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Geographic information-based data delivery in vehicular networks: A survey
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Geographic information-based data delivery in vehicular networks: A survey Sanghyun Ahn Department of Computer Science and Engineering, University of Seoul, Seoul, Republic of Korea Received 3 January 2017; accepted 14 March 2017 Available online xxxx
Abstract With the convergence of IT and automobile technologies, one of the key challenges is to effectively deliver Internet-based data through the vehicular network. The conventional topology-based data routing mechanisms are not suitable in the highly dynamic environment of vehicular networks. The geographic location information acquired from the GPS can help in efficiently finding routes to the destination in the vehicular network. Therefore, in this paper, we provide the survey on geographic addressing and forwarding mechanisms for the vehicular network, especially focusing on the close relationship between addressing and forwarding. c 2017 The Korean Institute of Communications Information Sciences. Publishing Services by Elsevier B.V. This is an open access article under ⃝ the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Geocast; Geographic addressing; Geographic forwarding; Geographic unicast; Vehicular network
1. Introduction Nowadays, vehicles are becoming increasingly akin to huge smartphones on the road, hence attracting the attention of major IT companies such as Google and Samsung. Many IT companies are trying to develop IT-based utilities for vehicles. From the perspective of communications, vehicles can be considered as computing and communication devices with no energy limits and high computing power, which is a good aspect of the vehicular network. One of the major challenges faced in transforming vehicles to sophisticated communication devices is the dynamic network topology due to the rapid changes in vehicular speeds. From the service point of view, appropriate information distribution about safety and location-based advertising is the most important aspect of the vehicular communication environment. Therefore, the geographic information-based data forwarding capability is one of the must-have functionalities in the dynamically changing vehicular network. The Internet is connecting everything on the globe, including vehicles. For vehicles to effectively communicate on the Internet, assigning Internet addresses to vehicles is essential. Due to the lack of IPv4 addresses, IPv6 [1] is the only available E-mail address: [email protected]. Peer review under responsibility of The Korean Institute of Communications Information Sciences.
and the ideal option. In this paper, we focus on how vehicles can effectively communicate on the Internet. There have been extensive studies on geographic forwarding (or routing) in vehicular networks [2]. However, geographic addressing [3] has attracted relatively less attention. Now, to support geographic information-based data delivery, geographic addressing has to be resolved on priority. Most of the work on geographic addressing deal with ways of including geographic information in addresses, but does not consider how the geographic address information is to be utilized in data forwarding. On the other hand, most geographic forwarding or routing mechanisms focus only on how to forward or route messages based on the given geographic information. In this paper, we explore the technologies for location-based data delivery in the vehicular network from the perspective of the inter-relationship between geographic addressing and forwarding. The rest of the paper is organized as follows. In Section 2, the vehicular network system is described in brief. The enabling technologies for geographic addressing and forwarding in the vehicular network are described in Sections 3 and 4, respectively. Finally, Section 5 concludes this paper. 2. Vehicular network system for geographic informationbased data delivery The Vehicular Network System (VNS) with geographic forwarding capable vehicles, GVNS, is composed of On-Board
http://dx.doi.org/10.1016/j.icte.2017.03.003 c 2017 The Korean Institute of Communications Information Sciences. Publishing Services by Elsevier B.V. This is an open access article under the 2405-9595/⃝ CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Units (OBUs), Roadside Units (RSUs) and Location Databases (LDBs). OBUs are vehicular gateways that can transmit and receive data from nearby OBUs or RSUs. RSUs are access points (APs), Base Stations (BSs) and the gateways for Roadside Sensor Networks. For an OBU to communicate with other OBUs and RSUs, the OBU has to be assigned with at least one address. Each vehicle is equipped with a GPS and the OBU obtains its geographic location information, such as longitude and latitude, from the GPS. For the proper operation of GVNS, the following functionalities are required to be provisioned: – Geographic addressing • Address assignment for geographic forwarding • Inclusion of geographic location information – Geographic forwarding • Geographic unicast • Geocast and geographic broadcast 3. Geographic addressing for GVNS These days, almost all the vehicles are equipped with GPSs, so they can be identified by their own geographic locations to some extent (but not with high accuracy because of the discrepancy between the geographic information obtained from the GPS and the real position information). This implies that the geographic location information of a vehicle can be used as a part of the vehicle identifier. 3.1. Address assignment for geographic forwarding The Mobile Ad hoc Network (MANET) and the Vehicular Ad hoc Network (VANET) are similar in terms of the nodes communicating via multiple wireless links without any wired infrastructure. However, VANET differs from MANET since it accounts for a large number of vehicles moving in semi-unbounded road layouts at relatively high speeds. Lots of vehicles require lots of addresses, so the 128-bit IPv6 addressing is a good scalable solution for a VANET. High speeds imply that the topology-based addressing such as the address assignment on the basis of the network prefix is not appropriate because a vehicle has to change its address upon encountering a new network prefix. M. Fazio et al. [4] proposed the Vehicular Address Auto-configuration (VAC), which is the address configuration mechanism based on the Dynamic Host Configuration Protocol (DHCP) and the concept of leaders for VANET. VAC selects a set of vehicles as the leaders, acting as the distributed DHCP servers, in a linear chain for efficient dynamic address assignment. However, VAC is a topology-based addressing mechanism not appropriate for geographic forwarding and the maintenance of the leader chain is not a trivial task. The Geographically Scoped stateless Address Configuration (GeoSAC) [5] includes longitude and latitude information in the IPv6 address by adapting the IPv6 Stateless Address Autoconfiguration (SLAAC) for VANET. Thus, GeoSAC is independent of topology (i.e., free from the network prefixes) and good for identifying a vehicle based on its geographic location information. However, the geographic location of a vehicle changes
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as the vehicle moves, so directly including the geographic location information as a part of the identifier (e.g., the IPv6 address) is not meaningful in the time-wise aspect regardless of the accuracy of the geographic location information. Similar to GeoSAC, W. Vandenberghe et al., in their research [3], proposed to include the geographic location information such as latitude, longitude, radius, heading, in the IPv6 address field. In the research by B. Meijerink et al. [6], the authors divide the Earth based on latitude and longitude into sections (rectangles), each of whose address can be included in the IPv6 destination address. This addressing scheme can be used not only in wireless ad hoc networks but also in fixed networks. This mechanism is good for addressing a geographical region, but not for a specific location. Thus, the usage of this addressing scheme is limited to geocasting. S. Ahn [7] proposed the usage of one or more address ranges for geographic forwarding, possibly from the address range with the prefix “100” [8]. In this case, IPv6 addresses in these address ranges are independent of topology and can be derived from globally unique identifiers assigned by automobile manufacturers. The major purpose of these address ranges is to indicate that the IPv6 datagrams with such addresses should be forwarded by geographic forwarding. One of the objectives of using the network prefix is topology-based routing. However, once we decide to use geographic forwarding, the network prefix is meaningless. Distinct from the above-mentioned schemes, the research by T. Fioreze et al. [9] proposes to extend the Domain Name System (DNS) so that it can answer geographically scoped queries, with geographic location information of those RSUs that cover the area, with the corresponding IPv6 addresses. The GeoNetworking (GeoNet) protocol [10] of ETSI configures addresses on the basis of the data link layer addresses by separating out the geographic location information from the addresses. In other words, the address does not reflect the geographic location information. 3.2. Inclusion of geographic location information For geographic forwarding to work, the geographic location information of the destination and/or that of the sender need to be specified in the message (e.g., the IP datagram) to be delivered. There are several ways to put the geographic location information in the message. As explained in Section 3. A, GeoSAC includes the geographic location information of the source and the destination in the IPv6 source and destination address fields. However, as explained in Section 4, the geographic forwarding mechanisms may require the geographic location information of the sender. In this regard, they do not have the capability to satisfy this requirement. The research by B. Meijerink et al. [6] specifies the translated geographic location information of a target region in the IPv6 destination address. The longitude and latitude information is translated into a binary bit pattern so that it can be included in the IPv6 address. This also does not have the
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capability of specifying the geographic location information of the sender. GeoNet defines a sub-IP layer whose corresponding header, the GeoNetworking header, is located between the layer 2 and the layer 3 headers. In the GeoNetworking header, the geographic location information of the source and that of the destination are specified. GeoNet is capable of specifying the geographic location of the sender, but the sender location information is not optional. More specifically, the GeoNet common header format has the sender position vector (SO PV, the sender geographic location), which is mandatory. This means that the GeoNetworking header is not optimized in terms of message size, which is not desirable for bandwidth-limited wireless network environments. Since GeoNet is not specifically proposed for IPv6, the GeoNetworking header has duplicate information such as hop limit. Therefore, we can say that GeoNetworking is not an optimal solution for supporting geographic forwarding in the IPv6-based GVNS even though an open source implementation with a network protocol stack combining IPv6 and GeoNet called CarGeo6 is already provided [11]. S. Ahn [7] proposed to include geographic location information for geographic forwarding in the IPv6 Hop-by-Hop Options extension header and defines two new IPv6 Hop-byHop options for geographic forwarding. The author proposed to utilize the IPv6 Hop-by-Hop Options extension header since the geographic location information of the destination and/or that of the sender have to be checked at each intermediate node for the decision of forwarding the datagram. To summarize, the geographic location information is specified at the 2.5th layer in GeoNetworking, at the 3rd layer in GeoSAC, the research by B. Meijerink et al. [6] and the research by S. Ahn [7] for GVNS. GeoSAC and the research by B. Meijerink et al. [6] are different from the research by S. Ahn [7] in that both of them utilize the fixed IPv6 header and the research by S. Ahn [7] utilizes the IPv6 Hop-by-Hop Options extension header. As pointed out in GeoSAC, the usage of the IPv6 extension headers may produce more overheads than using the fixed IPv6 header itself. However, GeoSAC and the research by B. Meijerink et al. [6] are limited in their capability to express geographic location information required for geographic forwarding in GVNS. 4. Geographic forwarding for GVNS In the vehicular network, vehicles move freely on the road. Hence, the geographic forwarding mechanisms are more suitable than the conventional topology-based routing mechanisms. Similar to the unicast, multicast, and broadcast of the topology-based routing mechanisms, the geographical forwarding mechanisms provide unicast, geocast and broadcast. Geocast means geographic multicast by which datagrams (e.g., datagrams with advertisement information) are delivered to all the nodes in the specified area using geographic unicast and geographic broadcast. 4.1. Geographic unicast Extensive studies have been conducted on geographic unicasts for VANET [2,12,13]. The Greedy Perimeter Stateless
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Routing (GPSR) [14] and the Contention-Based Forwarding (CBF) [15] are the representative geographic forwarding mechanisms proposed for MANET. Because MANET and VANET differ in many aspects such as road layout, many of VANET geographic unicast routing mechanisms adapt GPSR and CBF to the vehicular network environment. GPSR uses the greedy forwarding mechanism until the destination is reached or a void is confronted. In the latter case, the perimeter forwarding mode is applied to detour the void. The VANET geographic routing mechanisms based on GPSR take into consideration road conditions such as intersections, vehicular density. For example, C. Lochert et al. [16] considered intersections in figuring out the route to the destination and used source routing with the greedy forwarding on each road segment between two consecutive intersections. The VANET geographic routing mechanisms based on GPSR require the geographic location information of 1-hop neighbors by periodically exchanging beacon messages. Based on that information, the sender decides the next-hop node from its 1-hop neighbors which makes the longest progress to the destination. We categorize this type of geographic unicast forwarding as the sender-based approach in which the geographic information of the sender need not be specified in the forwarded message. The reason for this is that once the receiver has the message, it can figure out its next-hop node without the sender location information. Since periodic beaconing may incur frequent collisions in resource-constrained multi-hop wireless networks, CBF proposes a beaconless approach by having the sender just forward the message without specifying the receiver. In CBF, each of the recipients decides whether it can be the next forwarder or not, based on its timer value computed from the progress to the destination it will make. We call this the receiver-based approach in which the geographic location information of the sender must be contained in the datagram. 4.2. Geocast and geographic broadcast Geocast [17] is useful for disseminating announcement or advertising messages to all the nodes in a specific target area (or Zone of Relevance (ZOR)) and, for that, ZoR has to be specified in the geocast message. If ZoR is constrained in a road segment, the shape of ZoR nearly resembles a rectangle. However, if ZoR is sufficiently large to cover an intersection and its connected road segments, the shape of ZoR is not rectangular any more. Thus, the simplest way of specifying ZoR is the circle with the center point coordinates and the radius. A research by T. Jochle et al. [18] analyzes the impact of the geocast ZoR on communication efficiency and network overhead via simulations, and shows that a circular area performs the best. In order to accomplish geocasting in VANET, the geocast message has to be carried to any node in ZoR at first and subsequently, the message needs to be disseminated within ZoR. The dissemination within ZoR can be done by any (geographic) broadcast mechanisms such as flooding, InterVehicle Geocast (IVG) [19], probabilistic IVG (p-IVG) [20] Stateless Opportunistic Forwarding (SOF) [21] or GPS-Based
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Broadcasting (GBB) [22] In simple flooding, each node rebroadcasts the received geocast message to its neighbors only once, which may incur the broadcast storm problem in a dense network. In IVG, rebroadcasting is determined based on the timer value computed from the distance between the sender (the previous hop node) and the receiver (the candidate node for rebroadcast). p-IVG is an adapted version of IVG for dense vehicular networks, which probabilistically adjusts rebroadcast based on vehicular density. In SOF, the next forwarder is selected at random by the current forwarder (i.e., the sender) and, in GBB, the farthest neighbor is chosen as the next forwarder by the sender. The performance of geographic broadcast depends on the vehicular density within ZoR. For a dense area, the broadcast storm problem has to be resolved by using timer, probability, distance, priority or other factors to control rebroadcasting. In a sparse area, the dissemination speed and the dissemination ratio are the important performance factors to be considered. For that, Y. Li et al. [23] analyze the performance of dissemination mechanisms for intermittently connected vehicular networks and provides the analytic bounds on dissemination distance and hitting time of SOF and GBB. O. K. Tonguz et al. [24] propose a VANET broadcast mechanism which can be applied to both dense and sparse VANET environments. Independent of geocast, geographic broadcast can be used by a vehicle, an RSU or a sensor gateway to notify the vehicles heading towards critical or dangerous situations [24,25]. In this case, other factors may be additionally considered in rebroadcast decision. For example, the research by Rajendran et al. [25] considers the direction of dissemination in selecting the preferred forwarder. A variant of geocast is abiding geocast [26–28], which delivers datagrams to all the vehicles passing by ZoR during the specified time. The main issues of abiding geocast are reliability (i.e., message reception ratio), message transmission overheads etc. 5. Conclusions Vehicles are becoming more and more intelligent with support from IT and the Vehicle-to-Vehicle (V2V) communication is one of its most significant aspects. Owing to the dynamically changing topology of the vehicular network, geographic forwarding or routing mechanisms are desirable for delivering data via V2V communications. In this study, we surveyed geographic addressing and forwarding mechanisms for VANET. Geographic forwarding/routing has been studied more extensively in comparison to geographic addressing, and very few researchers have studied geographic addressing and forwarding simultaneously. Because geographic addressing and forwarding are very tightly coupled, we studied the available studies and tried to figure out the mechanisms from the perspectives of the researchers. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Koreas government (MSIP) (No. NRF-2015R1A2A2A04005646).
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Conflict of interest The authors declare that there is no conflict of interest in this paper. References [1] S. Deering, R. Hinden, Internet protocol, version 6 (IPv6) Specification, IETF RFC 2460, 1998. [2] P.G. Nirmal, A.R. Deshmukh, S.S. Dorle, A survey on geographical routing strategies in VANET, Int. J. Comput. Appl. (2016). [3] W. Vandenberghe, E.V. de Velde, C. Blondia, I. Moerman, P. Demeester, Vehicular ad hoc networking based on the incorporation of geographical information in the IPv6 header, EURASIP J. Wirel. Comm. Netw. (2012). [4] M. Fazio, C. Palazzi, S. Das, M. Gerla, Vehicular Address Configuration, in: Proc. IEEE Workshop on Automotive Networking and Applications, AutoNet, GLOBECOM, 2006. [5] R. Baldessari, C.J. Bernardos, M. Calderon, GeoSAC – Scalable address autoconfiguration for VANET using geographic networking concepts, in: Proc. IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, 2008. [6] B. Meijerink, M. Baratchi, G. Heijenk, An efficient geographical addressing scheme for the Internet, in: Proc. IFIP WG 6.2 International Conference on Wired/Wireless Internet Communications, WWIC, 2016. [7] S. Ahn, IPv6-based geographical forwarding in the vehicular network environment for ITS, in: Proc. International Conference on Information Networking, 2017. [8] R. Hinden, S. Deering, IP version 6 addressing architecture, IETF RFC 4291, 2006. [9] T. Fioreze, G. Heijenk, Extending the Domain Name System (DNS) to provide geographical addressing towards vehicular ad-hoc networks (VANETs), in: Proc. IEEE Vehicular Networking Conference, VNC, 2011. [10] “Intelligent transport systems (ITS); Vehicular communications; Geonetworking; Part4: Geographical addressing and forwarding for point-to-point and point-to-multipoint communications; Sub-part1: Media-independent functionality,” ETSI TS 102 636-4-1 V1.1.1, 2011. [11] CarGeo6 [Online]. Available: http://www.cargeo6.org/doku.php. [12] S. Parvin, H.A. Sarram, G. Mirjalily, F. Adibnia, A survey on void handling techniques for geographic routing in VANET network, Int. J. Grid Distrib. Comput. (2015). [13] H. Keshavarz, R. Md Noor, Beacon-based geographic routing protocols in vehicular ad hoc networks: A survey and taxonomy, in: Proc. IEEE Symposium on Wireless Technology and Applications, ISWTA, 2012. [14] B. Karp, H.T. Kung, GPSR: Greedy perimeter stateless routing for wireless networks, in: Proc. International Conference on Mobile Computing and Networking, Mobicom, 2000. [15] H. F¨ubler, J. Widmer, M. K¨asemann, M. Mauve, H. Hartenstein, Contention-based forwarding for mobile ad hoc networks, Ad Hoc Networks (2003). [16] C. Lochert, H. Hartenstein, J. Tian, H. F¨ussler, D. Hermann, M. Mauve, A routing strategy for vehicular ad hoc networks in city environments, in: Proc. IEEE Intelligent Vehicles Symposium, 2003. [17] J.C. Navas, T. Imielinski, Geocast – Geographic addressing and routing, in: Proc. ACM/IEEE International Conference on Mobile Computing and Networking, Mobicom, 1997. [18] T. Jochle, B. Wiedersheim, F. Schaub, M. Weber, Efficiency analysis of geocast target region specifications for VANET applications, in: Proc. IEEE Vehicular Networking Conference, 2012. [19] A. Bachir, A. Benslimane, A multicast protocol in ad hoc networks: intervehicle geocast, in: Proc. IEEE Vehicular Technology Conference, VTCSpring, 2003. [20] K. Ibrahim, M.C. Weigle, M. Abuelela, p-IVG: Probabilistic intervehicle geocast for dense vehicular networks, in: Proc. IEEE Vehicular Technology Conference, VTC-Spring, 2009. [21] C.-K. Chau, P. Basu, Analysis of latency of stateless opportunistic forwarding in intermittently connected networks, IEEE/ACM Trans. Netw. (2011). [22] M.-T. Sun, W.-C. Feng, T.-H. Lai, K. Yamada, H. Okada, K. Fujimura, GPS-based message broadcast for adaptive inter-vehicle communications, in: Proc. IEEE Vehicular Technology Conference, VTC-Fall, 2000.
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[26] C. Haihofer, T. Leinmuller, E. Schoch, Abiding geocast: Time-stable geocast for ad hoc networks, in: Proc. ACM International Workshop on Vehicular Ad Hoc Networks, VANET, 2005. [27] M. Fiore, C. Casetti, C.-F. Chiasserini, D. Borsetti, Persistent localized broadcasting in VANETs, IEEE J. Sel. Areas Commun. (2013). [28] X. Zhang, L. Yan, W. Li, Efficient and reliable abiding geocast based on carrier sets for vehicular ad hoc networks, IEEE Wirel. Commun. Lett. (2016).
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Geographic information-based data delivery in vehicular networks: A survey Sanghyun Ahn Department of Computer Science and Engineering, University of Seoul, Seoul, Republic of Korea Received 3 January 2017; accepted 14 March 2017 Available online xxxx
Abstract With the convergence of IT and automobile technologies, one of the key challenges is to effectively deliver Internet-based data through the vehicular network. The conventional topology-based data routing mechanisms are not suitable in the highly dynamic environment of vehicular networks. The geographic location information acquired from the GPS can help in efficiently finding routes to the destination in the vehicular network. Therefore, in this paper, we provide the survey on geographic addressing and forwarding mechanisms for the vehicular network, especially focusing on the close relationship between addressing and forwarding. c 2017 The Korean Institute of Communications Information Sciences. Publishing Services by Elsevier B.V. This is an open access article under ⃝ the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Geocast; Geographic addressing; Geographic forwarding; Geographic unicast; Vehicular network
1. Introduction Nowadays, vehicles are becoming increasingly akin to huge smartphones on the road, hence attracting the attention of major IT companies such as Google and Samsung. Many IT companies are trying to develop IT-based utilities for vehicles. From the perspective of communications, vehicles can be considered as computing and communication devices with no energy limits and high computing power, which is a good aspect of the vehicular network. One of the major challenges faced in transforming vehicles to sophisticated communication devices is the dynamic network topology due to the rapid changes in vehicular speeds. From the service point of view, appropriate information distribution about safety and location-based advertising is the most important aspect of the vehicular communication environment. Therefore, the geographic information-based data forwarding capability is one of the must-have functionalities in the dynamically changing vehicular network. The Internet is connecting everything on the globe, including vehicles. For vehicles to effectively communicate on the Internet, assigning Internet addresses to vehicles is essential. Due to the lack of IPv4 addresses, IPv6 [1] is the only available E-mail address: [email protected]. Peer review under responsibility of The Korean Institute of Communications Information Sciences.
and the ideal option. In this paper, we focus on how vehicles can effectively communicate on the Internet. There have been extensive studies on geographic forwarding (or routing) in vehicular networks [2]. However, geographic addressing [3] has attracted relatively less attention. Now, to support geographic information-based data delivery, geographic addressing has to be resolved on priority. Most of the work on geographic addressing deal with ways of including geographic information in addresses, but does not consider how the geographic address information is to be utilized in data forwarding. On the other hand, most geographic forwarding or routing mechanisms focus only on how to forward or route messages based on the given geographic information. In this paper, we explore the technologies for location-based data delivery in the vehicular network from the perspective of the inter-relationship between geographic addressing and forwarding. The rest of the paper is organized as follows. In Section 2, the vehicular network system is described in brief. The enabling technologies for geographic addressing and forwarding in the vehicular network are described in Sections 3 and 4, respectively. Finally, Section 5 concludes this paper. 2. Vehicular network system for geographic informationbased data delivery The Vehicular Network System (VNS) with geographic forwarding capable vehicles, GVNS, is composed of On-Board
http://dx.doi.org/10.1016/j.icte.2017.03.003 c 2017 The Korean Institute of Communications Information Sciences. Publishing Services by Elsevier B.V. This is an open access article under the 2405-9595/⃝ CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Units (OBUs), Roadside Units (RSUs) and Location Databases (LDBs). OBUs are vehicular gateways that can transmit and receive data from nearby OBUs or RSUs. RSUs are access points (APs), Base Stations (BSs) and the gateways for Roadside Sensor Networks. For an OBU to communicate with other OBUs and RSUs, the OBU has to be assigned with at least one address. Each vehicle is equipped with a GPS and the OBU obtains its geographic location information, such as longitude and latitude, from the GPS. For the proper operation of GVNS, the following functionalities are required to be provisioned: – Geographic addressing • Address assignment for geographic forwarding • Inclusion of geographic location information – Geographic forwarding • Geographic unicast • Geocast and geographic broadcast 3. Geographic addressing for GVNS These days, almost all the vehicles are equipped with GPSs, so they can be identified by their own geographic locations to some extent (but not with high accuracy because of the discrepancy between the geographic information obtained from the GPS and the real position information). This implies that the geographic location information of a vehicle can be used as a part of the vehicle identifier. 3.1. Address assignment for geographic forwarding The Mobile Ad hoc Network (MANET) and the Vehicular Ad hoc Network (VANET) are similar in terms of the nodes communicating via multiple wireless links without any wired infrastructure. However, VANET differs from MANET since it accounts for a large number of vehicles moving in semi-unbounded road layouts at relatively high speeds. Lots of vehicles require lots of addresses, so the 128-bit IPv6 addressing is a good scalable solution for a VANET. High speeds imply that the topology-based addressing such as the address assignment on the basis of the network prefix is not appropriate because a vehicle has to change its address upon encountering a new network prefix. M. Fazio et al. [4] proposed the Vehicular Address Auto-configuration (VAC), which is the address configuration mechanism based on the Dynamic Host Configuration Protocol (DHCP) and the concept of leaders for VANET. VAC selects a set of vehicles as the leaders, acting as the distributed DHCP servers, in a linear chain for efficient dynamic address assignment. However, VAC is a topology-based addressing mechanism not appropriate for geographic forwarding and the maintenance of the leader chain is not a trivial task. The Geographically Scoped stateless Address Configuration (GeoSAC) [5] includes longitude and latitude information in the IPv6 address by adapting the IPv6 Stateless Address Autoconfiguration (SLAAC) for VANET. Thus, GeoSAC is independent of topology (i.e., free from the network prefixes) and good for identifying a vehicle based on its geographic location information. However, the geographic location of a vehicle changes
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as the vehicle moves, so directly including the geographic location information as a part of the identifier (e.g., the IPv6 address) is not meaningful in the time-wise aspect regardless of the accuracy of the geographic location information. Similar to GeoSAC, W. Vandenberghe et al., in their research [3], proposed to include the geographic location information such as latitude, longitude, radius, heading, in the IPv6 address field. In the research by B. Meijerink et al. [6], the authors divide the Earth based on latitude and longitude into sections (rectangles), each of whose address can be included in the IPv6 destination address. This addressing scheme can be used not only in wireless ad hoc networks but also in fixed networks. This mechanism is good for addressing a geographical region, but not for a specific location. Thus, the usage of this addressing scheme is limited to geocasting. S. Ahn [7] proposed the usage of one or more address ranges for geographic forwarding, possibly from the address range with the prefix “100” [8]. In this case, IPv6 addresses in these address ranges are independent of topology and can be derived from globally unique identifiers assigned by automobile manufacturers. The major purpose of these address ranges is to indicate that the IPv6 datagrams with such addresses should be forwarded by geographic forwarding. One of the objectives of using the network prefix is topology-based routing. However, once we decide to use geographic forwarding, the network prefix is meaningless. Distinct from the above-mentioned schemes, the research by T. Fioreze et al. [9] proposes to extend the Domain Name System (DNS) so that it can answer geographically scoped queries, with geographic location information of those RSUs that cover the area, with the corresponding IPv6 addresses. The GeoNetworking (GeoNet) protocol [10] of ETSI configures addresses on the basis of the data link layer addresses by separating out the geographic location information from the addresses. In other words, the address does not reflect the geographic location information. 3.2. Inclusion of geographic location information For geographic forwarding to work, the geographic location information of the destination and/or that of the sender need to be specified in the message (e.g., the IP datagram) to be delivered. There are several ways to put the geographic location information in the message. As explained in Section 3. A, GeoSAC includes the geographic location information of the source and the destination in the IPv6 source and destination address fields. However, as explained in Section 4, the geographic forwarding mechanisms may require the geographic location information of the sender. In this regard, they do not have the capability to satisfy this requirement. The research by B. Meijerink et al. [6] specifies the translated geographic location information of a target region in the IPv6 destination address. The longitude and latitude information is translated into a binary bit pattern so that it can be included in the IPv6 address. This also does not have the
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capability of specifying the geographic location information of the sender. GeoNet defines a sub-IP layer whose corresponding header, the GeoNetworking header, is located between the layer 2 and the layer 3 headers. In the GeoNetworking header, the geographic location information of the source and that of the destination are specified. GeoNet is capable of specifying the geographic location of the sender, but the sender location information is not optional. More specifically, the GeoNet common header format has the sender position vector (SO PV, the sender geographic location), which is mandatory. This means that the GeoNetworking header is not optimized in terms of message size, which is not desirable for bandwidth-limited wireless network environments. Since GeoNet is not specifically proposed for IPv6, the GeoNetworking header has duplicate information such as hop limit. Therefore, we can say that GeoNetworking is not an optimal solution for supporting geographic forwarding in the IPv6-based GVNS even though an open source implementation with a network protocol stack combining IPv6 and GeoNet called CarGeo6 is already provided [11]. S. Ahn [7] proposed to include geographic location information for geographic forwarding in the IPv6 Hop-by-Hop Options extension header and defines two new IPv6 Hop-byHop options for geographic forwarding. The author proposed to utilize the IPv6 Hop-by-Hop Options extension header since the geographic location information of the destination and/or that of the sender have to be checked at each intermediate node for the decision of forwarding the datagram. To summarize, the geographic location information is specified at the 2.5th layer in GeoNetworking, at the 3rd layer in GeoSAC, the research by B. Meijerink et al. [6] and the research by S. Ahn [7] for GVNS. GeoSAC and the research by B. Meijerink et al. [6] are different from the research by S. Ahn [7] in that both of them utilize the fixed IPv6 header and the research by S. Ahn [7] utilizes the IPv6 Hop-by-Hop Options extension header. As pointed out in GeoSAC, the usage of the IPv6 extension headers may produce more overheads than using the fixed IPv6 header itself. However, GeoSAC and the research by B. Meijerink et al. [6] are limited in their capability to express geographic location information required for geographic forwarding in GVNS. 4. Geographic forwarding for GVNS In the vehicular network, vehicles move freely on the road. Hence, the geographic forwarding mechanisms are more suitable than the conventional topology-based routing mechanisms. Similar to the unicast, multicast, and broadcast of the topology-based routing mechanisms, the geographical forwarding mechanisms provide unicast, geocast and broadcast. Geocast means geographic multicast by which datagrams (e.g., datagrams with advertisement information) are delivered to all the nodes in the specified area using geographic unicast and geographic broadcast. 4.1. Geographic unicast Extensive studies have been conducted on geographic unicasts for VANET [2,12,13]. The Greedy Perimeter Stateless
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Routing (GPSR) [14] and the Contention-Based Forwarding (CBF) [15] are the representative geographic forwarding mechanisms proposed for MANET. Because MANET and VANET differ in many aspects such as road layout, many of VANET geographic unicast routing mechanisms adapt GPSR and CBF to the vehicular network environment. GPSR uses the greedy forwarding mechanism until the destination is reached or a void is confronted. In the latter case, the perimeter forwarding mode is applied to detour the void. The VANET geographic routing mechanisms based on GPSR take into consideration road conditions such as intersections, vehicular density. For example, C. Lochert et al. [16] considered intersections in figuring out the route to the destination and used source routing with the greedy forwarding on each road segment between two consecutive intersections. The VANET geographic routing mechanisms based on GPSR require the geographic location information of 1-hop neighbors by periodically exchanging beacon messages. Based on that information, the sender decides the next-hop node from its 1-hop neighbors which makes the longest progress to the destination. We categorize this type of geographic unicast forwarding as the sender-based approach in which the geographic information of the sender need not be specified in the forwarded message. The reason for this is that once the receiver has the message, it can figure out its next-hop node without the sender location information. Since periodic beaconing may incur frequent collisions in resource-constrained multi-hop wireless networks, CBF proposes a beaconless approach by having the sender just forward the message without specifying the receiver. In CBF, each of the recipients decides whether it can be the next forwarder or not, based on its timer value computed from the progress to the destination it will make. We call this the receiver-based approach in which the geographic location information of the sender must be contained in the datagram. 4.2. Geocast and geographic broadcast Geocast [17] is useful for disseminating announcement or advertising messages to all the nodes in a specific target area (or Zone of Relevance (ZOR)) and, for that, ZoR has to be specified in the geocast message. If ZoR is constrained in a road segment, the shape of ZoR nearly resembles a rectangle. However, if ZoR is sufficiently large to cover an intersection and its connected road segments, the shape of ZoR is not rectangular any more. Thus, the simplest way of specifying ZoR is the circle with the center point coordinates and the radius. A research by T. Jochle et al. [18] analyzes the impact of the geocast ZoR on communication efficiency and network overhead via simulations, and shows that a circular area performs the best. In order to accomplish geocasting in VANET, the geocast message has to be carried to any node in ZoR at first and subsequently, the message needs to be disseminated within ZoR. The dissemination within ZoR can be done by any (geographic) broadcast mechanisms such as flooding, InterVehicle Geocast (IVG) [19], probabilistic IVG (p-IVG) [20] Stateless Opportunistic Forwarding (SOF) [21] or GPS-Based
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Broadcasting (GBB) [22] In simple flooding, each node rebroadcasts the received geocast message to its neighbors only once, which may incur the broadcast storm problem in a dense network. In IVG, rebroadcasting is determined based on the timer value computed from the distance between the sender (the previous hop node) and the receiver (the candidate node for rebroadcast). p-IVG is an adapted version of IVG for dense vehicular networks, which probabilistically adjusts rebroadcast based on vehicular density. In SOF, the next forwarder is selected at random by the current forwarder (i.e., the sender) and, in GBB, the farthest neighbor is chosen as the next forwarder by the sender. The performance of geographic broadcast depends on the vehicular density within ZoR. For a dense area, the broadcast storm problem has to be resolved by using timer, probability, distance, priority or other factors to control rebroadcasting. In a sparse area, the dissemination speed and the dissemination ratio are the important performance factors to be considered. For that, Y. Li et al. [23] analyze the performance of dissemination mechanisms for intermittently connected vehicular networks and provides the analytic bounds on dissemination distance and hitting time of SOF and GBB. O. K. Tonguz et al. [24] propose a VANET broadcast mechanism which can be applied to both dense and sparse VANET environments. Independent of geocast, geographic broadcast can be used by a vehicle, an RSU or a sensor gateway to notify the vehicles heading towards critical or dangerous situations [24,25]. In this case, other factors may be additionally considered in rebroadcast decision. For example, the research by Rajendran et al. [25] considers the direction of dissemination in selecting the preferred forwarder. A variant of geocast is abiding geocast [26–28], which delivers datagrams to all the vehicles passing by ZoR during the specified time. The main issues of abiding geocast are reliability (i.e., message reception ratio), message transmission overheads etc. 5. Conclusions Vehicles are becoming more and more intelligent with support from IT and the Vehicle-to-Vehicle (V2V) communication is one of its most significant aspects. Owing to the dynamically changing topology of the vehicular network, geographic forwarding or routing mechanisms are desirable for delivering data via V2V communications. In this study, we surveyed geographic addressing and forwarding mechanisms for VANET. Geographic forwarding/routing has been studied more extensively in comparison to geographic addressing, and very few researchers have studied geographic addressing and forwarding simultaneously. Because geographic addressing and forwarding are very tightly coupled, we studied the available studies and tried to figure out the mechanisms from the perspectives of the researchers. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Koreas government (MSIP) (No. NRF-2015R1A2A2A04005646).
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