Vehicular Ad hoc NETworks (VANETs) allow vehicles to form a self-organized network without the need for a permanent infrastructure. As a prerequisite to communication, an efficient route between network nodes must be established, and it must adapt to the rapidly changing topology of vehicles in motion. This is the aim of VANET routing protocols. In this paper, we discuss the design factors of unicast routing protocols for VANETs, and present a timeline of the development of the existing unicast routing protocols. Moreover, we classify and characterize the existing unicast routing protocols for VANETs, and also provide a qualitative comparison of them. This classification and characterization gives a clear picture of the strengths and weaknesses of existing protocols in this area and also throws light on open issues that remain to be addressed. Multicast routing protocols are also very important in VANETs; however, they are outside the scope of this paper. (C) 2008 Elsevier B. V. All rights reserved.
