The Ad
Hoc On-Demand Distance Vector (AODV) routing protocol Perkins 1999 is basically,
a combination of DSDV and DSR. It borrows the basic on-demand mechanism of
Route Discovery and Route Maintenance from DSR, plus the use of hop-by-hop
routing, sequence numbers, and periodic beacons from DSDV. AODV minimizes the
number of required broadcasts by creating routes on an on-demand basis, as
opposed to maintaining a complete list of routes as in the DSDV algorithm.
Authors of AODV classify it as a pure on demand route acquisition system since
nodes that are not on a selected path, do not maintain routing information or
participate in routing table exchanges. It supports only symmetric links with
two different phases:

       
I.           
Route Discovery, Route
Maintenance; and

     II.           
Data forwarding.

When a
source node desires to send a message and does not already have a valid route
to the destination, it initiates a path discovery process to locate the
corresponding node. It broadcasts a route request (RREQ) packet to its
neighbors, which then forwards the request to their neighbors, and so on, until
either the destination or an intermediate node with a “fresh enough” route to
the destination is located. Figure 3(a) illustrates the propagation of the
broadcast RREQs across the network. AODV utilizes destination sequence numbers
to ensure all routes are loop-free and contain the most recent route
information. Each node maintains its own sequence number, as well as a
broadcast ID. The broadcast ID is incremented for every RREQ the node
initiates, and together with the node’s IP address, uniquely identifies an
RREQ. Along with the node’s sequence number and the broadcast ID, the RREQ
includes the most recent sequence number it has for the destination.
Intermediate nodes can reply to the RREQ only if they have a route to the
destination whose corresponding destination sequence number is greater than or
equal to that contained in the RREQ.

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During
the process of forwarding the RREQ, intermediate nodes record in their route

tables
the address of the neighbor from which the first copy of the broadcast packet
is

received,
thereby establishing a reverse path. If additional copies of the same RREQ are
later received, these packets are discarded. Once the RREQ reaches the
destination or an intermediate node with a fresh enough route, the
destination/intermediate node responds by unicasting a route reply (RREP)
packet back to the neighbor from which it first received the RREQ (Figure
3(b)). As the RREP is routed back along the reverse path, nodes along this path
set up forward route entries in their route tables that point to the node from
which the RREP came. These forward route entries indicate the active forward
route. Associated with each route entry is a route timer which causes the
deletion of the entry if it is not used within the specified lifetime. Because
the RREP is forwarded along the path established by the RREQ, AODV only
supports the use of symmetric links.

 

Routes
are maintained as follows. If a source node moves, it is able to reinitiate the
route discovery protocol to find a new route to the destination. If a node
along the route moves, its upstream neighbor notices the move and propagates a
link failure notification message (an RREP with infinite metric) to each of its
active upstream neighbors to inform them of the breakage of that part of the
route. These nodes in turn propagate the link failure notification to their
upstream neighbors, and so on until the source node is reached. The source node
may then choose to re-initiate route discovery for that destination if a route
is still desired. An additional aspect of the protocol is the use of hello
messages, periodic local broadcasts by a node to inform each mobile node of
other nodes in its neighborhood. Hello messages can be used to maintain the
local connectivity of a node. However, the use of hello messages may not be
required at all times. Nodes listen for re-transmission of data packets to
ensure that the next hop is still within reach. If such a re-transmission is
not heard, the node may use one of a number of techniques, including the use of
hello messages themselves, to determine whether the next hop is within its
communication range. The hello messages may also list other nodes from which a
mobile node has recently heard, thereby yielding greater knowledge of network connectivity.

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