wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

4-10 Industrial Communication Systems
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[S, E, F, J, D] [E, F, J, D]
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[C, S]
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[J, F, E, S]
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FIGURE 4.4
Example of the route cache in the DSR protocol.
4.5.4 ad Hoc On-Demand Distance Vector Routing Protocol
The AODV protocol is built on the destination-sequenced distance-vector (DSDV) protocol. The
improvement here is to minimize the number of transmissions required to create routes, because
as they are on demand, the nodes that are not placed in the chosen path do not have to maintain
the route or participate in table interchanges. Several studies on this protocol have been carried out
[PR99,RP99,RP00], and it has been modified [SWL03] several times. Currently, it is a protocol in
experimental state [PBD03].
When a node wants to transmit to a destination and it does not have a valid route, it must start the
path discovery process. To do this, in the first place, it broadcasts a RREQ message to its neighbors,
which in turn sends it to their own neighbors and so on, until it gets to the destination or to an intermediate
node that has a route to the destination. Like DSDV, it uses sequence numbers to identify the most
recent routes and delete loops. Each node maintains two counters: the node’s sequence number (to avoid
loops) and Broadcast_ID that is increased when a transmission is initiated in the node.
To identify a single RREQ, it uses the Broadcast_ID and the IP address of the source node. The
RREQ contains the following fields: source_addr, number_sequence_#, broadcast_id, dest_addr, dest_
sequence_#, and hop_cnt. Intermediate nodes only reply to RREQ messages if they have a route to the
destination with a sequence number greater or equal to the one stored in the RREQ, in other words, only
if they have equal routes (in age) or more recent. While the RREQ is sent, intermediate nodes increase
the “hop_cnt” field, and they also register in their routing table the address of the neighbor from which
they first received the message, to establish the Reverse Path, where RREQs are still sent, and at the same
time, the Reverse Path is being established. The copies of the same RREQ received after that coming
from other neighbors are dismissed.
Once the “destination node/intermediate node with recent route” has been found, it replies with a unicast
packet known as RREP to the neighbor from which it received the first RREQ. The RREP uses the
links that had been established before as reverse path. The RREP contains the following fields: source_addr,
dest_addr, dest_sequence_#, hop_cnt, and lifetime or expiration time for the Reverse Path. The RREP uses
the reverse path established to the source node. In its route, all the nodes the RREP passes through write
the Reverse Path as the most recent route to the destination node. From that we can conclude that the
AODV holds only bidirectional links. In Figure 4.5, the AODV route discovery procedure is shown.
If a source node moves, it is able to restart the discovery protocol to find a new route to the destination.
If an intermediate node moves, its previous neighbor (in the source-destination direction) broadcasts
(until the source node is reached) a not-requested RREP with a “recent” sequence number (in other words,
greater than the sequence number known) and with a number of hops to the destination equal to infinite.
That way, the source node restarts the route discovery process in the case that it still needs the route.
When the link breakage happens, the node must invalidate the existing route in the routing table
entry. The node must list the affected destinations and determine which neighbors can be affected by
© 2011 by Taylor and Francis Group, LLC