## 2 Literature Review
### Clustering Approach  
The paper [26] presents a moving-zone-based architecture and routing protocol for data transmission in vehicle-to-vehicle communication. A head vehicle is assigned for each area, and it is in charge of managing the information of other member vehicles as well as the message dissemination. This mechanism does not work with vehicle-to-infrastructure (vehicle-to-RSU) communication because the captain vehicle itself acts as a centralized unit. Clusters are formed by vehicles having similar moving patterns. In each cluster, one vehicle is elected as the section captain. The captain node is responsible for managing other nodes' information and handling data transmission. The sender node forwards its data to the captain. If the captain node detects that the receiver end is outside its zone, it forwards the data to a member node that is the nearest to the destination end. The chance of bottleneck occurrence for the captain node is high, which could result in having longer end-to-end delay and a higher chance of packet loss. Additionally, this scheme imposes high energy costs on the captain node.  
To conquer the issues of the traffic-aware approach, authors in [27] proposed a delay-aware grid-based geographical clustering method. The novelty of this work is to address desirable performance in both scenarios of dense and spare vehicular congestion. Different backbone nodes compose backbone links in each street segment. Inter-grid and intra-grid mechanisms have been proposed to accommodate the data transmission in all traffic models. In addition to the dependency of protocol on online maps and GPS, the method requires complicated calculations.  
In [28], the author proposed a clustering technique to transmit emergency messages such as road accidents with a minimum possible delay. This clustering technique handles broadcast storm problems to reduce network congestion. Simulation of this mechanism provides better results only at low speed. When the vehicle speed increases, it results in poor performance of the system as the network connectivity decreases.  
Many clustering protocols overlook the road structure in their design. In [29], the proposed method tries to overcome the possible upcoming issues at the intersections related to change of direction and mobility speed with the assistance of RSU nodes. Two adjacent intersections and the corresponding road is considered as road segment. The connectivity of each road segment is studied based on the average end-to-end delay in both scenarios of high and low-density road traffic. In this paper, the sender node decides the next forwarding RSU positioned at a nearby intersection. Then the data load is forwarded to the next intersection, which is close to the destination node through intermediate nodes in the next adjacent road sections until it reaches the destination vehicle.  
The protocols under the category of clustering mainly provide a high rate of packet delivery. Additionally, scalability is another feature of this group. These protocols can adapt to different environments with different predictable mobility patterns. However, clustering approach protocols increase the latency in consequence of the communications involved between the sender node and the cluster head, and this consumes a processing time.