## 3 Overview of Routing Protocols
### Proactive Routing Protocols
The route maintenance phase involves inspection for broken links in the network and updating the routing tables. In this work, we will accentuate two conventional reactive routing protocols DSR and AODV. * DSR Route Discovery and Route Maintenance  
The dynamic source routing protocol (DSR) is mainly based on the concept of source routing algorithm to discover routes, where a sender node specifies in the packet header the whole list of nodes that the packet must bypass to reach the destination node [15]. This basically means that every node just needs to forward the packet to its next hop specified in the header, and there is no need to verify its routing table, as in table-driven algorithm. Route discovery layout is shown simply in Figure 2, where the source node broadcasts route request packet (RREQ) to all its neighbors. Each neighboring node, in turn, rebroadcasts the route request to its neighbors if it has not already done so or if it is not the destination node, considering that the time to live (TTL) packet has not been exceeded. Each RREQ carries a sequence number generated by the source node and the path it has passed through; where the midway nodes upon receiving a RREQ packet, it needs to check the sequence number on that control packet before forwarding it, so it is forwarded only if it is not a duplicate RREQ. Upon reaching the destination node, it sends a reply packet (RREP) on that path back to the sender; this reply packet contains the route to that destination. The destination node chooses the best route received first and caches the other routes for future use. A route cache is preserved at every node, so that if an intermediate node that receives a RREQ has a route to the destination node in its own route cache, then it replies to the source node directly without broadcasting the RREQ anymore. The route maintenance procedure, as shown in [16], is accomplished whenever there is no link between any two nodes. When an intermediate node in the path moves away,  
Figure 2: DSR route discovery. causing a wireless link to break, a route error packet (RERR) is sent by the node that was connected to it to other nodes in the network so they will remove the routes that use that hop, so all routes including that broken link are removed from the node cache. As well, the source node reinitiates the route discovery procedure to find an alternative route to the destination. The route maintenance layout for DSR protocol is illustrated in Figure 3.  
The main benefit that characterizes DSR is the sufficient recovery when route modification is detected in the network and there is no need for any periodic routing advertisement messages. On the other hand, the use of route caches is a good mechanism to reduce propagation delay, but as size of the network increases, the routing overhead also increases since each packet has to carry the entire route to the destination. The main limitation of DSR is that whenever a link break is declared, the RERR packet propagates to the original source, which in turn starts a new route discovery process where the out of order link is not repaired locally. DSR was designed mainly to work well in networks with up to two hundred nodes and a high rate of node mobility. 1. AODV Route Discovery and Route Maintenance  
The ad-hoc on-demand distance vector (AODV) routing protocol employs a reactive approach to discover routes and a proactive mechanism for identifying the most up-to-date paths as in [17]. Moreover, it is designed to reduce overhead data traffic to improve network scalability and performance. It is proposed for mobile ad-hoc networks with capacities up to thousands of mobile nodes and can handle a relatively high mobility rate, as well as a diversity of data traffic levels. The schematic plan for the AODV route discovery process is shown in Figure 4, where the source node broadcasts RREQ packets similar to DSR.