# A New SDN-Based Routing Protocol for Improving Delay in Smart City Environments
## 2 Background and Related Work
### SDN-Based Routing Algorithms  
Since routing is the fundamental key in any communication network, several prior works have been designed for a wide selection of systems and applications with various challenges, such as minimizing delay transmission, maximizing packets delivery, enhancing reliability, etc. According to the listed works below, the structure of SDN provides a global view of the network, which eases computing optimal routing paths and guarantee efficient transmission. Besides, this section highlights the new routing contributions using the SDN paradigm. [PERSON] et al. [15] proposed an SDN-enabled connectivity-aware geographic-aware routing protocol (SCGRP), in which the SDN controller calculates routing measurements in real-time using cellular networks. SCGRP performance scores an enhancement in link deterioration, transmission delay, and packet delivery ratio. Moreover, SCGRP optimizes the transmission of data packets using an extended SDN architecture to vehicular networks in urban areas. The proposed solution aims to take  
Figure 1: SDN architecture in Smart City Environment. advantage of a global view of the network topology and heterogeneous traffic conditions maintained by SDN. [PERSON] et al. [16] suggested a new routing protocol for Software-Defined Vehicular Networking (SDVN) that optimizes the routing scheme with a source routing based flow instantiation (FI) operation. The routing protocol analyzes the link quality fluctuations due to dynamic topology that causes short lifetime links and then packet losses. Based on an incremental packet allocation scheme, the solution solves the routing problem in a less time complexity by finding multiple shortest paths. The built routes are collectively stable, which guarantees delivery of packets without any loss. [PERSON] et al. [17] designed a new approach by combining the SDN with the small-cell deployments. They implemented the Multipath TCP connections to establish link reliability between vehicles, increase throughput, and reduce packet loss. Similarly, [PERSON] et al. [18] considered using SDN in a Vehicular Ad hoc NETwork (VANET) setting as the best key to ensure a routing solution's efficiency. The SDN controller builds better paths based on a global view over the network. They used a new metric named \"minimum optimistic time\" to switch between multi-hop forwarding and carry-and-forward models according to the network density. The routing protocol aims to decrease delivery delay time and overhead. Other work introduced a new approach to resolve the routing problem [19]. [PERSON] et al. [19] put forward the routing solution that used a road-aware approach by selecting paths using road segment id to ensure path durable. The SDN controller computes path estimation model using speed, position, and road id information for finding the shortest routes. The data is gathered by a dedicated controller, i.e., the edge controller, which processes the real-time data coming from the vehicles. The usage of this controller aims to reduce the response time and packet overhead. The cellular network is employed to transmit packets between the SDN controller and vehicles with less bandwidth and low latency. [PERSON] et al. [20] proposed a new method to build multiple paths and to simplify the traffic construction procedure. The protocol used a new criterion based on average path channel load, channel loading, path length, and channel length to select a path from the available routes. The solution employs a modified channel state wave algorithm in path selection that allows a more uniform capacity of information transmission. The protocol manages the shortest paths between the intermediate and final nodes to weed out the recalculation of new routes in the transmission process. Consequently, the solutions presented in this section are based on the broadcasting approach to discover the destination if the controller did not know it beforehand. This target discovery will generate an additional delay in building routes and data transmission.