A2- 2
use for long time are deleted from the table [56]. A routing table entry is
expired if not used recently. A set of predecessor nodes is maintained for
each routing table entry, indicating the set of neighboring nodes which use
that entry to route data packets [57]. These nodes are notified with Route
Errors (RERR) packets when the next-hop link breaks. Each predecessor
node, in turn, forwards the RERR to its own set of predecessors, thus,
effectively erasing all routes using the broken link. Route error propagation
in AODV can be visualized conceptually as a tree whose root is the node at
the point of failure and all sources using the failed link as the leaves [ 58 ].A 2. 3 Temporally Ordered Routing Algorithm (TORA)
TORA is adaptive and scalable routing algorithm based on the
concept of the link reversal. It finds multiple roots from source to
destination in a highly dynamic mobile networking environment. An
important design concept of TORA is that control messages are localized to
a small set of nodes nearby a topological change. Nodes maintain routing
information about their immediate one hop neighbors [59]. TORA has three
basic functions, namely: route creation, route maintenance and route
erasure. Nodes use a “height” metric to establish a Directed Acyclic Graph
(DAG) rooted at the destination during the route creation and route
maintenance phases. The link can be either an upstream or downstream,
based on the relative height metric of the adjacent nodes [60]. Route
maintenance is necessary when any of the links in DAG is broken. The
strength of this protocol is the way it handles link failures. TORA‟s
reaction to link failures is so optimistic that it will reserve the links to re-
position the DAG for searching for an alternative path. One drawback of
TORA is that the algorithm may also produce temporary invalid routes
[60].