Download 2.5.1.1 Proactive routing algorithm

In flat routing algorithms
2.5.1 Flat routing
In flat routing algorithms all the nodes share the responsibility of forwarding
packets destined to other nodes by acting as routers. Most of the flat routing
algorithms implement the process of flooding or shortest path algorithm,
where the sender sends a copy of the message to its neighbor from where it
is then propagated to all the neighbors except to the one from which they
have received the message. Until the network is entirely flooded with the
message this process is repeated. If the destination node is in the same
partition as of the source, the message is sure to reach the destination.
2.5.1.1 Proactive routing algorithm (Table-Driven):
In proactive routing protocols [7], the routing information to reach all the other
nodes in a network is always maintained at every node in the format of the
routing table. When there is a change in network topology, such changes in
link states are announced to all the nodes in a network. Thus, the routes to all
possible destinations are discovered in advance of packet transmissions. If a
proactive protocol is used for MANETs, an immediate problem is that rapid
changes in network topology might overwhelm the network with control
messages (messages for updating the routing table at every node) and the
excess messaging overhead will compromise the throughput of actual data
transmissions. The settling time for a network is extremely high when
proactive type is used, and the number of messages exchanged in order to
maintain route information can grow large very quickly, limiting the scalability
of such algorithms. Examples of proactive protocols are distance vector (DV)
protocol, destination sequenced distance vector (DSDV) protocol, and fisheye state routing (FSR) protocol. These three protocols are also called table-
driven protocols since the routing table will be updated for each change in link
states in a network and routes are discovered using information stored in
routing tables. The Destination Sequenced Distance Vector Routing is
discussed below.
� Destination
Sequenced Distance Vector (DSDV)
In proactive routing scheme the destination sequence distance vector is the
best known protocol. It is based on the classical Distributed Bellman Ford
(DBF) routing algorithm for wired networks. DSDV is a table driven routing
algorithm, like every other table driven routing algorithm, the nodes here
maintain routing tables, which provide information about every possible
destination within the network.
By exchanging these routing tables, which each node maintains DSDV
determines the topology information and the route information. Whenever a
node detects a change in topology the nodes here exchange routing updates.
When a node receives an update packet, it checks the sequence number in
the packet. The packet is discarded if the information in the packet is older
than the receiving node has in its routing tables. Otherwise, information is
updated appropriately in the receiving node's routing table. The update
packet is then forwarded to all other neighboring nodes (except the one from
which the packet came). In addition, the node also sends any new information
that resulted from the merging of the information provided by the update
packet. The updates sent out in this case, by nodes resulting from a change,
can be of two types that is either a full update or a partial update. In case of
full updates, the complete routing table is sent out and in case of a partial
updates only the changes since last full update are sent out.
2.5.1.2 Reactive routing algorithm (on demand):
This protocol discovers a route only when actual data transmission takes
place. When a node wants to send information to another node in a network,
a route discovery process is initiated by a source node. Once a route is
discovered, unless it expires or some event occurs (e.g., a link failure) that
requires another route discovery to start over again it is maintained in the
temporary cache at a source node. Reactive protocols when compared to
proactive protocols require less routing information at each node, as it is not
needed to obtain and maintain the routing information for all the nodes in a
network. Another advantage in reactive protocols is that the routing decisions
need not be made by intermediate nodes. A clear disadvantage in reactive
protocols is the delay caused due to route discovery, which is known as route
acquisition delay. Furthermore, if the routing information is changed
frequently, as is the case in MANETs, and if route discoveries are needed for
those changed routes, reactive protocols may result in a large volume of
messaging overhead, since route recoveries require global broadcasts. Here
as long as there is only light traffic and low mobility, Scalability is achieved.
Various popular reactive protocols currently available are Dynamic Source
Routing (DSR) protocol, Associatively Based Routing (ABR) protocol, and Ad
hoc on - demand Distance Vector (AODV) protocol. The Dynamic Source
Routing Protocol is discussed below.
� Dynamic
Source Routing (DSR)
DSR uses a modified version of source routing. The operation of DSR can be
differentiated in two functions�one is route discovery and the other is route
maintenance. The Route discovery operation is used at the time when routes
to unknown hosts are required to initiate the route discovery if a route fails
and to monitor correctness of established routes, the process of route
maintenance is used.
In DSR, when a node has to send a packet to a destination about which it
doesn't know about, then the route discovery will be initiated by the source
node. The node sends route discovery request to its neighbors. The
neighbors can either send a reply to the initiator or forward the route request
message to their neighbors after having added their address to the request
message (i.e., source routing). Every node which receives the route request
message does the following: