How does the MANETS AODV Reactive Routing Protocol work?

MANET stands for Mobile Ad-Hoc Network. It is an infrastructure-less collection of mobile nodes that can arbitrarily change their geographic locations, creating networks with dynamic topologies composed of bandwidth-constrained wireless links.

MANET nodes are equipped with wireless transmitters and receivers. Based on node positions, coverage designs, and transmission power levels, wireless connectivity exists between nodes in the form of a random, multi-hop graph. Current applications include defense services, emergency search and rescue, meetings, conventions, and scenarios requiring quick information sharing without fixed infrastructure.

Unpredictable topological changes make route discovery and maintenance challenging in these networks. Conventional routing protocols using distance-vector or link-state algorithms cannot be used effectively because routing traffic would consume excessive wireless bandwidth, and discovered routes would quickly become obsolete due to node mobility.

AODV Protocol Overview

AODV (Ad hoc On-Demand Distance Vector) is a reactive routing protocol that uses hop-by-hop routing. When a node needs to find a route to a specific destination, it creates a Route Request (RREQ) message. Intermediate nodes forward this request while simultaneously creating a reverse route back to the source.

When a node that knows the route to the destination receives the RREQ, it creates a Route Reply (RREP) containing the number of hops needed to reach the destination. Each node that helps forward this reply to the source establishes a forward route to the destination.

AODV Message Types

AODV uses three types of routing messages:

• Route Request (RREQ) − When a node needs to send a packet but doesn't know the route to the destination, it initiates route discovery by broadcasting an RREQ message. Neighboring nodes record where the message originated and forward it until it reaches the destination node.

• Route Reply (RREP) − The destination node responds with an RREP, which travels back to the source along the reverse path established by the RREQ. Forward routes are created in intermediate nodes as the RREP returns to the source. An intermediate node can also send an RREP if it already knows the route to the destination.

• Route Error (RERR) − This message indicates that a destination is unreachable. Nodes receiving this message mark the route as inactive and typically delete it. The next packet requiring this route will trigger new route discovery.

How AODV Works

The AODV protocol operates in the following sequence:

1. Route Discovery − Source node broadcasts RREQ to find path to destination

2. Path Establishment − Intermediate nodes create reverse routes and forward RREQ

3. Route Reply − Destination or intermediate node with valid route sends RREP back

4. Route Maintenance − Nodes monitor link status and send RERR when links break

Advantages and Disadvantages

Conclusion

AODV is a reactive routing protocol designed for MANETs that discovers routes on-demand using RREQ and RREP messages. It efficiently handles dynamic topologies by establishing routes only when needed and maintaining them through error detection, making it suitable for mobile wireless networks with moderate node mobility.