IPv6 Wireless Access in Vehicular Environments (IPWAVE): Problem Statement and Use Cases

3.1. V2V

The use cases of V2V networking discussed in this section include¶

• Context-aware navigation for safe driving and collision avoidance;¶
• Cooperative adaptive cruise control in a roadway;¶
• Platooning in a highway;¶
• Cooperative environment sensing;¶
• Collision avoidance service of end systems of Urban Air Mobility (UAM) [I-D.templin-ipwave-uam-its].¶

These five techniques will be important elements for autonomous vehicles, which may be either terrestrial vehicles or UAM end systems.¶

Context-Aware Safety Driving (CASD) navigator [CASD] can help drivers to drive safely by alerting them to dangerous obstacles and situations. That is, a CASD navigator displays obstacles or neighboring vehicles relevant to possible collisions in real-time through V2V networking. CASD provides vehicles with a class-based automatic safety action plan, which considers three situations, namely, the Line-of-Sight unsafe, Non-Line-of-Sight unsafe, and safe situations. This action plan can be put into action among multiple vehicles using V2V networking.¶

Cooperative Adaptive Cruise Control (CACC) [CA-Cruise-Control] helps individual vehicles to adapt their speed autonomously through V2V communication among vehicles according to the mobility of their predecessor and successor vehicles in an urban roadway or a highway. Thus, CACC can help adjacent vehicles to efficiently adjust their speed in an interactive way through V2V networking in order to avoid a collision.¶

Platooning [Truck-Platooning] allows a series (or group) of vehicles (e.g., trucks) to follow each other very closely. Trucks can use V2V communication in addition to forward sensors in order to maintain constant clearance between two consecutive vehicles at very short gaps (from 3 meters to 10 meters). Platooning can maximize the throughput of vehicular traffic in a highway and reduce the gas consumption because the leading vehicle can help the following vehicles to experience less air resistance.¶

Cooperative-environment-sensing use cases suggest that vehicles can share environmental information (e.g., air pollution, hazards/obstacles, slippery areas by snow or rain, road accidents, traffic congestion, and driving behaviors of neighboring vehicles) from various vehicle-mounted sensors, such as radars, LiDARs, and cameras, with other vehicles and pedestrians. [Automotive-Sensing] introduces millimeter-wave vehicular communication for massive automotive sensing. A lot of data can be generated by those sensors, and these data typically need to be routed to different destinations. In addition, from the perspective of driverless vehicles, it is expected that driverless vehicles can be mixed with driver-operated vehicles. Through cooperative environment sensing, driver-operated vehicles can use environmental information sensed by driverless vehicles for better interaction with the other vehicles and environment. Vehicles can also share their intended maneuvering information (e.g., lane change, speed change, ramp in-and-out, cut-in, and abrupt braking) with neighboring vehicles. Thus, this information sharing can help the vehicles behave as more efficient traffic flows and minimize unnecessary acceleration and deceleration to achieve the best ride comfort.¶

A collision avoidance service of UAM end systems in air can be envisioned as a use case in air vehicular environments. This use case is similar to the context-aware navigator for terrestrial vehicles. Through V2V coordination, those UAM end systems (e.g., drones) can avoid a dangerous situation (e.g., collision) in three-dimensional space rather than two-dimensional space for terrestrial vehicles. Also, UAM end systems (e.g., flying car) with only a few meters off the ground can communicate with terrestrial vehicles with wireless communication technologies (e.g., DSRC, LTE, and C-V2X). Thus, V2V means any vehicle to any vehicle, whether the vehicles are ground-level or not.¶

To encourage more vehicles to participate in this cooperative environmental sensing, a reward system will be needed. Sensing activities of each vehicle need to be logged in either a central way through a logging server (e.g., TCC) in the vehicular cloud or a distributed way (e.g., blockchain [Bitcoin]) through other vehicles or infrastructure. In the case of a blockchain, each sensing message from a vehicle can be treated as a transaction and the neighboring vehicles can play the role of peers in a consensus method of a blockchain [Bitcoin][Vehicular-BlockChain].¶

To support applications of these V2V use cases, the required functions of IPv6 include IPv6-based packet exchange and secure, safe communication between two vehicles. For the support of V2V under multiple radio technologies (e.g., DSRC and 5G V2X), refer to Appendix A.¶

3.2. V2I

The use cases of V2I networking discussed in this section include¶

• Navigation service;¶
• Energy-efficient speed recommendation service;¶
• Accident notification service;¶
• Electric vehicle (EV) charging service;¶
• UAM navigation service with efficient battery charging.¶

A navigation service, for example, the Self-Adaptive Interactive Navigation Tool (SAINT) [SAINT], using V2I networking interacts with a TCC for the large-scale/long-range road traffic optimization and can guide individual vehicles along appropriate navigation paths in real time. The enhanced version of SAINT [SAINTplus] can give fast moving paths to emergency vehicles (e.g., ambulance and fire engine) to let them reach an accident spot while redirecting other vehicles near the accident spot into efficient detour paths.¶

Either a TCC or an ECD can recommend an energy-efficient speed to a vehicle that depends on its traffic environment and traffic signal scheduling [SignalGuru]. For example, when a vehicle approaches an intersection area and a red traffic light for the vehicle becomes turned on, it needs to reduce its speed to save fuel consumption. In this case, either a TCC or an ECD, which has the up-to-date trajectory of the vehicle and the traffic light schedule, can notify the vehicle of an appropriate speed for fuel efficiency. [Fuel-Efficient] studies fuel-efficient route and speed plans for platooned trucks.¶

The emergency communication between accident vehicles (or emergency vehicles) and a TCC can be performed via either IP-RSU or 4G-LTE networks. The First Responder Network Authority (FirstNet) [FirstNet] is provided by the US government to establish, operate, and maintain an interoperable public safety broadband network for safety and security network services, e.g., emergency calls. The construction of the nationwide FirstNet network requires each state in the US to have a Radio Access Network (RAN) that will connect to the FirstNet's network core. The current RAN is mainly constructed using 4G-LTE for the communication between a vehicle and an infrastructure node (i.e., V2I) [FirstNet-Report], but it is expected that DSRC-based vehicular networks [DSRC] will be available for V2I and V2V in the near future.¶

An EV charging service with V2I can facilitate the efficient battery charging of EVs. In the case where an EV charging station is connected to an IP-RSU, an EV can be guided toward the deck of the EV charging station through a battery charging server connected to the IP-RSU. In addition to this EV charging service, other value-added services (e.g., air firmware/software update and media streaming) can be provided to an EV while it is charging its battery at the EV charging station.¶

A UAM navigation service with efficient battery charging can plan the battery charging schedule of UAM end systems (e.g., drone) for long-distance flying [CBDN]. For this battery charging schedule, a UAM end system can communicate with an infrastructure node (e.g., IP-RSU) toward a cloud server via V2I communications. This cloud server can coordinate the battery charging schedules of multiple UAM end systems for their efficient navigation path, considering flight time from their current position to a battery charging station, waiting time in a waiting queue at the station, and battery charging time at the station.¶

The existing IPv6 protocol must be augmented through protocol changes in order to support wireless multihop V2I communications in a highway where RSUs are sparsely deployed, so a vehicle can reach the wireless coverage of an RSU through the multihop data forwarding of intermediate vehicles. Thus, IPv6 needs to be extended for multihop V2I communications.¶

To support applications of these V2I use cases, the required functions of IPv6 include IPv6-based packet exchange, transport-layer session continuity, and secure, safe communication between a vehicle and an infrastructure node (e.g., IP-RSU) in the vehicular network.¶

3.3. V2X

The use case of V2X networking discussed in this section is for a pedestrian protection service.¶

A pedestrian protection service, such as Safety-Aware Navigation Application (SANA) [SANA], using V2I2P networking can reduce the collision of a vehicle and a pedestrian carrying a smartphone equipped with a network device for wireless communication (e.g., Wi-Fi) with an IP-RSU. Vehicles and pedestrians can also communicate with each other via an IP-RSU. An edge computing device behind the IP-RSU can collect the mobility information from vehicles and pedestrians, compute wireless communication scheduling for the sake of them. This scheduling can save the battery of each pedestrian's smartphone by allowing it to work in sleeping mode before the communication with vehicles, considering their mobility.¶

For Vehicle-to-Pedestrian (V2P), a vehicle can directly communicate with a pedestrian's smartphone by V2X without IP-RSU relaying. Light-weight mobile nodes such as bicycles may also communicate directly with a vehicle for collision avoidance using V2V.¶

The existing IPv6 protocol must be augmented through protocol changes in order to support wireless multihop V2X or V2I2X communications in an urban road network where RSUs are deployed at intersections, so a vehicle (or a pedestrian's smartphone) can reach the wireless coverage of an RSU through the multihop data forwarding of intermediate vehicles (or pedestrians' smartphones) as packet forwarders. Thus, IPv6 needs to be extended for multihop V2X or V2I2X communications.¶

To support applications of these V2X use cases, the required functions of IPv6 include IPv6-based packet exchange, transport-layer session continuity, and secure, safe communication between a vehicle and a pedestrian either directly or indirectly via an IP-RSU.¶

4.1. Vehicular Network Architecture

Figure 1 shows an example vehicular network architecture for V2I and V2V in a road network. The vehicular network architecture contains vehicles (including IP-OBU), IP-RSUs, Mobility Anchor, Traffic Control Center, and Vehicular Cloud as components. These components are not mandatory, and they can be deployed into vehicular networks in various ways. Some of them (e.g., Mobility Anchor, Traffic Control Center, and Vehicular Cloud) may not be needed for the vehicular networks according to target use cases in Section 3.¶

Existing network architectures, such as the network architectures of PMIPv6 [RFC5213], RPL (IPv6 Routing Protocol for Low-Power and Lossy Networks) [RFC6550], and OMNI (Overlay Multilink Network Interface) [I-D.templin-6man-omni], can be extended to a vehicular network architecture for multihop V2V, V2I, and V2X, as shown in Figure 1. Refer to Appendix B for the detailed discussion on multihop V2X networking by RPL and OMNI. Also, refer to Appendix A for the description of how OMNI can support the use of multiple radio technologies in V2X.¶

As shown in this figure, IP-RSUs as routers and vehicles with IP-OBU have wireless media interfaces for VANET. Furthermore, the wireless media interfaces are autoconfigured with a global IPv6 prefix (e.g., 2001:DB8:1:1::/64) to support both V2V and V2I networking. Note that 2001:DB8::/32 is a documentation prefix [RFC3849] for example prefixes in this document, and also that any routable IPv6 address needs to be routable in a VANET and a vehicular network including IP-RSUs.¶

In Figure 1, three IP-RSUs (IP-RSU1, IP-RSU2, and IP-RSU3) are deployed in the road network and are connected with each other through the wired networks (e.g., Ethernet). A Traffic Control Center (TCC) is connected to the Vehicular Cloud for the management of IP-RSUs and vehicles in the road network. A Mobility Anchor (MA) may be located in the TCC as a mobility management controller. Vehicle2, Vehicle3, and Vehicle4 are wirelessly connected to IP-RSU1, IP-RSU2, and IP-RSU3, respectively. The three wireless networks of IP-RSU1, IP-RSU2, and IP-RSU3 can belong to three different subnets (i.e., Subnet1, Subnet2, and Subnet3), respectively. Those three subnets use three different prefixes (i.e., Prefix1, Prefix2, and Prefix3).¶

Multiple vehicles under the coverage of an RSU share a prefix just as mobile nodes share a prefix of a Wi-Fi access point in a wireless LAN. This is a natural characteristic in infrastructure-based wireless networks. For example, in Figure 1, two vehicles (i.e., Vehicle2, and Vehicle5) can use Prefix 1 to configure their IPv6 global addresses for V2I communication. Alternatively, mobile nodes can employ a "Bring-Your-Own-Addresses (BYOA)" technique using their own IPv6 Unique Local Addresses (ULAs) [RFC4193] over the wireless network, which does not require the messaging (e.g., Duplicate Address Detection (DAD)) of IPv6 Stateless Address Autoconfiguration (SLAAC) [RFC4862].¶

In wireless subnets in vehicular networks (e.g., Subnet1 and Subnet2 in Figure 1), vehicles can construct a connected VANET (with an arbitrary graph topology) and can communicate with each other via V2V communication. Vehicle1 can communicate with Vehicle2 via V2V communication, and Vehicle2 can communicate with Vehicle3 via V2V communication because they are within the wireless communication range of each other. On the other hand, Vehicle3 can communicate with Vehicle4 via the vehicular infrastructure (i.e., IP-RSU2 and IP-RSU3) by employing V2I (i.e., V2I2V) communication because they are not within the wireless communication range of each other.¶

As a basic definition for IPv6 packets transported over IEEE 802.11-OCB, [RFC8691] specifies several details, including Maximum Transmission Unit (MTU), frame format, link-local address, address mapping for unicast and multicast, stateless autoconfiguration, and subnet structure.¶

An IPv6 mobility solution is needed for the guarantee of communication continuity in vehicular networks so that a vehicle's TCP session can be continued, or UDP packets can be delivered to a vehicle as a destination without loss while it moves from an IP-RSU's wireless coverage to another IP-RSU's wireless coverage. In Figure 1, assuming that Vehicle2 has a TCP session (or a UDP session) with a corresponding node in the vehicular cloud, Vehicle2 can move from IP-RSU1's wireless coverage to IP-RSU2's wireless coverage. In this case, a handover for Vehicle2 needs to be performed by either a host-based mobility management scheme (e.g., MIPv6 [RFC6275]) or a network-based mobility management scheme (e.g., PMIPv6 [RFC5213] and AERO [I-D.templin-6man-aero]). This document describes issues in mobility management for vehicular networks in Section 5.2.¶

4.2. V2I-based Internetworking

This section discusses the internetworking between a vehicle's internal network (i.e., moving network) and an EN's internal network (i.e., fixed network) via V2I communication. The internal network of a vehicle is nowadays constructed with Ethernet by many automotive vendors [In-Car-Network]. Note that an EN can accommodate multiple routers (or switches) and servers (e.g., ECDs, navigation server, and DNS server) in its internal network.¶

A vehicle's internal network often uses Ethernet to interconnect Electronic Control Units (ECUs) in the vehicle. The internal network can support Wi-Fi and Bluetooth to accommodate a driver's and passenger's mobile devices (e.g., smartphone or tablet). The network topology and subnetting depend on each vendor's network configuration for a vehicle and an EN. It is reasonable to consider the interaction between the internal network and an external network within another vehicle or an EN.¶

                                                 +-----------------+
                        (*)<........>(*)  +----->| Vehicular Cloud |
     (2001:DB8:1:1::/64) |            |   |      +-----------------+
+------------------------------+  +---------------------------------+
|                        v     |  |   v   v                         |
| +-------+          +-------+ |  | +-------+          +-------+    |
| | Host1 |          |IP-OBU1| |  | |IP-RSU1|          | Host3 |    |
| +-------+          +-------+ |  | +-------+          +-------+    |
|     ^                  ^     |  |     ^                  ^        |
|     |                  |     |  |     |                  |        |
|     v                  v     |  |     v                  v        |
| ---------------------------- |  | ------------------------------- |
| 2001:DB8:10:1::/64 ^         |  |     ^ 2001:DB8:20:1::/64        |
|                    |         |  |     |                           |
|                    v         |  |     v                           |
| +-------+      +-------+     |  | +-------+ +-------+   +-------+ |
| | Host2 |      |Router1|     |  | |Router2| |Server1|...|ServerN| |
| +-------+      +-------+     |  | +-------+ +-------+   +-------+ |
|     ^              ^         |  |     ^         ^           ^     |
|     |              |         |  |     |         |           |     |
|     v              v         |  |     v         v           v     |
| ---------------------------- |  | ------------------------------- |
|      2001:DB8:10:2::/64      |  |       2001:DB8:20:2::/64        |
+------------------------------+  +---------------------------------+
   Vehicle1 (Moving Network1)            EN1 (Fixed Network1)

   <----> Wired Link   <....> Wireless Link   (*) Antenna

As shown in Figure 2, as internal networks, a vehicle's moving network and an EN's fixed network are self-contained networks having multiple subnets and having an edge router (e.g., IP-OBU and IP-RSU) for the communication with another vehicle or another EN. The internetworking between two internal networks via V2I communication requires the exchange of the network parameters and the network prefixes of the internal networks. For the efficiency, the network prefixes of the internal networks (as a moving network) in a vehicle need to be delegated and configured automatically. Note that a moving network's network prefix can be called a Mobile Network Prefix (MNP) [RFC3963].¶

Figure 2 also shows the internetworking between the vehicle's moving network and the EN's fixed network. There exists an internal network (Moving Network1) inside Vehicle1. Vehicle1 has two hosts (Host1 and Host2), and two routers (IP-OBU1 and Router1). There exists another internal network (Fixed Network1) inside EN1. EN1 has one host (Host3), two routers (IP-RSU1 and Router2), and the collection of servers (Server1 to ServerN) for various services in the road networks, such as the emergency notification and navigation. Vehicle1's IP-OBU1 (as a mobile router) and EN1's IP-RSU1 (as a fixed router) use 2001:DB8:1:1::/64 for an external link (e.g., DSRC) for V2I networking. Thus, a host (Host1) in Vehicle1 can communicate with a server (Server1) in EN1 for a vehicular service through Vehicle1's moving network, a wireless link between IP-OBU1 and IP-RSU1, and EN1's fixed network.¶

For the IPv6 communication between an IP-OBU and an IP-RSU or between two neighboring IP-OBUs, they need to know the network parameters, which include MAC layer and IPv6 layer information. The MAC layer information includes wireless link layer parameters, transmission power level, and the MAC address of an external network interface for the internetworking with another IP-OBU or IP-RSU. The IPv6 layer information includes the IPv6 address and network prefix of an external network interface for the internetworking with another IP-OBU or IP-RSU.¶

Through the mutual knowledge of the network parameters of internal networks, packets can be transmitted between the vehicle's moving network and the EN's fixed network. Thus, V2I requires an efficient protocol for the mutual knowledge of network parameters.¶

As shown in Figure 2, the addresses used for IPv6 transmissions over the wireless link interfaces for IP-OBU and IP-RSU can be link-local IPv6 addresses, ULAs, or global IPv6 addresses. When global IPv6 addresses are used, wireless interface configuration and control overhead for DAD [RFC4862] and Multicast Listener Discovery (MLD) [RFC2710][RFC3810] should be minimized to support V2I and V2X communications for vehicles moving fast along roadways.¶

Let us consider the upload/download time of a vehicle when it passes through the wireless communication coverage of an IP-RSU. For a given typical setting where 1km is the maximum DSRC communication range [DSRC] and 100km/h is the speed limit in highway, the dwelling time can be calculated to be 72 seconds by dividing the diameter of the 2km (i.e., two times of DSRC communication range where an IP-RSU is located in the center of the circle of wireless communication) by the speed limit of 100km/h (i.e., about 28m/s). For the 72 seconds, a vehicle passing through the coverage of an IP-RSU can upload and download data packets to/from the IP-RSU.¶

4.3. V2V-based Internetworking

This section discusses the internetworking between the moving networks of two neighboring vehicles via V2V communication.¶

                        (*)<..........>(*)
     (2001:DB8:1:1::/64) |              |
+------------------------------+  +------------------------------+
|                        v     |  |     v                        |
| +-------+          +-------+ |  | +-------+          +-------+ |
| | Host1 |          |IP-OBU1| |  | |IP-OBU2|          | Host3 | |
| +-------+          +-------+ |  | +-------+          +-------+ |
|     ^                  ^     |  |     ^                  ^     |
|     |                  |     |  |     |                  |     |
|     v                  v     |  |     v                  v     |
| ---------------------------- |  | ---------------------------- |
| 2001:DB8:10:1::/64 ^         |  |         ^ 2001:DB8:30:1::/64 |
|                    |         |  |         |                    |
|                    v         |  |         v                    |
| +-------+      +-------+     |  |     +-------+      +-------+ |
| | Host2 |      |Router1|     |  |     |Router2|      | Host4 | |
| +-------+      +-------+     |  |     +-------+      +-------+ |
|     ^              ^         |  |         ^              ^     |
|     |              |         |  |         |              |     |
|     v              v         |  |         v              v     |
| ---------------------------- |  | ---------------------------- |
|      2001:DB8:10:2::/64      |  |       2001:DB8:30:2::/64     |
+------------------------------+  +------------------------------+
   Vehicle1 (Moving Network1)        Vehicle2 (Moving Network2)

   <----> Wired Link   <....> Wireless Link   (*) Antenna

Figure 3 shows the internetworking between the moving networks of two neighboring vehicles. There exists an internal network (Moving Network1) inside Vehicle1. Vehicle1 has two hosts (Host1 and Host2), and two routers (IP-OBU1 and Router1). There exists another internal network (Moving Network2) inside Vehicle2. Vehicle2 has two hosts (Host3 and Host4), and two routers (IP-OBU2 and Router2). Vehicle1's IP-OBU1 (as a mobile router) and Vehicle2's IP-OBU2 (as a mobile router) use 2001:DB8:1:1::/64 for an external link (e.g., DSRC) for V2V networking. Thus, a host (Host1) in Vehicle1 can communicate with another host (Host3) in Vehicle2 for a vehicular service through Vehicle1's moving network, a wireless link between IP-OBU1 and IP-OBU2, and Vehicle2's moving network.¶

As a V2V use case in Section 3.1, Figure 4 shows the linear network topology of platooning vehicles for V2V communications where Vehicle3 is the leading vehicle with a driver, and Vehicle2 and Vehicle1 are the following vehicles without drivers.¶

     (*)<..................>(*)<..................>(*)
      |                      |                      |
+-----------+          +-----------+          +-----------+
|           |          |           |          |           |
| +-------+ |          | +-------+ |          | +-------+ |
| |IP-OBU1| |          | |IP-OBU2| |          | |IP-OBU3| |
| +-------+ |          | +-------+ |          | +-------+ |
|     ^     |          |     ^     |          |     ^     |
|     |     |=====>    |     |     |=====>    |     |     |=====>
|     v     |          |     v     |          |     v     |
| +-------+ |          | +-------+ |          | +-------+ |
| | Host1 | |          | | Host2 | |          | | Host3 | |
| +-------+ |          | +-------+ |          | +-------+ |
|           |          |           |          |           |
+-----------+          +-----------+          +-----------+
   Vehicle1               Vehicle2               Vehicle3

 <----> Wired Link   <....> Wireless Link   ===> Moving Direction
 (*) Antenna

As shown in Figure 4, multihop internetworking is feasible among the moving networks of three vehicles in the same VANET. For example, Host1 in Vehicle1 can communicate with Host3 in Vehicle3 via IP-OBU1 in Vehicle1, IP-OBU2 in Vehicle2, and IP-OBU3 in Vehicle3 in the VANET, as shown in the figure.¶

In this section, the link between two vehicles is assumed to be stable for single-hop wireless communication regardless of the sight relationship such as line of sight and non-line of sight, as shown in Figure 3. Even in Figure 4, the three vehicles are connected to each other with a linear topology, however, multihop V2V communication can accommodate any network topology (i.e., an arbitrary graph) over VANET routing protocols.¶

     (*)<..................>(*)<..................>(*)
      |                      |                      |
+-----------+          +-----------+          +-----------+
|           |          |           |          |           |
| +-------+ |          | +-------+ |          | +-------+ |
| |IP-OBU1| |          | |IP-RSU1| |          | |IP-OBU3| |
| +-------+ |          | +-------+ |          | +-------+ |
|     ^     |          |     ^     |          |     ^     |
|     |     |=====>    |     |     |          |     |     |=====>
|     v     |          |     v     |          |     v     |
| +-------+ |          | +-------+ |          | +-------+ |
| | Host1 | |          | | Host2 | |          | | Host3 | |
| +-------+ |          | +-------+ |          | +-------+ |
|           |          |           |          |           |
+-----------+          +-----------+          +-----------+
   Vehicle1                 EN1                  Vehicle3

 <----> Wired Link   <....> Wireless Link   ===> Moving Direction
 (*) Antenna

As shown in Figure 5, multihop internetworking between two vehicles is feasible via an infrastructure node (i.e., IP-RSU) with wireless connectivity among the moving networks of two vehicles and the fixed network of an edge network (denoted as EN1) in the same VANET. For example, Host1 in Vehicle1 can communicate with Host3 in Vehicle3 via IP-OBU1 in Vehicle1, IP-RSU1 in EN1, and IP-OBU3 in Vehicle3 in the VANET, as shown in the figure.¶

For the reliability required in V2V networking, the ND optimization defined in MANET [RFC6130] [RFC7466] improves the classical IPv6 ND in terms of tracking neighbor information with up to two hops and introducing several extensible Information Bases, which serves the MANET routing protocols such as the difference versions of Optimized Link State Routing Protocol (OLSR) [RFC3626] [RFC7181] [RFC7188] [RFC7722] [RFC7779] [RFC8218] and the Dynamic Link Exchange Protocol (DLEP) with its extensions [RFC8175] [RFC8629] [RFC8651] [RFC8703] [RFC8757]. In short, the MANET ND mainly deals with maintaining extended network neighbors. However, an ND protocol in vehicular networks shall consider more about the geographical mobility information of vehicles as an important resource for serving various purposes to improve the reliability, e.g., vehicle driving safety, intelligent transportation implementations, and advanced mobility services. For a more reliable V2V networking, some redundancy mechanisms should be provided in L3 in the case of the failure of L2.¶

5.1. Neighbor Discovery

IPv6 ND [RFC4861][RFC4862] is a core part of the IPv6 protocol suite. IPv6 ND is designed for link types including point-to-point, multicast-capable (e.g., Ethernet) and Non-Broadcast Multiple Access (NBMA). It assumes the efficient and reliable support of multicast and unicast from the link layer for various network operations such as MAC Address Resolution (AR), DAD, MLD and Neighbor Unreachability Detection (NUD).¶

Vehicles move quickly within the communication coverage of any particular vehicle or IP-RSU. Before the vehicles can exchange application messages with each other, they need to be configured with a link-local IPv6 address or a global IPv6 address, and run IPv6 ND.¶

The requirements for IPv6 ND for vehicular networks are efficient DAD and NUD operations. An efficient DAD is required to reduce the overhead of the DAD packets during a vehicle's travel in a road network, which can guarantee the uniqueness of a vehicle's global IPv6 address. An efficient NUD is required to reduce the overhead of the NUD packets during a vehicle's travel in a road network, which can guarantee the accurate neighborhood information of a vehicle in terms of adjacent vehicles and RSUs.¶

The legacy DAD assumes that a node with an IPv6 address can reach any other node with the scope of its address at the time it claims its address, and can hear any future claim for that address by another party within the scope of its address for the duration of the address ownership. However, the partitioning and merging of VANETs makes this assumption frequently invalid in vehicular networks. The merging and partitioning of VANETs frequently occurs in vehicular networks. This merging and partitioning should be considered for the IPv6 ND such as IPv6 Stateless Address Autoconfiguration (SLAAC) [RFC4862]. Due to the merging of VANETs, two IPv6 addresses may conflict with each other though they were unique before the merging. An address lookup operation may be conducted by an MA or IP-RSU (as Registrar in RPL) to check the uniqueness of an IPv6 address that will be configured by a vehicle as DAD. Also, the partitioning of a VANET may make vehicles with the same prefix be physically unreachable. An address lookup operation may be conducted by an MA or IP-RSU (as Registrar in RPL) to check the existence of a vehicle under the network coverage of the MA or IP-RSU as NUD. Thus, SLAAC needs to prevent IPv6 address duplication due to the merging of VANETs, and IPv6 ND needs to detect unreachable neighboring vehicles due to the partitioning of a VANET. According to the merging and partitioning, a destination vehicle (as an IPv6 host) needs to be distinguished as either an on-link host or an off-link host even though the source vehicle can use the same prefix as the destination vehicle [I-D.ietf-intarea-ippl].¶

To efficiently prevent IPv6 address duplication due to the VANET partitioning and merging from happening in vehicular networks, the vehicular networks need to support a vehicular-network-wide DAD by defining a scope that is compatible with the legacy DAD. In this case, two vehicles can communicate with each other when there exists a communication path over VANET or a combination of VANETs and IP-RSUs, as shown in Figure 1. By using the vehicular-network-wide DAD, vehicles can assure that their IPv6 addresses are unique in the vehicular network whenever they are connected to the vehicular infrastructure or become disconnected from it in the form of VANET.¶

For vehicular networks with high mobility and density, the DAD needs to be performed efficiently with minimum overhead so that the vehicles can exchange driving safety messages (e.g., collision avoidance and accident notification) with each other with a short interval suggested by NHTSA (National Highway Traffic Safety Administration) [NHTSA-ACAS-Report]. Since the partitioning and merging of vehicular networks may require re-perform the DAD process repeatedly, the link scope of vehicles may be limited to a small area, which may delay the exchange of driving safety messages. Driving safety messages can include a vehicle's mobility information (i.e., position, speed, direction, and acceleration/deceleration) that is critical to other vehicles. The exchange interval of this message is recommended to be less than 0.5 second, which is required for a driver to avoid an emergency situation, such as a rear-end crash.¶

ND time-related parameters such as router lifetime and Neighbor Advertisement (NA) interval need to be adjusted for vehicle speed and vehicle density. For example, the NA interval needs to be dynamically adjusted according to a vehicle's speed so that the vehicle can maintain its neighboring vehicles in a stable way, considering the collision probability with the NA messages sent by other vehicles. The ND time-related parameters can be an operational setting or an optimization point particularly for vehicular networks.¶

For IPv6-based safety applications (e.g., context-aware navigation, adaptive cruise control, and platooning) in vehicular networks, the delay-bounded data delivery is critical. IPv6 ND needs to work to support those IPv6-based safety applications efficiently.¶

From the interoperability point of view, in IPv6-based vehicular networking, IPv6 ND should have minimum changes with the legacy IPv6 ND used in the Internet, including the DAD and NUD operations, so that IPv6-based vehicular networks can be seamlessly connected to other intelligent transportation elements (e.g., traffic signals, pedestrian wearable devices, electric scooters, and bus stops) that use the standard IPv6 network settings.¶

5.2. Mobility Management

The seamless connectivity and timely data exchange between two end points requires efficient mobility management including location management and handover. Most vehicles are equipped with a GPS receiver as part of a dedicated navigation system or a corresponding smartphone App. Note that the GPS receiver may not provide vehicles with accurate location information in adverse environments such as a building area or a tunnel. The location precision can be improved with assistance of the IP-RSUs or a cellular system with a GPS receiver for location information.¶

With a GPS navigator, efficient mobility management can be performed with the help of vehicles periodically reporting their current position and trajectory (i.e., navigation path) to the vehicular infrastructure (having IP-RSUs and an MA in TCC). This vehicular infrastructure can predict the future positions of the vehicles from their mobility information (i.e., the current position, speed, direction, and trajectory) for efficient mobility management (e.g., proactive handover). For a better proactive handover, link-layer parameters, such as the signal strength of a link-layer frame (e.g., Received Channel Power Indicator (RCPI) [VIP-WAVE]), can be used to determine the moment of a handover between IP-RSUs along with mobility information.¶

By predicting a vehicle's mobility, the vehicular infrastructure needs to better support IP-RSUs to perform efficient SLAAC, data forwarding, horizontal handover (i.e., handover in wireless links using a homogeneous radio technology), and vertical handover (i.e., handover in wireless links using heterogeneous radio technologies) in advance along with the movement of the vehicle.¶

For example, as shown in Figure 1, when a vehicle (e.g., Vehicle2) is moving from the coverage of an IP-RSU (e.g., IP-RSU1) into the coverage of another IP-RSU (e.g., IP-RSU2) belonging to a different subnet, the IP-RSUs can proactively support the IPv6 mobility of the vehicle, while performing the SLAAC, data forwarding, and handover for the sake of the vehicle.¶

For a mobility management scheme in a domain, where the wireless subnets of multiple IP-RSUs share the same prefix, an efficient vehicular-network-wide DAD is required. If DHCPv6 is used to assign a unique IPv6 address to each vehicle in this shared link, the DAD is not required. On the other hand, for a mobility management scheme with a unique prefix per mobile node (e.g., PMIPv6 [RFC5213]), DAD is not required because the IPv6 address of a vehicle's external wireless interface is guaranteed to be unique. There is a tradeoff between the prefix usage efficiency and DAD overhead. Thus, the IPv6 address autoconfiguration for vehicular networks needs to consider this tradeoff to support efficient mobility management.¶

Even though the SLAAC with classic ND costs a DAD during mobility management, the SLAAC with [RFC8505] does not cost a DAD. SLAAC for vehicular networks needs to consider the minimization of the cost of DAD with the help of an infrastructure node (e.g., IP-RSU and MA). Using an infrastructure prefix over VANET allows direct routability to the Internet through the multihop V2I toward an IP-RSU. On the other hand, a BYOA does not allow such direct routability to the Internet since the BYOA is not topologically correct, that is, not routable in the Internet. In addition, a vehicle configured with a BYOA needs a tunnel home (e.g., IP-RSU) connected to the Internet, and the vehicle needs to know which neighboring vehicle is reachable inside the VANET toward the tunnel home. There is nonnegligible control overhead to set up and maintain routes to such a tunnel home over the VANET.¶

For the case of a multihomed network, a vehicle can follow the first-hop router selection rule described in [RFC8028]. For example, an IP-OBU inside a vehicle may connect to an IP-RSU that has multiple routers behind. In this scenario, because the IP-OBU can have multiple prefixes from those routers, the default router selection, source address selection, and packet redirect process should follow the guidelines in [RFC8028]. That is, the vehicle should select its default router for each prefix by preferring the router that advertised the prefix.¶

Vehicles can use the TCC as their Home Network having a home agent for mobility management as in MIPv6 [RFC6275] and PMIPv6 [RFC5213], so the TCC (or an MA inside the TCC) maintains the mobility information of vehicles for location management. IP tunneling over the wireless link should be avoided for performance efficiency. Also, in vehicular networks, asymmetric links sometimes exist and must be considered for wireless communications such as V2V and V2I.¶

Therefore, for the proactive and seamless IPv6 mobility of vehicles, the vehicular infrastructure (including IP-RSUs and MA) needs to efficiently perform the mobility management of the vehicles with their mobility information and link-layer information. Also, in IPv6-based vehicular networking, IPv6 mobility management should have minimum changes for the interoperability with the legacy IPv6 mobility management schemes such as PMIPv6, DMM, LISP, and AERO.¶

6.1. Security Threats in Neighbor Discovery

For the classical IPv6 ND, the DAD is required to ensure the uniqueness of the IPv6 address of a vehicle's wireless interface. This DAD can be used as a flooding attack that uses the DAD-related ND packets disseminated over the VANET or vehicular networks. [RFC6959] introduces threats enabled by IP source address spoofing. This possibility indicates that vehicles and IP-RSUs need to filter out suspicious ND traffic in advance. [RFC8928] introduces a mechanism that protects the ownership of an address for 6loWPAN ND from address theft and impersonation attacks. Based on the SEND [RFC3971] mechanism, the authentication for routers (i.e., IP-RSUs) can be conducted by only selecting an IP-RSU that has a certification path toward trusted parties. For authenticating other vehicles, the cryptographically generated address (CGA) can be used to verify the true owner of a received ND message, which requires to use the CGA ND option in the ND protocols. For a general protection of the ND mechanism, the RSA Signature ND option can also be used to protect the integrity of the messages by public key signatures. For a more advanced authentication mechanism, a distributed blockchain-based approach [Vehicular-BlockChain] can be used. However, for a scenario where a trustable router or an authentication path cannot be obtained, it is desirable to find a solution in which vehicles and infrastructures can authenticate each other without any support from a third party.¶

When applying the classical IPv6 ND process to VANET, one of the security issues is that an IP-RSU (or an IP-OBU) as a router may receive deliberate or accidental DoS attacks from network scans that probe devices on a VANET. In this scenario, the IP-RSU can be overwhelmed for processing the network scan requests so that the capacity and resources of IP-RSU are exhausted, causing the failure of receiving normal ND messages from other hosts for network address resolution. [RFC6583] describes more about the operational problems in the classical IPv6 ND mechanism that can be vulnerable to deliberate or accidental DoS attacks and suggests several implementation guidelines and operational mitigation techniques for those problems. Nevertheless, for running IPv6 ND in VANET, those issues can be more acute since the movements of vehicles can be so diverse that it leaves a large room for rogue behaviors, and the failure of networking among vehicles may cause grave consequences.¶

Strong security measures shall protect vehicles roaming in road networks from the attacks of malicious nodes, which are controlled by hackers. For safe driving applications (e.g., context-aware navigation, cooperative adaptive cruise control, and platooning), as explained in Section 3.1, the cooperative action among vehicles is assumed. Malicious nodes may disseminate wrong driving information (e.g., location, speed, and direction) for disturbing safe driving. For example, a Sybil attack, which tries to confuse a vehicle with multiple false identities, may disturb a vehicle from taking a safe maneuver.¶

To identify malicious vehicles among vehicles, an authentication method may be required. A Vehicle Identification Number (VIN) and a user certificate (e.g., X.509 certificate [RFC5280]) along with an in-vehicle device's identifier generation can be used to efficiently authenticate a vehicle or its driver (having a user certificate) through a road infrastructure node (e.g., IP-RSU) connected to an authentication server in the vehicular cloud. This authentication can be used to identify the vehicle that will communicate with an infrastructure node or another vehicle. In the case where a vehicle has an internal network (called Moving Network) and elements in the network (e.g., in-vehicle devices and a user's mobile devices), as shown in Figure 2, the elements in the network need to be authenticated individually for safe authentication. Also, Transport Layer Security (TLS) certificates [RFC8446][RFC5280] can be used for an element's authentication to allow secure E2E vehicular communications between an element in a vehicle and another element in a server in a vehicular cloud, or between an element in a vehicle and another element in another vehicle.¶

6.2. Security Threats in Mobility Management

For mobility management, a malicious vehicle can construct multiple virtual bogus vehicles, and register them with IP-RSUs and MA. This registration makes the IP-RSUs and MA waste their resources. The IP-RSUs and MA need to determine whether a vehicle is genuine or bogus in mobility management. Also, the confidentiality of control packets and data packets among IP-RSUs and MA, the E2E paths (e.g., tunnels) need to be protected by secure communication channels. In addition, to prevent bogus IP-RSUs and MA from interfering with the IPv6 mobility of vehicles, mutual authentication among them needs to be performed by certificates (e.g., TLS certificate).¶

6.3. Other Threats

For the setup of a secure channel over IPsec or TLS, the multihop V2I communications over DSRC or 5G V2X (or LTE V2X) is required in a highway. In this case, multiple intermediate vehicles as relay nodes can help forward association and authentication messages toward an IP-RSU (gNodeB, or eNodeB) connected to an authentication server in the vehicular cloud. In this kind of process, the authentication messages forwarded by each vehicle can be delayed or lost, which may increase the construction time of a connection or some vehicles may not be able to be authenticated.¶

Even though vehicles can be authenticated with valid certificates by an authentication server in the vehicular cloud, the authenticated vehicles may harm other vehicles. To deal with this kind of security issue, for monitoring suspicious behaviors, vehicles' communication activities can be recorded in either a central way through a logging server (e.g., TCC) in the vehicular cloud or a distributed way (e.g., blockchain [Bitcoin]) along with other vehicles or infrastructure. To solve the issue ultimately, we need a solution where, without privacy breakage, vehicles may observe activities of each other to identify any misbehavior. Once identifying a misbehavior, a vehicle shall have a way to either isolate itself from others or isolate a suspicious vehicle by informing other vehicles. Alternatively, for completely secure vehicular networks, we shall embrace the concept of "zero-trust" for vehicles in which no vehicle is trustable and verifying every message is necessary. For doing so, we shall have an efficient zero-trust framework or mechanism for vehicular networks.¶

For the non-repudiation of the harmful activities of malicious nodes, a blockchain technology can be used [Bitcoin]. Each message from a vehicle can be treated as a transaction and the neighboring vehicles can play the role of peers in a consensus method of a blockchain [Bitcoin] [Vehicular-BlockChain]. For a blockchain's efficient consensus in vehicular networks having fast moving vehicles, a new consensus algorithm needs to be developed or an existing consensus algorithm needs to be enhanced.¶

To prevent an adversary from tracking a vehicle with its MAC address or IPv6 address, especially for a long-living transport-layer session (e.g., voice call over IP and video streaming service), a MAC address pseudonym needs to be provided to each vehicle; that is, each vehicle periodically updates its MAC address and its IPv6 address needs to be updated accordingly by the MAC address change [RFC4086][RFC8981]. Such an update of the MAC and IPv6 addresses should not interrupt the E2E communications between two vehicles (or between a vehicle and an IP-RSU) for a long-living transport-layer session. However, if this pseudonym is performed without strong E2E confidentiality (using either IPsec or TLS), there will be no privacy benefit from changing MAC and IPv6 addresses, because an adversary can observe the change of the MAC and IPv6 addresses and track the vehicle with those addresses. Thus, the MAC address pseudonym and the IPv6 address update should be performed with strong E2E confidentiality.¶