Networking Reference
In-Depth Information
overhead and may lead to congestion in the network. Some protocols try to reduce
the flooding overhead by utilizing techniques to limit the range of transmission or
to limit the number of forwarding nodes. For example, the OLSR protocol depends
on the use of Multi-Point Relays (MPRs) to distribute the routing information
through the network. The MPR set of each node contains the minimum number of
direct neighbors that cover all the two-hop neighbors of that node. By using this
technique, the number of nodes involved in disseminating the routing information
is reduced; hence, the flooding overhead is mitigated. Another example is the
Core-Extraction Distributed Ad Hoc Routing (CEDAR) protocol [ 14 ] where a
subset of the nodes in the network are identified as the ''core'' nodes. These core
nodes are determined using a distributed algorithm which ensures that each node
has at least one adjacent core node. The link state information is propagated only
through the core nodes.
- the number of routing tables used by each protocol
Protocols may utilize different number of tables to support their operation. For
example, DSDV uses only one routing table where it stores the cost to each
possible destination in the network along with the next hop to this destination and
a sequence number that is assigned by the destination to specify how fresh the
route is. Unlike DSDV, WRP maintains four different tables; a routing table, a
distance table, a link-cost table, and a message retransmission list.
For route discovery, the reactive protocols employ different approaches for
sending the route request (RREQ) packets through the network:
- Some protocols flood the whole network with the RREQ packets (e.g., the
Dynamic Source Routing (DSR) protocol [ 15 ]). In DSR, when a node receives a
RREQ, if it is the intended destination, it returns a route reply (RREP) packet
carrying the whole accumulated route that it gets from the received RREQ. If it
is an intermediate node and has a route for that destination stored in its cache, it
concatenates the part of the route it has to the part in the received RREQ then
sends the whole route in a RREP back to the source. Otherwise, the intermediate
node forwards this RREQ to its neighbors after appending its address to the
route list in the request. Usually, The RREP follows the reverse route back to
the source. This is only possible if symmetric links are available. Otherwise, to
send the RREP back to the source, the responding node initiates a route dis-
covery process and piggybacks the RREP to the new RREQ.
- Other protocols try to reduce the flooding overhead by limiting the number of
forwarding nodes or the dissemination area. For example, the Ad Hoc On-
Demand Distance Vector (AODV) routing protocol [ 16 ] employs the ring-
search technique to handle cases where the destination is quite near the source.
In such cases, flooding RREQ packets through the whole network is wasteful.
To handle that, the idea of the ring search scheme is based on searching larger
areas successively. First, the RREQ is disseminated in the area around the
source. If the destination is not found, the searching area is widened and so on
until the destination is found. To control the area of searching, the TTL field of
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