I. Introduction
As an important component of smart cities and intelligent transportation systems (ITSs), vehicular ad hoc networks (VANETs) have been attracting increasing attention from both the research and industrial communities, and they are considered to be a special type of mobile ad hoc network (MANET) [1]. In the environments of VANETs, all vehicles can act as routers to communicate with each other without relying on fixed infrastructure supports. There are two types of wireless communication, i.e., vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) [2] for various applications, which are classified into safety and infotainment applications [3], such as collision warning, driver assistance, Internet access, and so on. Compared with other MANETs, VANETs have special characteristics [4] involving the high mobility, self-organization, road pattern restrictions, no energy constraints, large-scale network sizes, and so on. Although we have achieved remarkable developments in VANETs, there are still many challenges in the fields of designs and implementations, such as security, privacy, routing, scalability, and quality of service (QoS). In this paper, we mainly focus on the network routing problems with QoS constraints.