Routing protocols in Vehicular Adhoc Networks (VANETs): A comprehensive survey

Vehicular Ad-hoc Networks (VANETs) are pivotal for realizing smart cities' vision due to their rich services in road traffic safety, surveillance, traffic control, and traffic monitoring. However, vehicle links are subject to frequent disconnections due to high mobility, sparse connectivity, road-side obstacles, and a shortage of roadside units. For this purpose, various routing protocols have been proposed in the literature leveraging the network's topology, the vehicles' positions, and the message dissemination mechanism, i.e., unicast, multicast, broadcast, etc. This paper comprehensively surveys the VANETs routing protocols and analyzes their working principles, architecture, and application areas. Additionally, it presents a taxonomy and a systematic classification of these protocols based on various performance metrics. Furthermore, it presents a critical comparison of these routing protocols according to different criteria, identifies critical shortcomings in each protocol type, and outlines new directions and recent trends in the design of routing protocols.