In Vehicular Ad-hoc Network, DG-CASTOR works based on the principles of linkage availability and the node inference. The node communicates amongst them self provided they have the same status to communicate with the source. The Rendezvous boundary represents the Geocast routing area and the Rendezvous nodes communicates between the source and the neighbours’ nodes in which the link availability was estimated (Chawla, 2014). The goal of the protocol is to route data packets through the nearest nodes from the source node that have the greatest chance to communicate with the source considering their communication ability and the distance between their locations (Gulliver, 2015).
2.7.1 Distributed robust geocast routing protocol (DRG)
The DRG routing protocol works based on the forwarding algorithm on the principles of select and relays scheme. Each node receiving a geocast message to checks the message relevance based on its location. Two basic approaches were used as the zone of relevance (ZOR) to represent the set of geographic criteria a node must satisfy principles criteria in order for the geocast message to be relevant to that node, while the zone of forwarding (ZOF) is the set of geographic criteria a node must satisfy in order to forward a geocast message (Harshvardhan P. Joshi, 2006). If the node that belongs to ZOR reads the message then it will forwards the message that belongs to ZOF range else the packet will not be sent to destination. DRG protocol is based on flooding technique, hence, this can produce significant network overhead (Gulliver, 2015).
2.8 Cluster Based Routing Protocols
Cluster Based Routing Protocols divides the network into small groups called clusters, the nodes having the same characteristics such as moving with same velocity in the network can form a cluster and share information. The communication among the clusters occurs via pre-selected nodes called Cluster Heads (CHs). CHs are responsible for coordinating the cluster members; the CH finds the destination route; by propagating routing overhead which is proportional to the number of the clusters instead of the number of nodes. The objectives of using clusters are to minimize the control overhead, and increase the network scalability (Ahmad Abuashour, 2016). Few routing protocols are CBDRP and CBLTR.