A Secure and Service-Oriented Network Control Framework for WiMAX Networks

Transcription

1 WIRELESS BROADBAND ACCESS: WIMAX AND BEYOND A Secure and Service-Oriented Network Control Framework for WiMAX Networks Kejie Lu and Yi Qian, University of Puerto Rico Hsiao-Hwa Chen, National Sun Yat-Sen University ABSTRACT WiMAX, Worldwide Interoperability for Microwave Access, is an emerging wireless communication system that can provide broadband access with large-scale coverage. As a cost-effective solution, multihop communication is becoming more and more important to WiMAX systems. To successfully deploy multihop WiMAX networks, security is one of the major challenges that must be addressed. Another crucial issue is how to support different services and applications in WiMAX networks. Since WiMAX is a relatively new standard, very little work has been presented in the literature. In this article we propose a secure and service-oriented network control framework for WiMAX networks. In the design of this framework we consider both the security requirements of the communications and the requirements of potential WiMAX applications that have not been fully addressed previously in the network layer design. The proposed framework consists of two basic components: a service-aware control framework and a unified routing scheme. Besides the design of the framework, we further study a number of key enabling technologies that are important to a practical WiMAX network. Our study can provide a guideline for the design of a more secure and practical WiMAX network. INTRODUCTION WiMAX (Worldwide Interoperability for Microwave Access) is an emerging wireless communication system that is expected to provide high data rate communications in metropolitan area networks (MANs) [1]. In the past few years, the IEEE working group has developed a number of standards for WiMAX. The first standard was published in 2001, which aims to support the communications in the GHz frequency band. In 2003 IEEE a was introduced to provide additional physical layer specifications for the 2 11 GHz frequency band. These two standards were further revised in 2004 (IEEE ). Recently, IEEE e has also been approved as the official standard for mobile applications. In the physical (PHY) layer, IEEE supports four PHY specifications for the licensed bands. These four specifications are Wireless- MAN-SC (single carrier), -SCa, -OFDM (orthogonal frequency-division multiplexing), and -OFDMA (orthogonal frequency-division multiple access). In addition, the standard also supports different PHY specifications (-SCa, -OFDM, and -OFDMA) for the unlicensed bands: wireless high-speed unlicensed MAN (WirelessHUMAN). Most PHYs are designed for non-line-of-sight (NLOS) operation in frequency bands below 11 GHz, except -SC, which is for operation in the GHz frequency band. To support multiple subscribers, IEEE supports both time-division duplex (TDD) and frequency-division duplex (FDD) operations. In the medium access control (MAC) layer, IEEE supports two modes: point-to-multipoint (PMP) and mesh. The former organizes nodes into a cellular-like structure consisting of a base station (BS) and subscriber stations (SSs). The channels are divided into uplink (from SS to BS) and downlink (from BS to SS), and both uplink and downlink channels are shared among the SSs. PMP mode requires all SSs to be within the transmission range and clear line of sight (LOS) of the BS. On the other hand, in mesh mode an ad hoc network can be formed with all nodes acting as relaying routers in addition to their sender and receiver roles, although there may still be nodes that serve as BSs and provide backhaul connectivity. As a cost-effective solution, multihop communication is becoming more and more important to WiMAX system. To successfully deploy multihop WiMAX networks, security is one of the major challenges that must be addressed /07/$ IEEE

2 Base station Subscriber station Wireless link Figure 1. WiMAX network architectures: a) PMP mode; b) mesh mode. Another important issue is how to support different services and applications in WiMAX networks. Since WiMAX is a relatively new standard, very little work has been conducted in the literature. In [2] the authors provided a survey on the security schemes used in the IEEE standard [1]. They further analyzed the security flaws in the standard. Several improvements have been proposed since then. Nevertheless, we notice that the security mechanism of IEEE is mainly focused on security in the MAC layer, which may not be able to provide sufficient security in multihop scenarios and satisfy the requirements of emerging applications in WiMAX networks. In this article we propose a secure and service-oriented network control framework for WiMAX networks. The main motivation of our study is that we need to take into account both the security concerns and the requirements of potential WiMAX applications. The proposed framework consists of two important components: A service-aware control framework A unified routing scheme In our study we show that the proposed framework is suitable for various application scenarios and can provide sufficient security from the network perspective. In addition to the design of the framework, we also study a number of key enabling technologies that are very important to the framework. Our study can provide a guideline to the design of a more secure and practical WiMAX network. The rest of the article is organized as follows. We firstbriefly review the architectures of WiMAX networks. We then study the challenges and requirements in the design of the network layer for WiMAX networks. Based on the analysis, we propose and elaborate on a secure and service-oriented control framework. Within the proposed framework, we identify a number of important technologies, followed by conclusions. WIMAX NETWORK ARCHITECTURES According to the IEEE standard [1], WiMAX technology supports two operation modes: PMP and mesh. A WiMAX PMP network aims at providing last-mile access to a broadband Internet service provider (ISP). An (a) Relay station Mobile station (b) example of the network topology is illustrated in Fig. 1a, where the WiMAX network includes one BS and a number of SSs. On the other hand, mesh mode implies the requirement of supporting multihop ad hoc networking by SSs. An example of a WiMAX mesh network is illustrated in Fig. 1b. Notice that in this figure, we assume that BS can provide access to the Internet; a relay station (RS) is a special type of SS that can forward traffic flows to BSs or other RSs; and a mobile station (MS) is an SS that can move in the network. Compared to PMP mode, mesh mode is more flexible and can be used to quickly deploy the infrastructure. In fact, a more general term for mesh mode, wireless mesh network, has attracted great attention in the past few years [3 5]. Currently, different IEEE standard groups are working on providing wireless mesh support in a variety of networks, including wireless personal area networks and wireless local area networks [5]. It is worth noting that a concept of mobile multihop relay (MMR) networking has also been introduced for PMP mode in IEEE e, which may expand the coverage area and enhance throughput through multihop paths [5]. One of the motivations for the MMR scheme is to overcome the problem that mesh mode is not compatible with PMP mode. To summarize the existing architectures of WiMAX networks, we observe that multihop communications will become a basic feature in the near future. CHALLENGES AND REQUIREMENTS IN NETWORK LAYER DESIGN In the previous section we provide a brief review of existing and proposed architectures for WiMAX networks. In reality, to successfully deploy WiMAX networks, a number of challenging issues must be addressed. In this article we focus on two major issues in network layer design: security, and support of existing and future applications. In the rest of this section we first discuss the common requirements of security in WiMAX networks and possible attacks to WiMAX networks. We then address the current and potential application scenarios of WiMAX networks. It is worth noting that a concept of mobile multihop relay (MMR) networking has also been introduced for PMP mode in IEEE e, which may expand the coverage area and enhance throughput through multihop paths. 125

3 Server 3 Server 2 Figure 2. An example of manycast in a WiMAX network. Server 4 SECURITY CHALLENGES IN WIMAX NETWORKS General Requirements In general, a secure WiMAX network shall satisfy the following security concerns: Confidentiality: Confidentiality is necessary to ensure that sensitive information is well protected and not revealed to unauthorized third parties. The confidentiality objective is required in a WiMAX network environment to protect information traveling between different stations, since an adversary having the appropriate equipment may eavesdrop on the communication. Authentication: As in conventional systems, authentication techniques verify the identity of the participants in a communication, thus distinguishing legitimate users from intruders. Integrity: From the integrity perspective, there is the danger that information could be altered when exchanged over insecure networks. Lack of integrity could result in many problems since the consequences of using inaccurate information could be disastrous. Integrity controls must be implemented to ensure that information will not be altered in any unexpected way. Availability: In a wireless network, availability can be a major security issue since an adversary can launch a denial-of-service (DoS) attack to block a legitimate user s access to the network. Security Issues As a relatively new standard, security in WiMAX networks has only been addressed in a few previous studies. Early work can be found in [2], which provides a survey on the security schemes used in IEEE standards [1] and analyzes the security flaws in the standard. For instance, one of the flaws is that there is a lack of mutual authentication. In other words, a BS can authenticate an SS, but the SS cannot authenticate the BS. This authentication problem may cause significant security problems. In the literature a number of previous studies have suggested mutual authentication for both communication parties [2]. Although these discussions are very important to the security of the whole system, we notice that the security mechanism of IEEE is mainly focused on the MAC layer. Apparently, only the security in the MAC layer may not be sufficient to protect multihop communications, which will become common in WiMAX mesh or MMR networks. Particularly in WiMAX networks, a security mechanism in the MAC layer can only prevent eavesdropping by other parties through securing every message between adjacent stations. However, in practice an adversary can capture an RS or deploy a corrupted RS [6]. The latter case may be more significant if a broadband wireless service provider wants to choose a customer premises equipment (CPE) station to increase service coverage at low cost. In such scenarios, existing security approaches of IEEE are not sufficient to protect confidentiality in multihop communications. In [6] a transit access point (TAP) is used to represent a station that can perform relay functions. As suggested in [6], how to identify corrupt or compromised RSs is one of the most important security issues in a wireless mesh network. On the other hand, a common threat in wireless networks is a DoS attack, in which an adversary can block radio transmission in a certain area. Although a DoS attack is relatively simple to launch, it is virtually impossible for a WiMAX network to guard against in the PHY and MAC layers. To deal with the above issues, we believe that secure routing [7] will be crucial for multihop communication scenarios from the network perspective. Clearly, such routing problems must be addressed in the design of the network control framework. For example, careful network design can mitigate the compromised RS problem and DoS attacks by choosing one or more different paths for the same traffic flow. APPLICATION SCENARIOS In the previous discussion we address the network design issue from the security perspective. In practice, a good system design also depends on understanding the applications that will be carried in the networks. In this subsection we survey the existing application scenarios and several potential scenarios that have been proposed in the literature. It is important to note that we will also elaborate on the functions that shall be provided in the network layer so as to fulfill the requirements of these applications. Internet Access: Evidently, Internet access will still be the major demand in WiMAX networks, especially when they are newly deployed. To support Internet access, a straightforward method is to provide a unicast connection between SSs (including RSs and MSs) and the BS, which has the link toward the Internet. Group Communications: Since WiMAX networks can cover a relatively large area, it is natural to imagine that many group communications, such as videoconferences, will be important applications in WiMAX networks. To support such communication scenarios, multicast is the key technology. In the past, Internet multicast has not been successful due to its complexity and, more important, because Internet multicast requires global deployment, which is virtually impossible. In a WiMAX network, however, since all nodes are located inside, implementing 126

4 such group communication becomes possible. Metropolitan Area Distributed Service: With the deployment of WiMAX networks, more and more value-added services can be provided in a metropolitan area. To efficiently support a large number of customers, distributed services can be enabled. In other words, a customer can access the service from any of the servers in the network in which these servers are distributed to serve the entire metropolitan area. To enable distributed service, the authors in [8] propose a novel routing framework in the network layer, manycast routing. In this scheme the customer does not need to specify the exact address of a server in the network. Instead, it only needs to indicate the service it wants to access. Moreover, in such a communication scenario, the client (i.e., the customer) can communicate with a subset of all the servers in order to achieve better reliability and/or security. An example of manycast is illustrated in Fig. 2, in which the MS has decided to access servers 1 and 4 simultaneously. Content-Based Distribution: The contentbased routing scheme is a service-oriented communication model [9]. In this scheme the sender of a message does not need to explicitly specify its destination(s). The network layer will automatically deliver the message to receivers that are interested in the content of the message. In [9] the authors proposed to design an overlay network based on broadcast service of the existing network. Quality Guaranteed Applications: For many applications, it is desirable that the network layer can provide a sufficient quality of service (QoS) guarantee, usually in terms of bandwidth, data rate, delay, and delay jitter. However, wireless communications are naturally error-prone; thus, it is difficult to provide such a guarantee in a wireless network. To address this issue, in the literature multipath routing has been studied in many previous works. In general, multipath routing can provide better quality than single-path routing [10]. An example of multipath routing is illustrated in Fig. 3, in which the MS accesses server 3 via two different paths. Multihoming Applications: Multihoming [11] is a technology that can provide services similar to those of multipath routing. The main difference between these two schemes is that in multihoming, one station has two or more IP addresses and generally has the same number of interfaces. In this manner, the station can have multiple paths to access the same resources. In short, the application layer requirements must be addressed in the network layer design. In the next section we provide a solution to satisfy the above applications while providing sufficient security. A SECURE AND SERVICE-ORIENTED NETWORK CONTROL FRAMEWORK In this section we elaborate on a novel control framework to address the security requirements in WiMAX networks, and fulfill the demands of existing and future application scenarios discussed above. Server 3 Server 2 Figure 3. An example of multipath routing in a WiMAX network. COMPONENTS In this framework there are two major components. Service-Aware Control Scheme To efficiently support different applications, the network layer control scheme shall be aware of the availability of different services. In general, the service can be either located in a single node in the network or distributed in multiple locations in the network. To provide these services, the servers must register the type and availability of service to the control framework. Moreover, the availability information shall be updated periodically or based on predefined events. Upon receiving these messages, the control framework will also be responsible for distributing such message to nodes in the network. Unified Routing Scheme With the availability information of the service, a unified routing scheme shall be designed such that all the application scenarios discussed in the last section shall be supported. The packets of a certain flow will be forwarded based on the service and security requirements. CASE STUDIES To illustrate the behaviors of the framework, we use the following cases as examples. Case 1 We first assume that a BS has been registered as the gateway for Internet access in the control framework. Now suppose a regular best-effort Internet access request arrives at the control framework; a single-path unicast routing scheme can be set up for such a request. Notice that in such a scenario, the single path routing scheme cannot defend compromised RSs in a multihop situation. Case 2 For the same application scenario as in case 1, if the customer requires a better QoS for Internet access, a unicast multipath routing scheme can be set up. In this case, since the major concern is to provide better transmission Server 4 127

5 Path 1 Path 1 Path 3 Path 2 Path 2 Path 4 (a) (b) Figure 4. Examples of case 3: a) a single path; b) multipath. quality, multiple paths are not necessarily node or link disjoint. Case 3 For the same scenario as case 1, if the customer requires a higher level of security, another unicast multipath routing scheme can be set up to provide better confidentiality and maintain the integrity of the message. In such a case, the paths may be link disjoint or even node disjoint. An example of this case can be found in Fig. 4, where we use server to indicate that the node can provide Internet access. Clearly, in this case multiple paths can prevent a single compromised node from decoding the whole message. The use of these multiple paths can be flexible. One possible scenario is that multiple paths are used simultaneously, and the message transmitted through a single path may contain only a subset of the total information, as illustrated in Fig. 4a. At the receiver side, the whole message can be recovered if all or a subset of the messages are received. Another scenario is that only one path will be used at a time, and paths can be used alternatively. Similar to the previous scenario, the forwarding scheme can be designed so that a single node cannot receive the whole message. As shown in Fig. 4b, we can set up four paths between the source-destination pair. All packets can be numbered in sequence, and we can let the packet with sequence number 4k + m travel through path m, where k is any integer and m can be an integer in {1, 2, 3, 4}. Case 4 In this case we assume that a distributed service has been provided in the network, and a number of servers have been registered as nodes that can provide service. In such a scenario, if the application does not have security or reliability concerns, any one of the servers can be selected by the control framework, and a unicast path can be set up. Case 5 For distributed service, if reliability is important to the application, multiple servers can be selected and the manycast routing scheme can be utilized, as illustrated in Fig. 2. Case 6 To address security concerns in distributed service, the manycast and multipath schemes can be combined. An example is shown in Fig. 5, where four paths have been established to accommodate the application. Similar to case 3, these paths can be utilized in different ways. In the first scenario, different nodes act independently, and thus provide the same type of service if they receive the same set of messages. In the second scenario, different nodes can operate in a cooperative manner through control channels, which can be established in advanced. Consequently, only one of the servers is needed to prepare the service, while all others just verify whether the received information is the original information sent from the client. From the perspective of the client, the returned messages from different servers can be compared and verified. ENABLING TECHNOLOGIES To deploy the proposed framework, a number of key technologies must be addressed. In the rest of this section we address these issues. PLACEMENT OF BSS AND RSS In our framework, the placement of BSs and RSs is very important for a broadband wireless service provider to offer a secure communication platform. For example, in Fig. 6a, if there is only one path between one MS and a server, it is not possible to guarantee the security of the communication since a single RS in the path can damage the confidentiality and integrity of the information transmission, or block the traffic flow and affect the availability of the service. On the other hand, if there are two or more paths available, secure communication channels are more likely to exist between the MS and the server, as shown in Fig. 6b. One important issue related to the placement of BSs and RSs is the cost. Apparently, with increasing numbers of BSs and RSs of a service provider, security and availability will increase while cost will also soar. In such a case, it becomes a trade-off between security and cost. On the other hand, given the constraint of cost, the placement of BSs and RSs can be formulated as an optimization problem, which shall be further investigated. SECURITY MANAGEMENT In the proposed framework the security management scheme is very important to the system. Similar to [6], we consider the security management scheme responsible for monitoring the operation of the network and quickly identifying possible security attacks and threats. 128

6 KEY MANAGEMENT In addition to the MAC layer, key management is also important to the network layer. To provide a secure communication channel between the end user and the server, it is important to develop a key management scheme to establish a unique key for each session. In such a scenario the proposed framework can be directly utilized to improve the reliability and security of the key distribution. For instance, an MS can send key material through multiple paths to the server. Since each path may contain only a portion of all the information, the probability of the key material being intercepted by an adversary can be significantly reduced. Another potential research topic in key management is the key distribution scheme for group communication. One simple solution for group communication is to utilize a single group key. However, such an approach is not efficient because of two factors: the key will be exposed if a single group member is compromised; and the communication overhead will become a big burden if a member can frequently join or leave the group, a typical scenario for a system with mobile nodes. Traditionally, a key graph can be introduced to deal with such a scenario. However, we notice that existing solutions are generally based on a tree-like graph, which may not be directly applied to our framework, in which multiple paths can be set up for the communication. Server 3 Path 3 Path 1 Path 2 Server 2 Figure 5. An example of case 6. (a) Path 4 Server 4 SECURE ROUTING In the literature secure routing has been discussed in a number of previous studies [7]. However, we notice that none of them fully address the scenarios discussed in the previous section. In our scheme the routing algorithm takes into account the following issues. Multiple-radio and multiple-channel: In the near future, each node may be equipped with multiple radio interfaces. Therefore, the routing scheme shall take this into account. Multiple destinations: In our framework, an application can require multiple destinations in the network. Although several schemes were discussed in [8], we notice that security concerns were not addressed. For example, there is no requirement for selecting node disjoint paths in these schemes, which may not be sufficient to defend against compromised RS nodes. Multipath routing: As shown in the previous section, the multipath scheme is different from existing methods. First, multipath routing may need to forward messages to different destinations. Second, more paths may need to be set up, as illustrated in Fig. 4b. Heterogeneity of user devices: In practice, the capabilities of user devices (e.g., data rate) are highly heterogeneous. Several application layer schemes and middleware schemes have been proposed recently. However, it is appropriate for the network layer to consider such differences because the capability information of end users can be utilized to help choose the routing method used. To address the secure routing issue, we believe that the emerging network coding technique should be applied because it can provide Figure 6. The importance of the placement of BSs and RSs: a) a single path; b) multiple paths. the optimum solution and reduce the computational complexity for many problems [12]. CONCLUSION (b) WiMAX is a promising wireless communication technology for wireless MANs. In this article we address the design issue in multihop WiMAX networks. Specifically, we propose a secure and service-oriented network control framework in which both security concerns and the requirements of potential WiMAX applications are taken into account. In the framework there are two major components: a service-aware control framework and a unified routing scheme. We then demonstrate how these schemes can provide the required service from the network layer perspective. In addition to the design of the framework, we also study several enabling technologies for the framework, including the deployment of BSs and key management, and secure routing. We believe that our study can provide a guideline for the design of a more secure and practical WiMAX network. 129

7 To address the secure routing issue, we believe that the emerging network coding technique shall be applied because it can provide the optimum solution and can reduce the computational complexity for many problems. ACKNOWLEDGMENTS The work reported in this article was partially supported by the U.S. National Science Foundation under the NSF Award Number , a NSF EPSCoR start-up grant in Puerto Rico, and National Science Council, Taiwan, under the grants NSC E and NSC E REFERENCES [1] S. J. Vaughan-Nichols, Achieving Wireless Broadband with WiMAX, IEEE Comp., vol. 37, issue 6, June 2004, pp [2] D. Johnston and J. Walker, Overview of IEEE Security, IEEE Sec. & Privacy, vol. 02, no. 3, May-June 2004, pp [3] Akyildiz and X. Wang, A Survey on Wireless Mesh Networks, IEEE Commun. Mag., vol. 43, no. 9, pp. S23 S30, Sept [4] R. Bruno, M. Conti, and E. Gregori, Mesh Networks: Commodity Multihop Ad Hoc Networks, IEEE Commun. Mag., vol. 43, no. 3, Mar. 2005, pp [5] M. 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Theory, vol. 52, no. 6, June 2006, pp BIOGRAPHIES KEJIE LU (lukejie@ece.uprm.edu) received B.S. and M.S. degrees in telecommunications engineering from Beijing University of Posts and Telecommunications, Beijing, China, in 1994 and 1997, respectively. He received a Ph.D. degree in electrical engineering from the University of Texas at Dallas in In 2004 and 2005 he was a postdoctoral research associate in the Department of Electrical and Computer Engineering, University of Florida. Currently, he is an assistant professor in the Department of Electrical and Computer Engineering, University of Puerto Rico at Mayaguez. His research interests include architecture and protocols design for computer and communication networks, performance analysis, network security, and wireless communications. YI QIAN [M] (yqian@ece.uprm.edu) is an assistant professor in the Department of Electrical and Computer Engineering, University of Puerto Rico at Mayaguez. Prior to joining UPRM in July 2003, he worked for several startup companies, consulting firms, and a large telecommunications equipment manufacture company in the areas of voice over IP, fiber optical switching, Internet packet video, network optimizations and network planning for wireless and satellite networks, as a technical advisor, senior consultant, and senior member of scientific staff. He received a Ph.D. degree in electrical engineering from Clemson University. His current research interests include network security, network management, network modeling, simulation, and performance analysis for next-generation wireless networks, wireless sensor networks, broadband satellite networks, optical networks, high-speed networks, and the Internet. He has publications and patents in all these areas. He has been on numerous conference technical committees including serving as General Chair of the International Symposium on Wireless Pervasive Computing 2007, Technical Program Co-Chair of the IEEE GLOBECOM 2006 Symposium on Wireless Communications and Networking, and Technical Program Co-Chair of the Workshop on Information Assurance 2006 and He is a member of Sigma Xi, ACM, IEICE, IEEE Computer Society, and IEEE Vehicular Technology Society. HSIAO-HWA CHEN (hshwchen@ieee.org) is currently a full professor at the Institute of Communications Engineering, National Sun Yat-Sen University, Taiwan. He has been a visiting professor at the Department of Electrical Engineering, University of Kaiserslautern, Germany, in 1999, Institute of Applied Physics, Tsukuba University, Japan, in 2000, Institute of Experimental Mathematics, University of Essen, Germany, in 2002 (under a DFG Fellowship), Department of Information Engineering, Chinese University of Hong Kong, 2003, and Department of Electronics Engineering, City University of Hong Kong, His current research interests include wireless networking, MIMO systems, and next-generation CDMA technologies for future wireless communications. He has authored or co-authored over 200 technical papers in major international journals and conferences, and five books in the areas of communications, including Next Generation Wireless Systems and Networks (Wiley). He has served or is serving as International Steering Committee member, Symposium Chair, and General Co-Chair of more than 50 international conferences, including IEEE VTC, IEEE ICC, IEEE GLOBECOM, and IEEE WCNC. He has served or is serving as Editor, Editorial Board member, or Guest Editor of many international journals, including IEEE Communications Magazine, IEEE JSAC, IEEE Vehicular Technology, IEEE Wireless Communications Magazine, IEEE Network Magazine, IEEE Transactions on Wireless Communications, IEEE Communications Letters, Wiley s Wireless Communications and Mobile Computing Journal, and Wiley s International Journal of Communication Systems. He is an adjunct professor of Zhejiang University and Shanghai Jiao Tung University, China. Currently he is also Chair of the Radio Communications Committee, IEEE Communications Society. 130