Fast handovers without DAD using Sector-based Vehicular Mobile IPv6

Transcription

1 Fast handovers without DAD using Sector-based Vehicular Mobile IPv6 Laurence Banda, Mjumo Mzyece and Guillaume Noel French South African Institute of Technology and Department of Electrical Engineering, Tshwane University of Technology, Private Bag 680X, Pretoria 0001, RSA Tel: +27 (0) , Fax: +27 (0) Abstract-One of the major challenges faced in vehicular networks is the provision of seamless and continuous IP connectivity for on-board Internet users. This mainly emanates from long handover delays and packet loss problems. These hiccups can have severe effects such as Quality of Service (QoS) depreciation for delay-sensitive and throughput-sensitive services like real-time multimedia services. Most current IP mobility management solutions focus on pedestrian mobile users moving with relatively low speeds and thus fail to provide mobility support for high mobility nodes in vehicular networks. In this paper, we theoretically formulate a Network Layer Fast handover scheme in vehicular networks without Duplicate Address Detection (DAD) using sector based Vehicular Mobile IPv6. Sectorisation of radio coverage cells and Global Positioning Systems (GPS) mounted on vehicles aid in accurately predicting vehicle directions. The fast discovery of the target Access Router (AR) and the total elimination of DAD procedure could reduce the handover latency and packet losses thereby improving the overall QoS in vehicular networks. road segments. DSRC or IEEE p, an extension of the standards for inter-vehicle communication by the IEEE working group, operates in the GHz band with a bandwidth of 75MHz. The two types of wireless communication found in vehicular networks are vehicle-to-vehicle (V2V) which involves vehicles exchanging information through OBUs and vehicle-to- Infrastructure (V2I) where OBUs exchange information with RSUs [2]. V2I communication is the main source of on-board Internet access and real time services such as push-to-talk, instant messaging, instant gaming, etc. In V2V communication, the On Board Units (OBUs) mounted on vehicles exchange information among vehicles thereby creating a Vehicular Ad hoc Network (VANET), a dynamic form of a Mobile Ad hoc Network (MANET). Figure 1 below shows a general model of vehicular networks. Index Terms Vehicular Networks, Fast handovers, DAD, VMIPv6 I. INTRODUCTION The Intelligent Transportation System (ITS) has in recent years attracted a lot of research interests from both the industry and academia. ITS is the application of information and communication technologies to improve the safety, efficiency and environmental sustainability of transportation systems. A vehicular communication network was developed under the ITS to provide safety related applications as well as non safety applications. Safety ITS applications concerns information sharing on accidents, traffic congestion, road works, etc while non safety applications involve infotainment services like Internet access [1]. From the networking point of view, vehicular networks are considered to contain two types of communicating nodes which are both Dedicated Short Range Communication (DSRC) devices. These are: (i) On-Board Units (OBUs) mounted on vehicles and interlinked with Application Units (AUs) such as laptops, Personal Digital Assistants (PDAs), etc. used by passengers and (ii) Road Side Units (RSUs) found on fixed infrastructure such as Access Points (APs), Base Stations (BSs), etc. located along Figure 1: General model of vehicular networks [3] One of the major challenges of vehicular networks is to support the provision of seamless or interruption free real time and delay sensitive ITS applications. With the core network of the roadside wireless access network evolving to an All-IP network, Mobile IPv6 (MIPv6) has been proposed for mobility management of mobile nodes (MN) in wireless IPv6 by the Internet Engineering Task Force (IETF) [4]. However, MIPv6 has inherent limitations when it comes to mobility support for real time and delay sensitive applications. That is, the long handover latency and packet loss problems of MIPv6 depreciate the Quality of Services (QoS) for multimedia service applications. To address these problems, the IETF proposed the Fast handover for MIPv6

2 (FMIPv6) [5]. In FMIPv6, when the MN detects a possibility of subnet change, it creates a new address for use in the target network and receives data through tunneling in advance. Therefore, through this address preconfiguration process, the address resolution time is reduced and thus the overall handover latency and packet loss problem is reduced. Nonetheless, FMIPv6 still has setbacks in accomplishing fast and smooth handovers in vehicular networks where node mobility speeds can vary from as low as 0 km/h (static nodes) to as high as 200 km/h (highway vehicles). Further, the address pre-configuration becomes ineffective if the vehicle moves into a network different from the predicted one. In this paper, we propose and formulate a novel Fast handover scheme without Duplicate Address Detection (DAD) using sector-based Vehicular Mobile IPv6 (VMIPv6). In this scheme, the DAD procedure which takes maximum 1 second in standard MIPv6 is completely eliminated and the inaccurate predictions which sometimes affect FMIPv6 scheme are enhanced through sectorisation. The rest of this paper is organized as follows. In section II, the background information on MIPv6 and FMIPv6 is outlined. Section III presents related work. In section IV, we theoretically formulate the proposed sector-based Vehicular Mobile IPv6 model and section V concludes the paper. II. BACKGROUND AND RELATED WORK 1. Handover process in a wireless IP network. In wireless IP networks, handover can be defined as a process of switching and maintaining an active MN's connection from one Base Station (BS) or Access Point (AP) to another, as the MN moves out-of-range of an old BS (or AP) to the service area of a new BS (or AP) [6]. The handover process comprises three distinct phases and these are; (1) Handover Initiation which involves the need for a handover to take place due to various effects such as signal strength deterioration, signal quality degradation, distance from the transmitter, etc; (2) Handover decision this phase involves the process of either granting or denying a handover; (3) Handover execution this is the final stage which takes place after the two preceding conditions are satisfied and involves the actual switching of an active session from one BS to another [3]. From the network level (layer) point of view, a handover can either be intra-subnet or inter-subnet [3]. Intra subnet handover is one which takes place between two APs (or BSs) belonging to the same access router (AR). This involves switching at link layer only. Inter subnet handover on the other hand takes place when a MN moves between two APs (or BSs) belonging to two different ARs and involves switching at both the link and network layers. 2. Handovers in Mobile IPv6 A typical handover scenario in a MIPv6 based network is depicted in figure 2 below. AP Access Point AR Access Router HO Handover Internet AR1 AR2 AP1 Intra Subnet HO AP2 AP3 AP4 Inter Subnet HO Intra Subnet HO Subnet 1 Subnet 2 Figure 2: Mobile IPv6 handover scenario In figure 2, an intra subnet handover takes place when a MN accessing the Internet via Access Router 1 (AR1) changes its wireless link connection from Access Point 1 (AP1) to AP2. This handover process only involves layer 2 (L2) handover which is the link layer switching between AP1 and AP2. However, when the MN changes its point of attachment to the Internet from AR1 to AR2 by changing its wireless link connection from AP2 to AP3, an Inter subnet handover takes place and this involves both L2 and L3 handovers. L3 handover is the process of changing the point of attachment to the IP network (the Internet) and it requires a change in the IP address of the MN. Handover latency is the time interval between the last moment when a MN can send or receive packets through the previous AR (AR1) and the first moment when it can receive or send packets through the new AR (AR1). This corresponds to the time during which the MN can neither receive nor send IP traffic and is used as a measure of handover performance [4]. In MIPv6 networks, L2 handover precedes L3 handover and the total handover latency comprises both L2 and L3 handover latencies as by figure 3 below. Figure 3: Total handover latency in MIPv6 [7] L2 handover latency is the time taken for the MN to switch its link layer connection from one wireless AP to another and consists of three phases: scanning, authentication and re-authentication phase delays. L2 handover delay ranges from 50ms to about 400ms and always exhibit great variations since it is media or technology dependent [8]. The scanning phase, which largely contributes to L2 handover latency, can be minimized by reducing the beacon intervals from the access points or base stations to the MNs. However, this comes at a price of increased overhead in the link layer. L3 handover latency is the time taken for the MN to change its point of attachment to the IP network from one AR to another and is made up of; Router Discovery; new Care of Address (CoA) configuration and Duplicate Address Detection (DAD) processes; and Registration

3 (Binding Update and Binding Acknowledgement) phase delays. The total handover latency in MIPv6 is thus given by; and the network layer handover delay can be represented by; (2) Router discovery (RD) During the router discovery (Movement detection) phase, the MN sends Router Solicitation (RS) messages to the new AR. Upon reception of the RS, the new AR sends Router Advertisement (RA) messages. After receiving the RA, the MN will know that it has moved to the new AR s subnet. Therefore, router discovery delay is the total time taken by the MN to transmit RS and to receive RA messages and is about 100ms in standard MIPv6 [7]. Address Configuration and DAD procedure The address configuration phase involves the process of an MN acquiring a new temporal address called Care of Address (CoA) for use in the new subnet. The new CoA can either be obtained by stateful or stateless address configuration. In the stateful configuration, a central mechanism such as Dynamic Host Configuration Protocol version 6 (DHCPv6) server [12] is responsible for address assignment and keeps track of addresses in use and the free ones. In stateless address autoconfiguration, a new CoA is assigned by combining the address prefix of the new AR and the prefix of the MN [13]. The configuration period in the stateless process is much shorter than in the stateful process. The uniqueness and validity of the new assigned CoA must be tested to avoid contrasts and duplications with the already existing address in a particular subnet. This is achieved through the Duplicate Address Detection (DAD) test. The DAD test delay accounts for the biggest contribution (1 second) to the overall MIPv6 handover latency [14]. The DAD uniqueness check is based on sending an Address Request (AREQ) message to the new AR and expecting an Address Reply (AREP) if the address is not unique. When there is no AREP received, the uniqueness test is successful [15]. Registration During the registration phase, the MN registers its new CoA to its Home Network (HN) by sending Binding Update (BU) messages to the Home Agent (HA) and the Correspondent Nodes (CNs). The HA and CNs confirms receipt of the BU messages by responding through the Binding Acknowledgement (BAck) messages. The total registration delay in MIPv6 is about 140ms. Equation (2) above can be expressed in terms of the delays incurred by individual processes constituting the three main L3 handover stages as follows; (1) (3) 3. Fast handovers for Mobile IPv6 (FMIPv6) Fast handover for Mobile IPv6 (FMIPv6) was proposed by the IETF [5]. In this scheme, router discovery latency is reduced by providing the MN with the information about the new access point and the associated subnet prefix information when the MN is still connected to its current subnet. FMIPv6 can be implemented in either predictive or reactive mode. In the predictive case, preparations for L3 handover are completed before L2 handover starts. Using an L2 trigger through the scanning process, the MN is able to detect that the Received Signal Strength (RSS) from the neighbouring Access Point (AP) or Base Station (BS) is stronger than that from the currently serving AP or BS. Based on these results of L2 triggers, L3 handover is initiated. MN sends the Router Solicitation for Proxy (RtSolPr) message to its currently connected access router called Previous Access Router (PAR) in order to acquire information about the targeted access router called New Access Router (NAR). The PAR responds to the MN via the Proxy Router Advertisement (PrRtAdv) message containing the NAR information like its subnet prefix, etc. Whilst still connected to the PAR, the MN configures its new Care of Address (ncoa) based on the subnet prefix of the NAR contained in the PrRtAdv sent by the PAR.MN then transmits the generated ncoa using the Fast Binding Update (FBU) message to the PAR. The PAR then sends the Handover Initiation (HI) message containing the ncoa to the NAR for the validation of the ncoa using the DAD procedure and the creation of the tunnel between the PAR and NAR. The NAR replies to the PAR by sending the Handover Acknowledgement (HAck) message. The PAR checks the HAck message for a possible handover execution, and then sends the result (positive or negative) to the MN in the Fast Binding Acknowledgement (FBAck). At the same moment, the PAR transmits all the IP packets of the MN to the NAR using the bi-direction tunnel created in between them. Upon completion of L2 handover, MN notifies the NAR that it is now attached to a new wireless link in the new subnet using the Fast Neighbour Advertisement (FNA) message. The MN packets that were stored in the buffer zone of the NAR are then channeled to the MN and thereafter, the direct communication and packet transfer between the MN and the Correspondent Node (CN) is restored through the Binding Update (BU) procedure. FMIPv6 reactive mode procedure is similar to that of the predictive mode only that in this case, the connection between the MN and the PAR is ceased due to L2 handover execution. III. RELATED WORK In an attempt to reduce the handover latency contribution due to router discovery and DAD procedure delays, a lot of handover schemes have been proposed by various researchers in that respect. In [9], a Mobile IP handover scheme based on movement detection algorithm at Layer 3 is proposed. This scheme called Enhanced Lazy Cell Switching (ELCS) combines the effectiveness of the laziness within overlapping cell zones with the capability of obtaining good

4 performance in the case of adjacent cells. Furthermore, based on the ELCS, a new fast movement detection algorithm called Fast Detection Movement Layer 3 (FDML3) is proposed in [10]. This scheme aims at reducing the handover delay in MIPv6 by reducing the movement detection latency. An enhanced fast handover scheme with low latency for Mobile IPv6 [11] is proposed where each AR maintains a CoA table and generates new CoA for a MN that moves into its domain. At the same time, the BUs to the HA and CN are performed from the time point when the new CoA is known by the previous AR. Further, the DAD procedure is performed before the handover process starts and thus delay due to DAD procedure is eliminated. A Fast handover in Mobile IPv6 (FMIPv6) predictive mode deploying a temporary CoA is proposed in [12]. In this scheme, the MN with the use of Base Station ID creates a temporal CoA that guarantees uniqueness of the target subnet and the DAD procedure is in the process omitted In [13], a seamless and robust handover scheme that supports multimedia services in vehicular networks is proposed. The scheme allows the MN to continuously maintain the original CoA configured at the original AR and reduces the delays caused by the DAD process. At the intersection, the vehicle creates a new CoA to limit the packet forwarding hops between the original AR and the new AR and the background DAD scheme reduces the DAD delay at the intersection and also reduces the number of HA bindings. DAD-less MIPv6 for reduced handover latency is proposed in [14]. This scheme takes full advantage of the abundant address space in the MIPv6 domain and thus completely eliminates the DAD process from the MIPv6 handover process. IV. FORMULATION OF THE PROPOSED FAST HANDOVER SCHEME Based on the fact that router discovery and DAD procedure heavily contribute to the total handover latency in MIPv6, we propose a sector-based VMIPv6 which works in the predictive mode of FMIPv6 [5] and deploys the DAD-Less MIPv6 scheme in [14]. The accuracy of the MNs movement prediction is enhanced through the sectorisation [15] and the use of Global Positioning Systems (GPS) mounted on moving vehicles. 1_6 1_5 1_1 1_0 1_4 Subnet 1 AR 1_3 1_2 Internet 2_5 2_6 2_1 2_0 2_4 Subnet 2 AR Figure 4: System Architecture for sector-based VMIPv6 scheme the subnet s center AP which is assigned an ID of 0 then the AP in the true north direction of the center AP is assigned an ID of 1 then a clockwise pattern is followed until an AP ID of 6 is attained. For cell Id representation, the cell facing the true north direction of a particular AP is assigned an Id of 0. Similarly, a clockwise pattern is followed until a cell ID of 5 is attained. As an example, a cell represented by 1_2_5 belongs to subnet 1 with AP ID of 2 and cell ID of 5. The values of a can range from range 0 to a value (N-1), where N is the number of subnets present in a particular IP network. Similarly, b ranges from 0 to the some value (n-1), where n is the number of APs in a particular subnet. The value of c strictly ranges from 0 to 5. Figure 5 below shows the schematic representation of cells in the proposed model. The figure depicts an ideal geometrical layout of cells belonging to two different subnets. In figure 5, an MN connected to the Internet and moving from cell (1_2_3) to cell (1_0_1) experiences an intra subnet handover and this involves the link layer switching from AP (1_2) to AP (1_3). On the other hand, an MN accessing the Internet leaving cell (1_3_1) and entering cell (2_6_4) encounters an inter subnet handover and this consist of link switching between AP (1_3) and AP (2_6) and network layer switching between subnet 1 AR and subnet 2 AR (i.e. switching from subnet 1 and subnet 2). 1_0_5 1_0_0 1_0_1 1_2_5 1_2_4 1_2_0 1_2_3 1_2_1 1_2_2 2_6_5 2_6_0 2_2 2_3 2_6_1 1. Sector-based VMIPv6 System Architecture 1_0_4 1_0_2 1_3_0 2_6_4 2_6_2 2_0_0 1_0_3 1_3_5 1_3_1 2_6_3 2_0_5 2_0_1 Figure 4 shows the system architecture of the sectorbased VMIPv6 comprising two subnets each having seven APs. The coverage area of each AP is further divided into six sectors. Each of the six sectors forms a radio access area called a cell. A specific cell is represented by three values in the form a_b_c where: a is the subnet ID, b is the AP ID and c is the cell ID. In Figure 4, only a_b values are shown. The subnet Ids may be assigned by the network operator and can be based on geographical location, hierarchical cell structure, etc. The naming procedure of AP IDs starts with 1_4_5 1_4_4 1_4_0 1_4_3 1_4_1 1_4_2 1_3_4 1_3_3 1_3_2 2_5_5 2_5_4 2_5_0 2_5_3 2_5_1 2_5_2 2_0_4 2_0_3 2_0_2 Figure 5: Schematic representation of cells in sector-based VMIPv6 model 2. Formulation of Fast Router Discovery Procedures To increase the rate and accuracy of candidate router discovery, each sector is divided into two regions which are; the handover region (near the cell border) and the non

5 handover region (near the transmitter) as shown in figure 6 below. In the non handover region, the MN is under the full coverage of the serving AP and thus, there is no need for a handover. However, when it enters the handover region, the MN experiences different APs signal probes with varying signal strength and quality which can trigger the handover process. Modified Binding Update procedures Since the IP address of the MN is permanent, there is no new CoA registration. However, for the MN to be reachable when in a new subnet, the new (target) AR must register its IP address along with its subnet ID to the Home Agent of (HA) of the MN. Thus, packets meant for the MN can easily be channeled from the HA to the MN via the new AR. 4. Implementation and simulations to be conducted Non Handover Region Handover Region Figure 6: Handover and non handover regions in sector based VMIPv6 When a vehicle moves from the non handover region to the handover region as shown by the arrow in the figure 6, handover preparations start as the MN scans and observes potential candidates APs through probes. In the AP probes are neighbour cell information such as subnet ID, Access Point (AP) ID and cell ID. In addition, the attached GPS traces the position, speed and direction of the vehicle which are recorded and stored in the memory of the OBU. The target cell information (i.e., a_b_c) and the GPS information (i.e., position, speed and direction) are transmitted to the serving subnet AR via the serving AP. The serving subnet AR requests the target subnet AR (for inter-subnet handover) to reserve radio resources for the MN to use at the target AP. At the same time, a bidirectional tunnel is created between the two ARs and the serving subnet AR transfers data packets meant for the MN to the target subnet AR. Implementation The implementation process would require modifications to both the subnet ARs and the HAs of the MN. Each subnet AR should have an information table consisting of the local IP and MAC Addresses of the MNs present in its subnet, Subnet IDs of the entire network, AP and cell IDs for all APs and cells under its subnet. Table 1 shows a typical example of the subnet AR s information table. Table 1: An Example of the subnet AR s Information table The local IP address would be composed of the subnet AR s prefix (shown in bold) and the IPv6 address of the MN. From table 1, MN 3 has different subnet AR s prefix indicating that it is away from its home subnet. In addition, each subnet AR stores the information about the neighbouring subnets. Table 2: An Example of Neighbouring subnet information 3. Formulation of Non DAD Procedures In order to eliminate DAD completely, the MN s IP address must be kept unique at all times irrespective of its current location. A stateful address configuration is proposed in this approach. Permanent IP address assignment Every Internet Service Provider (ISP) or network operator has a block of addresses assigned by the Internet Assignment Numbers Authority (IANA). Taking advantage of the large address space of IPv6, the network operators can distribute addresses to each subscriber permanently through stateful address configuration using DHCPv6. Thus, the IP addresses are assigned in a manner similar to mobile numbers in cellular networks. Non DAD test When a MN moves from one subnet to another, there is no need for new CoA configuration and hence no DAD procedure since it has a unique global IP address with which it is reachable. The target subnet AR must register and store the IP address of the MN to its local address table. The HA would store the address current subnet AR to which the MN is attached rather than the new CoA as in standard MIPv6. Thus the binding cache entries of the HA would contain the subnet ARs addresses as proposed in [14]. Simulations study and performance evaluations The simulation study would involve an appropriate network simulator preferably OPNET. Simulation setup would consist of the following network elements operating according to IEEE standard technology and IPv6 as the network layer protocol; One MN, one CN, one current subnet AR, one target subnet AR and one HA. The simulation results would have to be compared with the existing schemes so as to ascertain the credibility of our proposed scheme.

6 V. CONCLUSION This paper has presented an analysis of the network layer handovers in vehicular networks. In order to meet the fast handover requirements in vehicular networks, the predictive mode of the FMIPv6 is seem to be more advantageous. The introduction of sectorisation and the mounting of GPS on moving vehicles would ensure accurate predictions of the MN direction, current position and speed. This reduces the time for discovering the target AR. Further, the demarcation of each sector into handover and non-handover regions would ensure ample time for radio resource allocation in the target subnet. The proposed elimination of the DAD procedure would significantly reduce the network layer handover. This paper basically presented a pure theoretical of the scheme. Actual model implementation and simulations for validation purposes will be conducted in future work. REFERENCES [1] H. Hartenstein and K. P. Laberteaux, "A tutorial survey on vehicular ad hoc networks," IEEE Communications Magazine, vol. 46, pp , [2] S. Kiran, et al., "Architectural Crises in Vehicular Ad Hoc Networks," Global Journal of Computer Science and Technology, vol. 10, pp , [3] K. Zhu, et al., "Mobility Support and Handoff Management in Vehicular Networks: A Survey," Wireless Communications and Mobile Computing, vol. 00, pp. 1-20, [4] D. Johnson, et al., "Mobility Support in IPv6," RFC 3775 ed, [5] R. Koodli, "Fast Handovers for Mobile IPv6," in RFC 4068 ed, [6] P.A. Shah and M. Yousaf, " End-to-end mobility management solutions for TCP: An analysis," in Computer and Information Technology, ICCIT th International Conference, 2008, pp [7] A. Wei, et al., "Improving Mobile IPv6 Handover in Wireless Networks with E-HCF," in IEEE, [8] V.Vassiliou and Z. Zinonos, "An Analysis of the Handover Latency Components in Mobile IPv6," Journal of Internet Engineering, vol. 3(1), pp , [9] N. Blefari-Melazzi, et al., "A Layer 3 Movement Detection Algorithm Driving Handovers in Mobile IP," in Wireless Networks, 2005, pp [10] Javier Carmona-Murillo and J.-L. Gonzalez- Sanchez, "Handover Performance Analysis in Mobile IPv6: A Contribution to Fast Detection Movement," presented at the International Conference on Wireless Information and Systems, [11] R. Li, et al., "An Enhanced Fast Handover with Low Latency for Mobile IPv6," IEEE Transaction on Wireless Communications, vol. 7 NO.7, pp , [12] J.S. Lee, et al., "Fast Handover Scheme Using Temporary CoA in Mobile WiMAX Systems," pp , [13] H. OH, et al., "VMIPv6: A Seamless and Robust Vehicular MIPv6 for Vehicular Wireless Networks and Vehicular Intelligent Transportation Systems (V-Winet/V-ITS)," Journal of Information Science and Engineering, vol. 26, pp , [14] Yu-xuan Hong, et al., "DAD-Less MIPv6 for Reduced Handover Latency," in 2011 Fifth International Conference on Innovative Mobile and Internet Services in Ubiquitous Computing, 2011, pp [15] M.W. Nkambule, et al., "Predicting Mobility on the Wireless Internet Using Sectored-Cell Approach," presented at the SATNAC, Laurence Banda received his BEng degree in Electrical and Electronic Engineering in 2006 from the University of Zambia. He is presently studying towards his dual Masters (MSc and MTech) degrees with F SATI and Tshwane University of Technology. His research interests include Vehicular Networks, Mobility management, Network Planning and Optimization.