Design and investigation of scalable multicast recursive protocols for wired and wireless ad hoc networks

2.4.3.1 Ad hoc on demand distance vector routing protocol (AODV)

AODV [16, 56, 54] is a reactive ad hoc protocol, that can be considered as a combination of
DSDV [53] and DSR [43]. It takes the on-demand mechanism for route discovery and route
maintenance from DSR [43], while the hop-by-hop and sequence number is taken from
DSDV [53].

AODV uses different set of messages for route discovery and route maintenance. Whenever a
node has data packets and needs to send these packets to another node it broadcasts a Route
Request message (RREQ) to all its neighbours. This RREQ message propagates through the
network until reaches the destination node or an intermediate node that posses a fresh route
to the destination. The route will be ready by unicasting a Route Reply message (RREP) to
the source from the destination.

Another kind of messages used by this algorithm is Hello messages. These messages are sent
periodically in order to keep each node and its neighbours connected and maintain the routes
valid. If one node does not receive a Hello message from its neighbour node, the later
assumes that its neighbour has moved away and marks that link to the node as broken then
notifies the affected nodes by sending a link failure notification to them.

The updated information about routes is kept in a table, this table called a route table. The
structure of this table and the operation implemented in AODV are as follows:-

- Route table management.

AODV maintains a route table at each node; the following information should remain tracked
for each route at this table:

• Destination sequence number: Sequence number for the destination known to
this node.

• Destination IP address: the destination node IP address.

• Next Hop: the neighbour node for this route toward the destination.

• Hop Count: number of hops to reach the destination.

• Life Time: the amount of time this route is valid.

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