# Impact of Direction Parameter in Performance of Modified AODV in VANET
## 2 AODV and Modified AODV Routing Algorithms  
The AODV [15] routing protocol is reactive, where routes are determined only when needed. Ad hoc On-Demand Vector is bandwidth efficient because it works only on demand. Figure 1 presents the AODV routing protocol message exchanges. Figure 1: AODV messaging. In this protocol, each mobile node identifies other neighborhood nodes by flooding or by receiving a local broadcast known as a Hello message. At this time, to provide an immediate response to the requester for establishing new routes, the routing tables of the neighboring nodes are modified with response time of local movements. The main objectives of this routing protocol are [15]:  
1. To broadcast discovery packets (Hello) and receive acknowledgment when necessary. 2. To distinguish between general topology maintenance (route management) and local connectivity management (detection of neighborhood). 3. To inform the neighboring mobile nodes about the changes in local connection. Hello messages [15] are utilized to detect and to observe the nearby neighbors. When Hello messages are used, every active node in the network periodically transmits a Hello message to its neighbors. These messages permit the detection of a connection or link break. The protocol uses different types of messages, shown below, to discover and maintain the links (broken links or selecting a different path). They are:  
1. Route Request (RREQ)
2. Route Reply (RREP)
3. Route Reply Acknowledgement (RREP-ACK)
4. Route Error (RERR)  
A source broadcasts [15] an RREQ to transfer data to an unknown destination. Therefore, a route from a node to the source is created at the intermediate node after the reception of an RREQ. If the receiving node does not receive the RREQ, the source rebroadcasts it. When the destination node is the receiving node or has a recent route from the source to the destination, it creates an RREP. As the RREP circulates, intermediate nodes generate routes to the destination. If the source receives the RREP, it registers the route to the destination in its routing table. Then it starts directing data. The source chooses the path with the shortest hop count after receiving multiple path information. When the data is transferred from the source to the destination, nodes update their timers, which is related to the time the route is used between the source and destination to maintain the routing table. When a route is inactive (for about 2000 ms), the node invalidates the route and removes it from its routing table. The RREP-ACK message [15] is generally sent in response to an RREP message to complete the route discovery cycle. If a link failure is detected while transferring the data, the node sends an RERR message to the source in a hop-by-hop approach. As the RERR is headed for the source, the intermediate nodes invalidate routes to inaccessible destinations. The source also invalidates the route after receiving the RERR and initiates the route discovery again. An AODV uses a destination sequence number (DSN) [15] to avoid counting to infinity to be loop free (no duplication of packet sending). The destination includes the destination sequence number and any route information when sending messages to requesting nodes. A node broadcasts an RREQ to all network nodes to discover a route as long as it finds the destination or another node with the most recent route to the destination. The requesting node then selects the best route based on the DSN (an AODV uses a DSN to determine the most recent path to a destination). The entries in the routing table are related to sequence numbers generated by the destination. A DSN acts as a route timestamp, confirming a fresh route. When an intermediate node receives an RREQ packet, it compares its DSN with that in the RREQ packet. If the stored DSN is greater than the current DSN in the RREQ packet, the existing route is considered to be up to date. At that point, an RREP is sent back to the source, and the route (discovered) is made available.