routing information at each node helps in handling any changes in the network
In the reactive protocols, discovered routes are usually stored in a route cache
in the nodes that are involved in a transmission on those routes. These routes
should be refreshed for the nodes to ensure their validity. Also, they should be
invalidated when expired in order not to use them in subsequent transmissions.
Also, failures of these routes should be handled either locally or by the source
nodes if these routes are still needed for further transmissions. The reactive pro-
tocols use various approaches to support all the processes related to the route
maintenance operation. For example:
- In DSR, each node should make sure that the packet is received by its next hop
by means of acknowledgment. 3 Otherwise, the node should retransmit the
packet (up to a certain limit). When the limit of retransmission is reached, the
sending node should look for an alternative path to the destination in its cache. If
there is no other alternative, the sending node sends a Route Error packet to the
source with information about the broken link. The source should remove the
broken link from its cache and start the route discovery process again.
- In AODV, to maintain local connectivity, if a node has not sent any packets to
its active neighbors for a predefined period, the node broadcasts a Hello mes-
sage to inform its neighbors that it is still there and alive. If any node along a
route has moved, this can be detected by its predecessor that will generate a link
failure notification message and sends this message to the active neighbors of
this route. These neighbors send this message to their active neighbors and so on
until it reaches the source which can reinitiate the route discovery process if it
still needs to send data to that destination.
- The CBRP protocol deploys a Local Repair mechanism. With this mechanism,
if a node detects that its next hop is unreachable, it tries to salvage the packet. It
checks the source information in the packet to get the hop after the next, 4 then it
checks its neighbor table and looks for a neighbor node that can reach the next
hop or the hop after the next. If any of these two hops is reachable by one of its
neighbors, the packet is forwarded through the new route.
- The Temporally Ordered Routing Algorithm (TORA) protocol [ 27 ] depends in
its operation on creating a Directional Acyclical Graph (DAG) from the source
to the destination. When a node loses its downstream link, it generates a new
reference level and broadcasts the reference to its neighbors. TORA also
involves a Route Erasure mechanism for retiring the invalid routes. The erase
operation floods CLR packets through the network to erase invalid routes [ 28 ].
As the hybrid protocols are a combination of both reactive and proactive
protocols, route maintenance in the hybrid protocols is also a combination of the
CBRP employs source routing, so, the whole path is stored in the data packet.