• Request Buffer (flag): a flag to show if this node passes on the request
message or not.
- Route discovery
When a node needs to transmit data packets to a destination, it broadcasts a RREQ message
if the route to the destination is unknown or the previous route expires. After RREQ
broadcast, the node waits a period of time in order to receive a RREP for its request. If the
node does not receive a reply it may rebroadcast the RREQ again or assumes that the
destination is no longer exists.
If there is no valid route for the destination carried by the RREQ at the intermediate nodes,
the intermediate nodes forward the RREQ by broadcasting. At the same time these nodes
keep a reverse route which contains the source IP address and the IP address from the RREQ
received. This reverse route is a temporary route in order to be used later to form a RREP to
the source. Figure 2.3 (a) shows the propagation of RREQ from source node H to find out a
route to the destination C, the message will pass through the intermediate nodes (F, G and
D).
A node generates RREP if it is either the destination node or a node with valid route to the
destination. While RREP is forwarded back, a route for the source node is created to the
destination and the route is ready for sending data packets. Figure 2.3 (b) shows the RREP
message sent by the destination (D) upon receiving the RREQ message from the source H.
- Route maintenance
A node will detect a link failure to one of its neighbours by link layer messages or by
periodical Hello messages. As soon as the node detects this failure it will remove the route
entry for that neighbour and creates a route failure message, then it sends this message to all
neighbouring nodes that are actively using this route, informing them that this route is no
longer valid. Each node in AODV protocol keeps a list of active neighbours in order to keep
track of neighbours that are using particular list of neighbours.
18
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