MANET Routing Protocols

Routing protocols designed for wired networks are not suitable for MANET, because of the mobile nature of the nodes, as routes will expire and change over time [45]. A whole set of different routing protocols has been designed specifically for MANET. This routing falls into two major categories, the proactive and reactive protocols. These protocols will be discussed further below.

2.3.3.1.

Proactive Protocols

A proactive routing protocol keeps an updated routing table by regularly requesting and sharing updated information with its neighbouring nodes. Mobile nodes that want to transmit packets already know the packets optimal route to their respective destination. This scheme requires a high number of messages to keep the routing tables up to date; as a result, the limited network resources such as bandwidth and battery life are used. Proactive protocols are often referred to as Distance-Vector or Link State protocol. An example of proactive protocol is the Destination-Sequenced Distance-Vector routing (DSDV) [16].

2.3.3.2.

Reactive Protocol

A reactive protocol establishes a route when a node wishes to transmit a packet, and there is no valid route in the routing table. Nodes maintain routes until the destination is not reachable, or the route is no longer needed. The advantage of this type of routing protocol is that, there is less traffic overhead compared to the proactive routing protocol; thus increasing the available bandwidth for sending more information. Examples of reactive protocols are Dynamic Source Routing (DSR) and Ad-hoc On-demand Distance Vector routing protocol (AODV) [46], both protocols are discussed below.

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A. Ad hoc On-demand Distance Vector Routing (AODV)

When using Ad hoc On-demand Distance Vector Routing (AODV) protocol, a route is established only when the source node needs it. This protocol was designed to be autonomous and adapt to a variety of different network behaviour such as node mobility, channel changes, link failures, and packet loss. AODV consists of two mechanisms, which are Route Discovery and Route Maintenance. Each node maintains at most one route to a destination. When the available route from a source to a destination fails, AODV protocol invokes a new route discovery. Route discovery is also invoked whenever there is a topology change; however, this can sometimes be inefficient. AODV uses the destination sequence to maintain an up to date path to the destination. Nodes update their routing table when the destination sequence of a received packet is greater than or equal to the stored destination sequence at the node with a smaller hop count. The routing table of nodes contains the most recent available route to the destination as confirmed by the source. AODV uses a broadcast identify number to prevent multiple broadcasts of similar packets. Intermediate nodes only transmit the first received copy of a packet and similar packets received are discarded [47]. Each routing table entry in AODV consists of the following fields:

 Destination IP Address: The IP address of the destination of the packet being transmitted

 Destination Sequence Number

 Next hop: This can be the IP address of the destination or an intermediate node configured to forward the packet to its destination.

 Hop Count: The number of hops required from the source to the destination

 Lifetime: The time, in which nodes receiving the RREP packet, considers a route to be valid and is in milliseconds

 Routing Flags: The current state of the route; down (not valid) or in repair, up (valid)

B. Dynamic Source Routing protocol (DSR)

DSR uses source routing instead of relying on routing tables as in AODV. Individual nodes maintain a routing log accumulated during the process of route discovery, where learned routes are stored. In order to send data to another node, the sender first checks the routing

Oche Alexander Egaji Page 24 log to determine if there is an available route. If there is an available route, the source appends this routing information to the packet header and sends it to the next available hop on its path. Each intermediate node on receiving the packet examines the header to determine the next available hop and then forwards the packet to the next node indicated in the packet route. When no route is found on the packet, it is sent to a buffer whilst the router tries to obtain a route using a route discovery process [48]. The route discovery and maintenance process are discussed below.

Route Discovery and Maintenance:

 In order to discover a route for a packet, a Route Request (RREQ) packet is broadcast from the source to all nodes within its radio transmission range. A route request contains the address of the source and destination nodes as well as a route record. The route record contains the entire path followed by the route request packet to get to the destination from the source node [48]. The node, which receives the route request does the following:

 It checks if the address matches the destination address of the route if so it is the destination; otherwise it is an intermediate node. If the node is receiving this route request for the first time, it checks its routing table to find a route to the destination. If a route is found, it creates a route reply packet with the route from its routing table and sends it back to the source. This type of reply is called intermediate-node reply. Otherwise, if the route to the destination is not available in the routing table; the node appends its address to the routing table, increment the hop count by one and rebroadcast the request. This process continues until the destination or a node with a valid route is reached. When a node acknowledges being the required destination on receiving the RREQ packet, the node sends a Route Reply packet (RREP) to the node that originated the RREQ message. This RREP follows a reverse route of the RREQ packet. On arriving at the originating node, the route path to the destination node is retrieved from the RREP packet [48][49].

When the RREP packet reaches the source, it adds the routing information to its routing table and then sends any pending data in its buffer to the destination via the new route. If the MAC

Oche Alexander Egaji Page 25 layer of the transmitting node detects any broken links, a route error packet is generated. This packet is sent back to the source through the reverse path to erase all entries in the routing table that contains the broken link.