Page 2 Lecture Notes in Computer Science 2865 Edited by G. Goos ...

28 P. Narayan and V.R. Syrotiuk2.2 An Overview of the DSR and AODV Routing ProtocolsDynamic Source Routing. The Dynamic Source Routing (DSR) protocol [11]is an on-demand routing protocol that allows nodes to dynamically discover asource route to any destination in the network. In source routing, the source isresponsible for computing the route that a packet should take.When a node wishes to communicate with another node, it employs routediscovery to flood a control packet, called a route request (RREQ), through thenetwork, in search of a route to the destination. A RREQ packet with a target oft is broadcast in the network. It is forwarded hop-by-hop from the node initiatingthe route discovery. When the RREQ reaches the destination t or a node thatis aware of a route to t, forwarding stops. A route reply (RREP) packet is sentback to the source s on the reverse path, including a full source route to thedestination t. This source route is included in the header of each data packetsent to t and enables stateless forwarding. Data sent to t by an application isbuffered at s until a route reply with a route to t is received, at which point,these packets are forwarded to t along the acquired route.The route maintenance mechanism monitors the status of source routes inuse, detects link failures and repairs routes with broken links. When route maintenanceindicates that a source route is broken, s can attempt to use any otherroute it happens to know to t, or the source s can invoke route discovery againto find a new route.Ad Hoc on Demand Distance Vector. The Ad hoc On Demand DistanceVector (AODV) [16] routing protocol is also an on-demand protocol. Similar totraditional distance vector protocols, AODV maintains routing tables with oneentry per destination.AODV builds routes using RREQ and RREP control packets similar to theroute discovery mechanism in DSR. A node receiving the RREQ packet maysend a RREP if it is either the destination or if it has a route to the destinationwith corresponding sequence number greater than or equal to that contained inthe RREQ. Otherwise, it rebroadcasts the RREP. As a RREP propagates backto the source, nodes set up forward path entries to the destination in their routetables. Once the source node receives the RREP it may begin to forward datapackets to the destination.There are several differences in the route discovery mechanisms of DSR andAODV. The source routing mechanism used in DSR enables s to learn routestowards each intermediate node on the route to t. Additionally, each intermediatenode on the path from s to t may learn routes to every other node on the route.In DSR, the destination node replies to all RREQs sent towards it during asingle request cycle. Thus, in DSR, the source learns many alternate routes tothe destination in contrast to AODV where the destination replies only to thefirst arriving request.