Ad Hoc Networks : Technologies and Protocols - University of ...

70 Routing in Mobile Ad Hoc Networks
with a higher distance) based on past history and uses it to avoid propagating
every improvement in distance information.
Wireless Routing Protocol (WRP) [41] is another distance vector protocol
optimized for ad hoc networks. WRP belongs to a class of distance vector protocols
called path finding algorithms. The algorithms of this class use the next
hop and second-to-last hop information to overcome the counting-to-infinity
problem; this information is sufficient to locally determine the shortest path
spanning tree at each node. In these algorithms, every node is updated with
the shortest path spanning tree of each of its neighbors. Each node uses the
cost of its adjacent links along with shortest path trees reported by neighbors to
update its own shortest path tree; the node reports changes to its own shortest
path tree to all the neighbors in the form of updates containing distance and
second-to-last hop information to each destination.
Path finding algorithms originally proposed for the Internet (e.g., [8]) suffer
from temporary routing loops even though they prevent the counting-to-infinity
problem. This happens because these algorithms fail to recognize that updates
received from different neighbors may not agree on the second-to-last hop to
a destination. WRP improves on the earlier algorithms by verifying the consistency
of second-to-last hop reported by all neighbors. With this mechanism,
WRP reduces the possibility of temporary routing loops, which in turn results
in faster convergence time. One major drawback of WRP is its requirement for
reliable and ordered delivery of routing messages.
3.3.2 Link State Protocols
Optimized Link State Routing (OLSR) [9] is an optimized version of
traditional link state protocol such as OSPF. It uses the concept of Multipoint
Relays (MPRs), discussed in the previous section, to efficiently disseminate
link state updates across the network. Only the nodes selected as MPRs by
some node are allowed to generate link state updates. Moreover, link state
updates contain only the links between MPR nodes and their MPR-Selectors in
order to keep the update size small. Thus, only partial topology information is
made available at each node. However, this information is sufficient for each
to locally compute shortest hop path to every other node because at least one
such path consists of only MPR nodes.
OLSR uses only periodic updates for link state dissemination. Since the total
overhead is then determined by the product of number of nodes generating the
updates, number of nodes forwarding each update and the size of each update,
OLSR reduces the overhead compared to a base link state protocol when the
network is dense. For a sparse network, OLSR degenerates to traditional link
state protocol. Finally, using only periodic updates makes the choice of update
interval critical in reacting to topology changes.