The objective of vehicular communication is to improve road safety and traffic efficiency through various cooperative intelligent transport system (C-ITS) services. These services allow information exchange between vehicles and other communication entities (e.g., vehicles, Infrastructure, pedestrians). In addition, many advanced services are envisaged to support autonomous vehicles and safety applications. Therefore, the performance requirements of such services are considered highly critical for road safety. However, all these services increase the channel load, and thus, it isn't easy to differentiate which service has a higher priority for accessing communication channels. In this paper, we focus on the classification of C-ITS services, which allows the cohabitation of all services considering the strict performance requirements for some services. The aim is to classify C-ITS services based on their packet delay requirements to define higher priority for critical services to ensure their dissemination, especially under congestion conditions. Then, we present protocols and quality of service (QoS) mechanisms that can map this classification to the available vehicular networks.