A Scalable Indirect Position-Based Causal Diffusion Protocol for Vehicular Networks

Emerging applications based on vehicular ad-hoc networks (VANETs) are focused on offering new services to intelligent transportation systems (ITSs), such as traffic safety and management. For such kind of services, there are intrinsic spatial and temporal dependencies in the communication among mobile and fixed components. However, the characteristics of VANETs, which include a rapidly changing topology and the lack of global temporal references, make it difficult to satisfy such communication constraints. Commonly, the communication protocols for VANETs assume high coupled communication links; nevertheless, for most ITSs, the entities to whom such information will be useful are determined at run time by the spatial and temporal context. In this paper, a causal position-based indirect communication protocol is proposed, which allows road side units (RSUs) to disseminate messages with an uncoupled communication. The proposed solution uses the vehicles as opportunistic carriers, leveraging the bounded movement in a specific geographical region to exchange data among RSUs even with unknown recipients. To preserve the coherence of the exchange of messages, they are ordered by establishing their causal dependencies without the requirement of global references or synchronized physical clocks. Furthermore, the proposed protocol leverages message redundancy to recover some lost messages during the transmission. It is analytically demonstrated that the protocol is scalable since the size of the control information depends only on the number of RSUs rather than on the total number of entities in the system.