environment. Furthermore, the data transmission capability of a
wireless network suffers from the overhead traffic related to IEEE 802.11 control and
management frames. This results in the reduction of the net throughput of a WLAN by 50 – 60
per cent of the nominal.
Implementation of a VPN over a wireless link to secure the transmission may further
diminish its performance and QoS levels. Therefore, it is highly important for wireless network
operators and application developers to understand the behavior trends of such QoS
parameters.
On the other hand, as wireless networks become widely accepted for voice, video and
data communications, wireless applications must be carefully designed to ensure reduced
latency, accurate delivery and prioritisation. Therefore, the QoS expectations of each of these
services and applications must be clearly defined and established. In many instances, such QoS
expectations may exceed the inherent limitations of the wireless environment.
Unfortunately, not many researchers have contributed towards defining the acceptable
performance and QoS levels of a wireless VPN setup. If such measures are established,
wireless network operators and application developers will be able to customise applications in
such a way that high availability and service reliability is achieved by a VPN.
1.4 Objectives
The objective of this thesis is to evaluate the performance and QoS levels of one of the most
popularly used wireless network security solutions. More specifically, this thesis presents the
analysis and experimental results for evaluation of the performance of a VPN implementation
over an IEEE 802.11b wireless infrastructure. The VPN tunnelling protocol considered for the
above study is IP Security (IPSec). The main focus of the research is to identify the major
performance limitations and their underlying causes for such VPN implementations under study.
Chapter 1 – Introduction
6
1.5 Approach
The experimentation and data collection involved in the study spans over a number of platforms
to suit a range of practical VPN implementations over a wireless medium. The collected data
includes vital QoS and performance measures such as the application throughput, packet loss,
jitter, and round-trip delay. It further investigates the contribution of the CPU, inter-packet
generation rate, payload data size, geographical distance and the number of simultaneously
operating VPNs. Once the baseline measure is established, a series of experiments are conducted
to analyse the behaviour of a single IPSec VPN operating over an IEEE 802.11b infrastructure,
after which the experimentation is extended by investigating the trends of the performance
metrics of a simultaneously operating multiple VPN setup. Finally, the work is extended to a
geographically spanned multi-campus site-to-site VPN. The two sites are connected via an IPSec
VPN tunnel, which is implemented over a public network infrastructure. Furthermore, the two
VPN tunnel end-points are connected to a wireless mesh network and an IEEE 802.11b wireless
ad hoc network. The performance measures for each of the above scenarios are comparatively
analysed for defining acceptable performance and QoS levels for a wireless VPN.
1.6 Contribution
The results of this study can be used in defining acceptable performance and QoS levels for a
wireless VPN configuration, under general enough conditions. By and large, this provides a
guideline for predicting the behaviour of performance parameters, under such circumstances.
Furthermore, these results can be used for estimating maximum possible throughput, percentage
of packet loss, round-trip delay, and jitter for a specific data flow over a wireless VPN.
Therefore, network operators and application developers can use these results to fine tune
application parameters for achieving optimal QoS levels and reliability over such wireless VPNs.
1.7 Outline of the Thesis
The next chapter provides an in-depth discussion on the current and future trends in research in
wireless network security. This discussion begins by introducing the IEEE 802.11 security
standard to the reader, after which the security flaws of the existing standard and alternative
Chapter 1 – Introduction
7
solutions are subsequently introduced. At this point the reader’s attention is drawn towards the
benefits of using a VPN over a wireless infrastructure for securing communications. Chapter 2
concludes by establishing the need for QoS of a wireless VPN and presenting some of the
current research trends in this area.
Once the purpose of the research is established, the stage is set for discussing the details
of the experimentation. Chapter 3 contains a detailed description of the experimental platforms,
setups and methodologies used for data collection. The objective behind the collection of such
data is to obtain a comprehensive set of measurements for analysis and evaluation of the QoS
measures of a wireless VPN.
Chapter 4 uses the previously collected performance results to perform a detailed
analysis. The behavioural trends of each of the performance metrics are analysed individually.
Chapter 5 uses these analysis results to identify the specific performance-limiting causes for
each QoS metric considered under the study. Finally, this thesis concludes by drawing
important conclusions based on the overall results of the research.