2.1 Connecting VANETs to the internet
In order to connect VANETs to the Internet, vehicles have to be provided with a full Internet Protocol (IP) stack, as IP is the basic building block for Internet communications. IPv6 has been adopted as the version of the IP protocol by all the previously mentioned standardization bodies and consortia and has been included in their communication architectures. We can identify three main functionalities required to bring IP into the vehicular networks: (a) the capability of vehicles to auto-configure an IP address, (b) IP mobility mechanisms suited for vehicular scenarios, and (c) mechanisms for an efficient transmission and forwarding of IP datagrams within the vehicular network. In this paper we focus on the first topic, that is the auto-configuration of IP addresses by nodes of a VANET. IPv6 provides some standardized mechanisms of IP address auto-configuration, both stateless [3, 4] and stateful [5] that cannot--or at the very least are hard to--be applied without any modification in vehicular environments. The main causes of this fact are the multi-hop nature of VANETs and their lack of a single multicast-capable link for signaling, which prevent current IP address auto-configuration-related protocol specifications from being used as is in VANETs. Therefore, a key research issue is how to auto-configure IPv6 addresses in a VANET. The same problem occurs in general in any unmanaged multi-hop network. Among these, Mobile Ad hoc Networks (MANETs) have received a remarkable attention in the research area for years, and there even exists a working group in the IETF,e called AUTO-CONF, that is chartered to work on the standardization of an address auto-configuration solution for MANETs [6].
Two main approaches that can be followed to integrate IP in a multi-hop vehicular network:
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1.
Making the IP layer fully aware of the multi-hop nature of VANETs. In this case, the VANET can be defined as a set of IP routers that are interconnected by a multitude of IP links. The high dynamics of each individual link strongly contributes to the overall addressing and routing management overhead. In particular, in order to understand this complexity, we recall the assumption underpinning IP routing, which requires IP addresses assigned to nodes terminating different links to belong to non-overlapping prefixes.
Two IP prefixes p::/l_p and q::/l_q are non-overlapping if and only if there is no IP address p::a/l_p configured from p::/l_p that also belongs to q::/l_q, and vice versa.f In order to enable IP routing, an overwhelming amount of short-lived routes is required, posing extremely challenging management issues.
An example of solution that falls in this category and is particularly designed for VANET environments is the Vehicular Address Autoconfigura-tion (VAC) solution, proposed by Fazio et al. in [7]. This solution exploits the VANETs topology and an enhanced DHCP service with dynamically elected leaders to provide a fast and reliable IP address configuration. VAC organizes leaders in a connected chain such that every node (vehicle) lies in the communication range of at least one leader. This hierarchical organization allows limiting the signaling overhead for the address management tasks. Only leaders communicate with each other to maintain updated information on configured addresses in the network. Leaders act as servers of a distributed DHCP protocol and normal nodes ask leaders for a valid IP address whenever they need to be configured. The main drawbacks of this solution are the assumption of linear topology and group movement which limits the applicability scope, the overhead due to the explicit management signaling (e.g., between leaders), and the possible security threat due to the critical tasks carried out by the leaders. Some of the solutions proposed for Mobile Ad Hoc Networks (MANETs) [6] may also be used for vehicular networks. Most of these solutions and VAC share the problem that they require modifications to the IP stack of the nodes, as they do not rely on existing standardized IPv6 address auto-configuration solutions.
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2.
Hiding the multi-hop nature of VANETs from the IP layer. In this approach, the concept of IPv6 link is extended to a set of nodes which might not be directly reachable within one physical hop. A protocol located below IP presents a flat network topology, ensuring that the link seen by the IP layer includes all the nodes of the extended set, even those that are not directly reachable. In this case, existing IP address auto-configuration mechanisms could be used with minor modifications--and even without any.
This last approach was followed by the European GeoNet project,g which contributed to the solution finally standardized by ETSI. Two similar solutions have been proposed:(a) Geographically Scoped stateless Address Configuration (GeoSAC) [8], initially proposed before GeoNet started, and further developed during the lifetime of the project; and (b) [9], that adopts this same concept but has many and essential differences in the realization. The latter solution does not assure compatibility with legacy IPv6 protocol implementations and requires the IPv6 protocol to be geo-aware.
In this paper we focus on the solution adopted by the ETSI TC ITS, which follows the second approach, hiding the multi-hop nature of the VANET from the IP layer. We next present this system architecture and define the terms used in the rest of the paper.
2.2 ETSI TC ITS IPv6 integration system architecture
ETSI TC ITS is developing a set of protocols and algorithms that define an harmonized communication system for European ITS applications taking into account industry requirements like in particular those coming from the Car-to-Car Communications Consortium. In the ETSI TC ITS network architecture [2], vehicles are equipped with devices called Communication and Control Units (CCUs), which implement the ETSI protocol stack (see Figure 1, in which only the part of the stack involved in IPv6 communications is shown). Vehicles can communicate with each other or with fixed roadside ITS stations (also called Roadside Units, RSUs) installed along roads. CCUs and RSUs implement the same network layer functionalities and form a self-organizing network. RSUs can be connected to a network infrastructure, most likely an IP-based network. On-board application hosts including passenger devices attached to the vehicle on-board system are called Application Units (AUs). Passenger devices are assumed to have a standard IPv6 protocol stack, whereas CCUs act as gateways for the in-vehicle network optionally enhanced with the Network Mobility Basic Support protocol [10].
The ETSI GeoNetworking (GN) protocol [11], currently under completion and expected to be published soon, plays the role of a sub-IP layer, offering a flat network view to the IPv6 layer and dealing with the multi-hop routing within the VANET (nodes within the same area--i.e., attached to the same IP link-- might not be directly reachable, but are portrayed as such by the sub-IP layer). The ETSI has standardized a protocol adaptation sub-layer referred to as the GN6ASL (GeoNetworking to IPv6 Adaptation SubLayer) [1] which allows for the transport of IPv6 packets by ETSI GeoNetworking protocol, enabling sub-IP multi-hop delivery of IPv6 packets. The ETSI GN geo-broadcasting capability is used by the GN6ASL in order to shape link-local multicast messages to geographical areas.
Figure 2 shows the subset of the ETSI TC ITS system which is relevant to understand how IPv6 is integrated in the ETSI geonetworking architecture and the way the ETSI GN layer is used to logically create links--called Geographical Virtual Links (GVLs)-- mapped to areas--called GVL Areas. We will explain this in more detail in Section 2.3. We want to highlight here how IP packets are sent in the system, using the scenario depicted in Figure 2. Let us suppose a device within Vehicle C wants to communicate with a node in the Internet. For that communication to happen, the Vehicle C has to send packets to the RSU of its area--that is the next hop at the IP layer--and this requires at the ETSI GN layer Vehicle C to send packets to Vehicle B, which forwards them to Vehicle A, that finally delivers them to the RSU. Note that this multi-hop forwarding is required because Vehicle C is not within the radio coverage of the RSU. This example shows that in a system architecture based on short-range communication devices, the effective provisioning of Internet-based applications over multi-hop communication strongly depends on mobility. Single-hop vehicular Internet access based on WLAN has already been investigated in highway scenarios [12], concluding that the link between CCU and RSU is stable enough to allow for several types of applications. When considering multi-hop communication, the applicability scope of Internet-based applications might need to be reduced to lower speed scenarios (e.g., urban or semi-urban), to a proper ratio of CCUs per installed RSU and to a realistic maximum number of hops (to be determined). Section 3 addresses these particular issues, assessing under which conditions it is realistically feasible to support IP unicast multi-hop communications.
2.3 IPv6 stateless address configuration over the ETSI TC ITS architecture
The ETSI specification devoted to the integration of IPv6 and the geonetworking architecture not only describes how IPv6 packets are exchanged between ITS stations and how the GN6ASL is presented to the IPv6 layer as a link-layer protocol, but also explains how IPv6 addresses can be automatically configured by ITS stations, namely CCUs. The specification [1] only considers the use of stateless address autocon-figuration schemes, as stateful ones present higher latencies (due to the several round-trip time signaling messages) and requires of greater management effort. Manual configuration is also not recommended.
The ETSI solution is based on the Geographically Scoped stateless Address Configuration (GeoSAC) solution [8], which can be considered as one particular realization of the ETSI standardized mechanism. In the rest of the paper we refer to the ETSI IPv6 address stateless autoconfiguration solution as ETSI SLAAC.
ETSI SLAAC adapts the standard IPv6 SLAAC (Stateless Address Auto-Configuration) mechanism so it can be used in multi-hop vehicular ad hoc networks, by taking advantage of the geographical location awareness capabilities of the vehicles. In ETSI SLAAC, the concept of IPv6 link is extended to a well-defined geographical area (i.e., GVL area) associated with a point of attachment to an infrastructure-based network that plays the role of the IPv6 Access Router (AR).
The GeoNetworking-IPv6 Adaptation Sub-Layer (GN6ASL) (see Figure 1) is a sub-IP layer sitting on top of the ETSI GN layer. The ETSI GN layer deals with ad hoc routing by using geographic location information, while the GN6ASL presents to the IPv6 layer a flat network topology. Consequently, the link seen by the IPv6 layer includes nodes that are not directly reachable but are portrayed as such by the sub-IP layer (see Figure 3). This layer provides IPv6 with a link-local multicast-capable link, the Geographical Virtual Link (GVL), which includes a non-overlapping partition of the VANET formed by all nodes within a certain geographical area (the GVL area).
Each GVL area is managed by at least one RSU that acts as an IPv6 Access Router and sends standard IPv6 Router Advertisements (RA), carrying the IPv6 prefix(es) inside the Prefix Information Option (PIO). Nodes receiving the RAs can then build a valid IPv6 address out of the included IPv6 prefix, following the standard SLAAC mechanism, i.e., the host generates an address by joining the prefix received from the RA and the network identifier derived by its MAC address.
The link-local multicast capability emulation is achieved by relying on the geo-multicast/geo-broadcast capabilities provided by the ETSI GN layer. In particular, in order to be link-local multicast capable, an IP link must provide symmetric reachability [3], which is normally not accomplished by virtual links spanning multiple physical links due to the lack of reference boundaries. Link-local multicast packets are forwarded with geographical knowledge, so that a node processes a packet only if it was addressed to the area where the node is located. The geographic scoping provides non-variable virtual link boundaries which enable symmetric reachability. For RAs, this means that RAs must be delivered to--and only to--the nodes that are part of the same IPv6 link, nodes that are actually connected via multiple wireless hops. If a multi-hop path exists, all the nodes within the area will receive a copy of the RA, and the IPv6 instance running above the geonetworking will process the message as if the node was directly connected to the access router that issued the message.
It is assumed that MAC addresses (or a different identifier that can be used for IPv6 address generation purposes) of vehicles are unique, at least within macro-regions where vehicles are sold and can potentially communicate with each other (e.g., a continent). This property in fact is highly desirable for security and liability reasons, as it would allow (i) forensic teams to rely on vehicular communications to reconstruct accident scenes or other critical situations and, (ii) to detect malicious nodes and reduce considerably the effects of network attacks. Despite uniqueness of identifiers, privacy of users can be protected by equipping vehicles with sets of unique identifiers to be used for limited intervals as pseudonyms[13].
These identifiers could be assigned by authorities and, when coupled with the usage of digital certificates and cryptographic protection [14], this mechanism can accomplish support for liability as well as privacy protection from malicious users (commonly referred to as revocable privacy). Assuming that the IPv6 prefix announced by the RSU is exclusively assigned to this area, the address uniqueness is verified, and therefore no Duplicate Address Detection (DAD) mechanism is required. Note that the proposed solution could be applied to multiple RSUs acting as bridges connected to one single Access Router. This might be a good deployment choice in scenarios where single-hop connectivity to the infrastructure is preferred while it is also required to reduce the number of IPv6 address changes (e.g., city environment).
A technique that maximizes the benefits of ETSI SLAAC consists in shaping the GVL areas assigned to the RSU in a adjacent and logically non-overlapping fashion, as depicted in Figure 3. By doing so, the following key advantages are obtained: (i) unequivocal gateway selection is achieved with the infrastructure having full control on it,h as only one RSU is assigned per geographical area; (ii) a network partitioning is obtained that supports movement detection procedures of IPv6 mobility and also allows for location-based services. In particular, a vehicle moving across regions served by different Access Routers experiences a sharp sub-net change, without traversing gray areas where Router Advertisements are received from multiple access points (potentially leading to ping-pong effects).
Before characterizing and analyzing the performance of the ETSI SLAAC solution, we next analyze under which conditions it is realistically feasible to support IP unicast multi-hop communications in a vehicular environment.
3 Effectiveness of vehicular multi-hop communications
Vehicular networks using short-range wireless technologies, such as IEEE 802.11-based ones, rely on multi-hop communications to extend the effective coverage of the RSUs deployed on the roadside. One of the main challenges that VANETs pose is the minimum degree of technology penetration that is needed in order to ensure that there is enough density of communication-enabled vehicles to support multi-hop connectivity between the intended peers (e.g., for the case of Internet communications, between the vehicle and the RSU). This problem becomes even more problematic during the time of the day when roads are less busy. In these environments, communications can become difficult because radio devices often operate at their design limits (large distances, multi-path signal propagation, critical packet length vs. channel coherence time ratio, etc.), which amplifies the effect of layer-2 inefficiencies due to hidden node scenarios. Furthermore, the probability of having a multi-hop path between two nodes is lower in sparse scenarios. On the other hand, when roads become more crowded, speeds are lower, links are more reliable, and the chances for two arbitrary nodes to be connected by at least one stable multi-hop path are higher.
Deploying vehicular networks without dead zones (i.e., areas not served by any RSU) is economically inefficient in non-urban locations. As we have mentioned above, in the ETSI TC ITS architecture, vehicles form a self-organized multi-hop network. This multi-hop network is used to forward packets between the RSU and the CCUs within the RSU's area of influence (i.e., associated GVL area), and therefore extends the effective coverage area of the RSU. In order to assess the feasibility of vehicular communications in practical scenarios, it is necessary to evaluate whether wireless multi-hop communications are possible in different vehicular situations. To do so, we model and analyze the probability of having a multi-hop path between a sender and a receiver, studying the impact of different parameters, such as vehicular speed and density, wireless radio coverage, etc. We present our mathematical model first and then validate it via simulations.
Given two nodes separated by a distance S, Pmhc(S) is the probability of having multi-hop connectivity (mhc) or, in other words, the probability that one chain of inter-connected vehicles between the two nodes exists. This probability depends--as we show below--on the distance between the two nodes, the radio coverage, and the vehicular density. Figure 4 shows an example of a chain of interconnected vehicles between a car and an RSU.
We model the distance D between consecutive vehicles (inter-vehicle spacing) as exponentially distributed [15, 16], with parameter β, with its Probability Density Function (PDF) given by:
(1)
where β is the vehicular density. Let R be the wireless coverage radius. The distance between two consecutive vehicles that are part of a connected multi-hop chain of vehicles (the inter-vehicle gap is smaller than R) follows a truncated exponential distribution [17]:
(2)
The length of a multi-hop connected chain of n + 1 vehicles (Y) can be represented as the sum of n independent exponential truncated variables. The PDF of Y can be obtained by the method of characteristic functions [17]:
(3)
where k0 = 0,1,..., n - 1, and b = (1 - e-βR).
Let , where is an integer, and 0 ≤ c < 1. The Cumulative Distribution Function (CDF) of Y evaluated at a is :
(4)
where Q[u, w] = P (χ2(w) < u) and χ2(w) is a chi-square variable with w degrees of freedom. Since the probability P(i) of having a connected chain of i hops is given by (1-e-βR) e-βR, the PDF and CDF of the length (L) of a connected multi-hop chain of vehicles can be derived using the law of total probability:
(5)
(6)
Based on this, Pmhc(S) is given by:
(7)
Another factor that should be considered to assess the feasibility of vehicular multi-hop communications is the number of available lanes in a road. Our previous analysis is valid regardless of the number of lanes, thanks to the properties of the exponential distribution. If we consider several lanes, and in each one we model the spacing between cars by an exponential distribution, not necessarily with the same mean (the different lanes can have different car densities), the resulting space between cars in the road (not matter in which lane) is exponentially distributed with mean the average of the means in each lane. Therefore, we do not assume any particular number of lanes throughout the rest of the paper, unless indicated explicitly. Note that we are approximating the car distribution assuming that there is no correlation between the lane geometry and the car distribution. This means that we disregard the spatial correlation introduced by traffic regulation and congestion. The consequences of this assumption are evaluated in the next section.
In order to validate our analysis of Pmhc, we performed a large amount of experiments via simulation under different traffic conditions. The simulatori was developed using Matlab and it implements the scenario described in this section, namely vehicles distributed in a one-dimensional road, traveling at a pre-defined and constant speed, with an exponential inter-vehicular distance and a maximum wireless radio coverage, assuming an ideal wireless technology (no packet losses nor collisions and infinite bandwidth). Although the simulator does not consider a real wireless model, we argue that it is enough to show the correctness of our mathematical model, as it fully implements the behavior we are modeling. Obtained results show that our mathematical analysis perfectly models the probability of having multi-hop connectivity (assuming the aforementioned simplifications). We do not show these validation results due to space constraints. Simulation and experimental results are shown in Section 5, where we use a more advanced simulator (OMNeT++) that does include a complete wireless model to validate our formulation of the configuration time of the ETSI SLAAC solution.
In the following, we focus on analyzing the scenarios in which unicast communications using a multi-hop vehicular network are feasible. There are three parameters that have an impact on the probability of having multi-hop connectivity between two nodes:
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The distance S between the nodes. The larger this distance is, the lower is the probability of having connectivity. If we focus on the vehicle-to-Internet scenario, this value would be related to the distance between a moving vehicle and the fixed RSU, and therefore it depends on how RSUs are deployed.
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The vehicular density β. The probability of having connectivity increases with the vehicular density. The density depends on the traffic conditions (i.e., the time of the day and road) and the type of road (i.e., there are roads more congested than others). Vehicles density and speed are usually correlated as well, since the minimum safety distance between vehicles depends on the speed [18].
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The wireless coverage radius R. The effective radius depends on the specific wireless access technology, the transmission power at the antenna, the antenna radiation pattern, and the instantaneous channel response. The probability of having multi-hop connectivity is obviously very much affected by R, shorter values leading to lower probabilities.
If we fix the value of R, which is equivalent to assuming a reference system, it is interesting to study which is the minimum vehicular density required to ensure a certain probability of multi-hop connectivity between two nodes, depending on their distance. Figure 5 depicts the simulation results obtained for three different values of R (150, 300, and 450 m), which represent a realistic range of wireless coverage radius for wireless access technologies expected to be used in vehicular communications [19]. The results are plotted in three dimensions, so it can be observed how the vehicular density β and the distance S between the two nodes affect the multi-hop connectivity probability. An horizontal plane for Pmhc = 0.9 is also depicted in the figures, so we can observe which are the combinations of β and S that result in values of Pmhc higher than 90%. The cut (intersection) of horizontal planes corresponding to probabilities of 0.7, 0.8, 0.9, and 0.95 and the 3D curve are shown in Figure 6. Using this figure we can find out which is the minimum vehicular density required to achieve a minimum multi-hop connectivity probability between two nodes separated by a given distance.
Let us take for example the reference value of S = 1,000 m. From the results in Figure 6, we can conclude that if the coverage radius R is 150 m, a vehicular density of approximately 35 veh/km or higher ensures that there is multi-hop connectivity in the 90% of the cases. Similarly, 15 veh/km are enough if R is 300 m, and 8 veh/km for R = 450 m. It is important to highlight that these densities are quite low and that, therefore, are likely to be found in realistic scenarios with typical traffic conditions.
In order to limit the number of results presented in the paper, we selected the following three scenarios which mostly cover a wide spectrum of potential traffic scenarios:
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Urban road: high vehicular density (β = 80 veh/km) and low speed (v = 50 km/h).
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City highway: moderate vehicular density (β = 50 veh/km) and moderate speed (v = 80 km/h).
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Motorway: low vehicular density (β = 35 veh/km) and high speed (v = 120 km/h).
As it can be observed from Figure 6, it is perfectly feasible to have multi-hop connectivity in these three scenarios for most of the potential deployments (i.e., inter-RSU distances).
The probability of multi-hop connectivity is not the only parameter that should be considered when assessing the feasibility of vehicular communications, as the number of hops also plays an important role (i.e., the larger this number is, the lower are throughput and reliability). Figure 7 shows the average, minimum, and maximum values of the number of traversed hops (only for those communications that can take place, i.e., where a multi-hop chain of vehicles exists) for the same scenarios. From these results we can also conclude that it is not efficient from a performance viewpoint to deploy RSUs which are separated by large distances, as the number of hops would get too high, impacting the performance of the communications. It should be noted that vehicles are expected to be equipped with one single wireless radio interface for multi-hop communications using a self-configured VANETj and therefore the effective throughput decreases with the number of traversed wireless hops in the VANET.