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AODV is a variation of Destination-SequencedDistance-Vector (DSDV) routing protocol which iscollectively based on DSDV and DSR. It aims to minimizethe requirement of system-wide broadcasts to its extreme. Itdoes not maintain routes from every node to every other nodein the network rather they are discovered as and when needed& are maintained only as long as they are required.The key steps of algorithm used by AODV forestablishment of unicast routes are explained below.A. Route DiscoveryWhen a node wants to send a data packet to a destinationnode, the entries in route table are checked to ensure whetherthere is a current route to that destination node or not. If it isthere, the data packet is forwarded to the appropriate next hoptoward the destination. If it is not there, the route discoveryprocess is initiated. AODV initiates a route discovery processusing Route Request (RREQ) and Route Reply (RREP). Thesource node will create a RREQ packet containing its IPaddress, its current sequence number, the destination’s IPaddress, the destination’s last sequence number andbroadcast ID. The broadcast ID is incremented each time thesource node initiates RREQ. Basically, the sequence numbersare used to determine the timeliness of each data packet andthe broadcast ID & the IP address together form a uniqueidentifier for RREQ so as to uniquely identify each request.The requests are sent using RREQ message and theinformation in connection with creation of a route is sent
back in RREP message. The source node broadcasts the
RREQ packet to its neighbours and then sets a timer to wait
for a reply. To process the RREQ, the node sets up a reverse
route entry for the source node in its route table. This helps to
know how to forward a RREP to the source. Basically a
lifetime is associated with the reverse route entry and if this
entry is not used within this lifetime, the route information is
deleted. If the RREQ is lost during transmission, the source
node is allowed to broadcast again using route discovery
mechanism.
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