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resources (services) as per the IOV user’s requirements. In a 5G-VANET milieu, the service discovery implementations include the VANET users and service providers.

      Service Consumers (SC) or VANET users: Service consumers request for the services.

      Service Providers (SP): SP publishes its services in the registry for access to SC.

      2.4.1 Service Discovery Methods

      The literature distinguished three types of service discovery methods such as,

      1 Push-based Service Discovery (PUSD)

      2 Pull-based Service Discovery (PLSD)

      3 Hybrid-based Service Discovery (HSD)

      PUSD: In PUSD, the service provider disseminates the services to all vehicles in the VANET milieu, does not wait for the requests from the VANET users. The PUSD is a proactive mode [15–22].

      PLSD: In PLSD, the vehicle user transmits a request towards the service provider to find the specific service. The PLSD is a reactive mode [23–25].

      HSD: The HSD uses both the PUSD and PLSD. HSD uses both proactive and reactive mode [26–29]. Table 2.1 depicts the characteristics of service discovery methods.

      2.4.2 A Framework of Service Discovery in the 5G-VANET Milieu

Figure 2.2 shows the framework of service discovery in the 5G-VANET milieu. A network layer is incorporated with the service unit and routing unit. The service unit processes the discovery mechanism. The discovery mechanism is the key mechanism in service discovery that discovers the desired services for each vehicle user as per their requirements. The routing unit processes the routing task, such as route discovery and hand-off.

      Table 2.1 Characteristics of service discovery methods.

Characteristics	PUSD	PLSD	HSD
Mode	Proactive	Reactive	Proactive and reactive
Architecture	Centralized	Centralized	Centralized
Routing	Broadcast	On-demand routing	Unicast/Broadcast/ Geocast
Mobility	Periodic update	Not required	Periodic update/event-driven update
Discovery	Spatial distribution	Context-based/Spatial distribution	Context-based/Spatial distribution
Bootstrapping	Broadcast	Geographical Routing/Computationbased	Geographic dissemination/Broadcast
Accuracy	High	High	High
Scalability	Yes	No guarantee	Yes
Reliability	Yes	Yes	Yes

      Table 2.2 illustrates the difference between the routing unit and service unit. Both the routing and service units come under the discovery agents but they differ in the form of processing behavior of an approach, service discovery works on the service line to accomplish the effective services to the users and the route discovery is majorly spotlight on the defining route from vehicle user to the target for smooth transversal between nodes. The two units such as, routing, and service are used for the overall improvement of the framework in the form of operational and conceptual deportment.

      The routing unit protocol is distinguished based on their characteristics are Table-Driven Routing Protocol (TDRP), On-Demand Routing Protocol (ODRP), and Hybrid Routing Protocol (HRP) [30]. The definition of routing unit protocols based on their characteristics is as follows:

      Figure 2.2 A framework of service discovery.

      Table 2.2 Comparison of routing unit and service unit.

Characteristics	Routing unit	Service unit
Protocols	Routing protocols	Service protocols
Leaned on	Process-Oriented	Resource-Oriented
Discover	Finds the superlative path	Finds the superlative services
Parameter for Discovery	Distance, energy, cost, and so on.	Accuracy and efficiency.
Mechanism	Route discovery mechanism	Service discovery mechanism
Information	Routing information	Service information
Layer	Network layer	Network layer
Protocols	PUSD, PLSD, and HSD [13–27].	TDRP, ODRP, and HRP [28].
Reliability	Yes	Yes

TDRP: This protocol reads the topological information continuously and updates it in the routing table.

      ODRP: This protocol reads the topological information when it requires.

      HRP: Hybrid uses the features of both TDRP and ODRP.

2.5 Service Discovery Architecture for 5G-VANET Milieu

      The few important network characteristics to be considered while designing the service discovery architecture for 5G-VANET are dense, mobility, scalability, collision, failure, congestion, reliability, service availability, and latency. The services provided by the service discovery mechanism are classified into two types such as fixed services and moving services. The definitions of fixed and moving services are as follows:

      Fixed Services:

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