## 2 Related Work
### Reliability and Traceability of Data in VANETs  
As explained in [26], VANET communications may be affected by nodes that alter the content of the messages that they should simply relay, or that generate false data. Some authors have proposed trust models as a first facility to address this problem, which can be classified into three categories:  
* Some approaches assess the reliability of the vehicles through a reputation system that relies on a centralized entity, which collects the opinions of neighboring nodes [44; 45; 46; 47]. In scenarios of high mobility, these approaches struggle to collect sufficient information to calculate the reputation scores for each node; furthermore, the centralized entity represents a single point of failure.
* Other approaches resort to the cooperation among several sources of information to assess the reliability of the data they receive [48; 49; 50]. The need to replicate packets onto several nodes increases the communication overhead and the latencies, and in cases of high mobility it is hard to ensure the necessary levels of collaboration.  
* Some proposals have combined the entity-centric and data-centric approaches, to jointly assess privacy and trust [51; 52].  
The literature of VANETs is rich in methods for authentication [53; 54], including privacy-preserving techniques [55] which are highly needed for the kind of services advocated in this paper. On top of these, blockchain technology comes as a way to achieve traceability of the data, due to the introduction of a public, decentralized, hardly corruptible, and practically immutable ledger [56]. The recorded transactions, after being validated and added to the ledger, can be consulted at any time, but they cannot be deleted or modified [57]. The following are some remarkable proposals for the integration of blockchain in VANETs.  
* The authors of [58] presented a reputation mechanism inspired by Bitcoin, in which unique cryptographic identifiers, known as the Intelligent Vehicle Trust Points (IV-TP), are created and delivered by authorized dealers. The higher the IV-TP achieved by a node, the more trustworthy it is considered.
* An anonymous blockchain-based reputation system was presented in [26], with two blockchains handled in parallel: the first one to store the certificates issued by a certification authority, together with the reputation scores of the corresponding nodes, and the second one to keep track of revoked public keys. When a communication is initiated between vehicles and/or roadside units, the validity of their certificates and public keys is checked by searching in the two blockchains. A similar approach was presented in [59], based on a lightweight blockchain protocol to keep track of all certificates (issued and revoked), whereas the authors of [60] made a proposal to speed up the authentication and revocation processes.
* The authors of [61] proposed a new type of blockchain to address aspects of security and reliability in the dissemination of emergency messages, which are relevant within a certain distance from the location of an incident. In order to validate the disseminated messages, the vehicles use certificates provided by the closest RSU, which guarantees that they were in a location close to the incident they are reporting.  
In all of these approaches, the integration of blockchain in VANETs has been seen as a separate problem from the reliable transmission of data packets through one-hop or multi-hop routes. However, latencies and packet losses represent a persistent problem that cannot be diminished to an extent that does not impinge heavily on the performance of the blockchain protocols above [4]. Our approach to solve this inconvenience is to architecturally entwine a virtualization layer (which addresses the problems of the communications) and the blockchain protocol (which deals with the properties of the data). Our proposal is described in detail in the next section.