31-07-2012, 03:19 PM
A Quick Guide to AODV Routing
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AODV is a method of
routing messages between
mobile computers. It allows
these mobile computers, or
nodes, to pass messages
through their neighbors to
nodes with which they
cannot directly
communicate. AODV does
this by discovering the
routes along which
messages can be passed.
AODV makes sure these
routes do not contain loops
and tries to find the shortest
route possible. AODV is
also able to handle changes
in routes and can create
new routes if there is an
error.
The diagram to the left
shows a set up of four
nodes on a wireless
network. The circles
illustrate the range of
communication for each
node. Because of the
limited range, each node
can only communicate with
the nodes next to it.
Sequence Numbers
Sequence numbers serve
as time stamps. They allow
nodes to compare how
“fresh” their information on
other nodes is. Every time a
node sends out any type of
message it increase its own
Sequence number. Each
node records the Sequence
number of all the other
nodes it talks to. A higher
Sequence numbers signifies
a fresher route. This it is
possible for other nodes to
figure out which one has
more accurate information.
In the example, Node 1 is
forwarding a RREP to Node
4. It notices that the route in
the RREP has a better
Sequence number than the
route in it’s Routing List.
Node 1 then replaces the
route it currently has with
the route in the Route Reply
Error Messages
The Route Error Message (RERR) allows
AODV to adjust routes when Nodes move
around.
Whenever a Node receives RERR it looks at
the Routing Table and removes all the routes
that contain the bad Nodes.
The diagrams to the left illustrate the three
circumstances under which a Node would
broadcast a RERR to its neighbors.
In the first scenario the Node receives a Data
packet that it is supposed to forward but it
does not have a route to the destination. The
real problem is not that the Node does not
have a route; the problem is that some other
node thinks that the correct Route to the
Destination is through that Node.
In the second scenario the Node receives a
RERR that cause at least one of its Route to
become invalidated. If it happens, the Node
would then send out a RERR with all the new
Nodes which are now unreachable
In the third scenario the Node detects that it
cannot communicate with one of its Neighbors.
When this happens it looks at the route table
for Route that use the Neighbor for a next hop
and marks them as invalid. Then it sends out a
RERR with the Neighbor and the invalid routes