## 5 Securing ITS Applications: A Practical Case Study
### ETSI TC ITS V2X Reference Architecture  
ETSI TC ITS has defined a reference architecture as shown in Figure 9, which is similar to the U.S. Architecture [7; 91]. It is based on a slightly modified IEEE 802.11p at the access layer, and enables new networking functionalities based on geographical addressing at the network layer, and new facilities layer on top that enables a set of rich messages that support different types of applications. Compared to the U.S. ITS Architecture, the ETSI TC ITS architecture includes more features at the network layer to support further communication scenarios, such as multi-hop forwarding. The facilities layer  
Figure 9: ETSI TC ITS reference architecture.  
functionalities are very similar in both architectures as most of them have been initially defined by the U.S. standard, then adopted and slightly adapted by the EU standard (ETSI).  
V2X is intended to enable critical safety applications first, where vehicles and road infrastructure cooperate by exchanging real-time information to be used for the prediction and the avoidance of accidents and, thus, to improve road safety. This type of application requires fast communication. Once the technology is deployed, it will also open the door to enable new traffic efficiency applications, as well as useful infotainment and added-value applications. The technology will support also the autonomous driving application, as it is important for an autonomous vehicle to communicate with other autonomous vehicles around it to negotiate the sharing of the road resources.  
The above applications require well-defined messages that could provide all of the required information in an efficient and reliable meaner. ETSI TC ITS has been working on defining key messages at the facilities layer, such as cooperative awareness messages (CAM) and decentralized environmental notification messages (DENM) [10]. The CAM is intended to be sent by each vehicle at least once every second and at most 10 per second, based on the vehicle dynamics [11]. Each CAM message includes a list of information about the location and status of the vehicle. The CAM exchange enables each vehicle to build a local map about all vehicles in the surrounding. While CAM is a proactive message, the DENM is a reactive message and triggered by an event, e.g., when an accident is detected, a DENM generation mechanism is triggered to initiate a related DENM to inform all vehicles within the relevant geographical area about the accident. As mentioned above, some messages like DENM need to be disseminated within a limited geographical area and to support that there was a need to enable new dissemination algorithms at the transport and network layers. The GeoNetworking functionalities at the network layer of the ETSI TC ITS have been introduced to support geographical-based routing mechanisms where a packet is forwarded based on geographical addressing schemes that use geospatial coordinates.  
Figure 10 shows the structure of a standard GeoNetworking packet, where the GeoNetworking header is divided into three mandatory sub-headers, _i.e._, basic, common and extended sub-headers. One of the motivations for splitting the common header into two sub-headers (_i.e._, basic and common) is to facilitate the application of security, as explained in the following subsection.