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Optimizing routes quality and scattering in the AODV routing protocol.
Guezouri, Mustapha
Ouamri, Abdelaziz
Pub Date:
Name: Journal of Computer Science & Technology Publisher: Graduate Network of Argentine Universities with Computer Science Schools (RedUNCI) Audience: Academic Format: Magazine/Journal Subject: Computers Copyright: COPYRIGHT 2007 Graduate Network of Argentine Universities with Computer Science Schools (RedUNCI) ISSN: 1666-6046
Date: Oct, 2007 Source Volume: 7 Source Issue: 3

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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


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.


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).


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.


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.


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.


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).


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).


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)



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:
Destination IP
   Source IP
   Broadcast id
   Expiration time for reverse path route entry
   Source node sequence number
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