ABSTRACT
An ad-hoc mobile network is a collection of mobile nodes that are
dynamically and arbitrarily located in such a manner that the
interconnections between nodes are capable of changing on a continual
basis. Routing protocols are used to discover routes between nodes. Many
mobile ad-hoc networks protocols such as AODV construct route only when
desired by the source node (reactively). The advantage hereof is that no
prior assumptions of the network topology are required. In highly mobile
networks this is an attractive property. Other used protocols (such as
OLSR) are said proactive. Such protocols maintain information about
routes to all destinations all times. The consequence of this approach
is that the amount of control traffic is independent of the actual
traffic and mobility in the network.
In this paper we describe three major optimization schemes for the
well-known AODV routing protocol in order to get some of the proactive
protocols features in it. The targeted characteristics are: traffic
independent control and shortest path routes.
Keywords: Manet, Ad-hoc networks, Mobile networks, Wireless
networks, Dynamic routing
1. INTRODUCTION
A mobile ad-hoc network (MANET) is a collection of nodes capable of
movement and connected dynamically in an arbitrary manner. Nodes of
theses networks function as routers which discover and maintain routes
to other nodes in the network.
The issue in MANETs is that routing protocols must be able to
respond rapidly to topological changes in the network. At the same time
the amount of control traffic generated by the routing protocols must be
kept at a minimum due to the limited available bandwidth through radio
interfaces.
Since the advent of DARPA packet radio networks in 1970's [9]
several protocols dealing with the problems of routing in mobile ad-hoc
networks have been developed. These protocols may generally be
categorized as (a) proactive or table driven [14], [4] and (b) reactive
or on demand driven.
Proactive routing protocols attempts to maintain consistent,
up-to-date routing information from each node to every other node all
times. Theses protocols require each node to maintain on or more tables
to store routing information and respond to topological changes by
propagating updates through the network.
Thus using a proactive protocol, a node is immediately able to
route or drop a packet. Examples of proactive protocols are TBRPF
"Topology Broadcast based on Reverse Path Forwarding" [18] and
OLSR [19] (Optimized Link State Routing protocol".
Reactive routing creates routes only when desired by the source
node. When a node requires a route to a destination, a query is flooded
on the network and replies containing possible routes to the destination
are returned. Examples of reactive protocols include AODV "Ad-hoc
On Demand Distance Routing protocol" [15] and DSR "Dynamic
Source Routing" [2].
In this paper three optimization schemes of the AODV will be
presented. Theses optimizations aim in on hand to render the amount of
control traffic independent of the actual traffic and mobility in case
of high utilization of the network and keep it as low as possible
otherwise. In a second hand ensure that the learned routes are the
shortest ones in term if hop count.
The reminder of this paper is organized as follows: in section 2, a
short overview of the AODV routing protocol is given, emphasizing on the
path setup stages and route maintenance. In section 3, we introduce
three optimization schemes for the previously described AODV protocol.
The paper is concluded in section 4.
2. AODV OVERVIEW
Ad-hoc On demand Distance Vector algorithm [15][16] is described by
its authors as a pure on-demand route acquisition system, as nodes that
are not on a selected path do not maintain routing information. A node
doesn't discover and maintain a route to another node until a
communication is needed. Local connectivity is maintained by the use of
local broadcast known as hello messages.
The path discovery process is initiated whenever a source node
needs to communicate with another node for which it has no routing
information. Path initiation is done by broadcasting a route request
RREQ packet to the neighbours, which then forward the request to their
neighbours, and so on, until either the destination or an intermediate
node with a route to the destination is located (fig 2.a). AODV utilizes
destination sequence numbers to ensure all routes are loop-free and
contain the most recent route information. The RREQ contains the
following fields:
.
broadcatid and source_sequence_# are incremented whenever the
source issues a new RREQ. The pair uniquely identifies a RREQ. The source node
includes in the RREQ the most recent sequence number it has for the
destination. Each neighbour either satisfies the RREQ by sending a route
reply (RREP) back to the source or broadcasts the RREQ to its neighbours
after increasing the hop count (hop_cnt).
[FIGURE 2.a OMITTED]
An Intermediate node can reply to the RREQ only if it has a route
to the destination whose corresponding destination sequence number is
greater than or equal to that contained in the RREQ. If a node cannot
satisfy the RREQ, it keeps track of the following information to
implement the reverse path setup as well as the forward path setup:
As the RREP is routed back along the reverse path, node along this
path setup up forward route entries pointing to the node from witch the
RREP came (fig 2.b).
A timer is associated with each route entry in order to delete it
if it is not used within a specified lifetime. If a source moves, it is
able to reinitiate a route discovery to find a new route. When either
the destination or some intermediate node moves, a special RREP is sent
to the affected source nodes.
To maintain local connectivity, the protocol uses periodic local
broadcasts of hello messages to inform each mobile node of the others
nodes in the neighbourhood. The use of hello messages is not necessary;
nodes can listen for retransmission of data packets to determine if the
next hop is within communication range.
[FIGURE 2.b OMITTED]
3--AODV Optimization schemes
In this section, we propose modified schemes for the AODV routing
algorithm. The objectives herein are: (a) reduce the amount of control
traffic during high network utilization and mobility periods and make it
as possible independent of the actual traffic and (b) get the shortest
path for a destination node. Theses schemes rely on modifying the rules
a node obey during the reverse and forward path setup stages.
3.1. Reverse path setup
In AODV, to set up a reverse path, a node records the address of
the neighbour from witch it received the first copy of the RREQ. This
only guarantees a fast setup, and not the shortest path to the source
(fig 3.a). In our modified scheme, a node updates the reverse path each
time it receives a RREQ request from the source with hop count less than
the stored one. This RREQ request is not forwarded.
[FIGURE 3.a OMITTED]
3.2. Forward path setup
In AODV, if an intermediate (possibly the destination it self) does
have a current route to the destination and if it has not been processed
previously then the node unicasts a route reply packet back to the
source. This also doesn't guarantee that the forward route to the
destination is the shortest one. To ensure the selection of the shortest
path, we propose a new scheme in which an intermediate node, replies
each time it receive a RREQ with hop count less than the one previously
processed.
3.3. Route Scattering
The third scheme we propose in this paper concerns route
scattering. Each node on the newly discovered route by the AODV
algorithm knows only how to reach the end points of the path and not the
other nodes on the same path (fig 3.b). Hence, if two nodes on an active
path need to communicate, the whole process must be restarted.
[FIGURE 3.b OMITTED]
To prevent this, we propose to modify the RREQ and RREP to contain
an additional field routers_list. Upon the receipt of a route request
RREQ, each node either satisfies the RREQ by sending a route reply RREP
back to the source with routers_list containing the IP addresses of all
the nodes from the source to the destination or rebroadcasts the RREQ to
its own neighbours after adding its address to the routers_list and
increasing the hop_cnt field.
As the RREQ travels from a source to a destination, it
automatically sets up reverse path to all the nodes back to the source
by using the routers_list field of RREQ (fig 3.c).
[FIGURE 3.c OMITTED]
The same way, as the RREP travels back to the source, each node
along the reverse path sets up a forward pointer to all nodes along the
way to the destination. This is also possible because of the
routers_list in RREP (fig 3.d).
[FIGURE 3.d OMITTED]
Finally, all nodes on the path from the source to the destination
broadcast their forward and reverse entries to their neighbours outside
the path. This, permit to those nodes in the neighbourhood of the path
to use it as a backbone (fig 3.e)
[FIGURE 3.e OMITTED]
4. CONCLUSION AND FURTHER WORKS
Non-optimal routes bring a non negligible overhead that is
proportional to the data load of the network. We have shown that the
AODV algorithm yields non-optimal routes and then proposed three
modified simple schemes with the goal of reducing route length overhead.
This is done by first modifying the node's behaviour face of RREQs
and RREPs and second by adding a new field to RREQ and RREP. Route
scattering is another presented scheme which aims to provide a backbone
to the nodes in the neighbourhood of an active path.
Currently, are about specifying the details of the proposed schemes
in an Internet Draft to be submitted to the IETF manet working group.
Simulation work is in progress to test theses new schemes under
different traffic and mobility scenarios.
Received: Jun. 2006. Accepted: Aug. 2007.
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Dr. Mustapha GUEZORI and Abdelaziz OUAMRI
Signal Image Laboratory, Department of Electronics, Faculty of
Electrical Engineering, University of Science and Technology (USTO),
P.O. Box 1505--El-m'naouar, Oran, ALGERIA. e-mail:
mguezouri@yahoo.frDestination IP
Source IP
Broadcast id
Expiration time for reverse path route entry
Source node sequence number