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ratio is considered. This protocol provides higher
packet delivery and lower routing overheads at
the expense of slightly longer end-to-end delays.
It is noteworthy that the aforementioned QoS
protocols utilize AODV as the underlying routing
mechanism with only the end-to-end bandwidth
and distance metrics. Moreover, most proactive
and reactive protocols choose a route based on
the number of hops to the destination that may
be inappropriate when there is congestion. On the
other hand, recent AODV modifications, although
considering traffic load as a route selector, do
not deal with a dynamic environment (e.g. flash
crowds, burst traffic, transient congestion and
mobile patients) once the path is fixed (Argyriou,
2006; Choi, 2003; Jung, 2004; Lee, 2005; Pham,
2004; Saigal, 2004; Yoo, 2004). Before present-
ing the main focus of this chapter, we will briefly
describe the AODV protocol mechanism by which
modification the proposed router cooperative
strategy is implemented.
Because of their limited range, each node
can only communicate with the nodes next to it.
A node keeps track of its neighbors by listening
for a HELLO message that each node broadcasts
periodically. When one node needs to send a mes-
sage to another node that is not its neighbor, it
initiates a path discovery phase by broadcasting a
route request (RREQ) packet to its neighbors. The
request (RREQ) message contains several fields
such as the source, destination and lifespan of
the message and a Sequence Number that serves
as a unique ID.
When intermediate nodes receive a RREQ
packet, they update their routing tables for a reverse
route to the source and, in the same way, when the
intermediate nodes receive a route reply (RREP),
they update the forward route to the destination.
If multiple RREPs are received by the source,
the route with the shortest hop count is chosen.
If a route is not used for some period of time,
a node cannot be sure whether the route is still
valid; consequently, the node removes this route
from its routing table.
Sequence Numbers serve as time stamps
allowing nodes to determine the timeliness of
each packet and to prevent the creation of loops.
Every time a node sends out any type of message
it increases its own “Sequence Number”. Each
node records the Sequence number of all the
other nodes. A higher Sequence Number refers
to a fresher route.
The Route Error Message (RERR) allows the
AODV to adjust routes when node/link failure
occurs.
Whenever a node receives a RERR, it looks at
the routing table and removes all the routes that
contain the bad nodes. When the next hop link
breaks, RERR packets are sent by the starting
node of the link to a set of neighboring nodes
that communicate over the broken link with the
destination.
If data is flowing and a link break is detected,
a Route Error (RERR) packet is sent to the source
of the data in a hop-by-hop fashion. As the RERR
AODV Protocol
The Ad-hoc On-Demand Distance Vector Rout-
ing Protocol (AODV) is one common routing
algorithm in ad hoc networks and is based on the
principle of discovering routes as needed. AODV
is a reactive algorithm that has a low network uti-
lization, processing and memory overheads. The
request is made on-demand rather than in advance,
to take into account the dynamic changing of a
network structure. In the AODV routing algorithm,
the source node issues a route request packet to
the destination node at the time a path is needed
and this allows mobile nodes to pass messages
through their neighbors to nodes with which they
cannot directly communicate. AODV does this by
discovering the routes (the discovery phase) 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.
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