wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Routing in Wireless Networks 4-9
in highly mobile networks. TBRPF performs neighbor discovery using “differential” HELLO messages
that report only changes in the status of neighbors. This results in HELLO messages that are much
smaller than those of other link-state routing protocols.
4.5.3 Dynamic Source Routing Protocol
This protocol was created in 1996 [JM96,JM01], and like other protocols, it has been widely studied and
also modified [DRL03]. Currently, its state is experimental [JHM07], like that of OLSR. DSR is a reactive
routing protocol. An important characteristic to consider is that this protocol requires that each message
sent contains the complete address from the source node to the destination node (source based).
When a node needs to know the path to the destination, it sends a Route Request (RREQ) message and
the route discovery procedure is initiated. Each node adds its own identifier when the RREQ is resent,
and in this way, the route record is created.
To detect RREQ duplicates, each node in the network maintains a list with the pair [source address,
request_id]. When a node receives a RREQ message, it follows the following steps: (1) If the pair
[source address, request_id] for this route discovery are in the recent search list of this node, it dismisses
it and does not process the RREQ message. (2) If the address of this node is in the route record
of the RREQ message, it dismisses it and does not process it. (3) If the search objective is the node
address, the route record already contains the route from source to destination and the route reply
(RREP) message is sent. (4) Otherwise, this node adds its own address in the route record of the RREQ
message and resends the search.
If the node that receives the RREQ is the destination or it contains information about the destination,
it replies with a RREP message. The RREP message is sent through the same path that has been
discovered, and this same message holds the route from the source node to the destination. If the
link is not bidirectional, a new route discovery is carried out to send the RREP message. When the
source node receives the RREP, it stores the route from source to destination included in the RREP
in the route cache. While RREQ and RREP messages pass through intermediate nodes, they store the
information in their route cache. While waiting for the route discovery to finish, the node can continue
sending and receiving messages to/from other nodes. When the source node receives the RREP
message, it can send data packets to the destination node, and the header of those packets includes the
route they have to follow. This way, intermediate nodes use the route included in the packet to decide
which node they have to send the packet to. When the packet reaches its destination, it is delivered to
the network layer.
Each node has a route cache where the path to a destination node is stored. Each entry in the route cache
has an associated lifetime after which the entry is deleted from the cache. The use of this route cache in
each node offers two advantages: Firstly, more speed in the route discovery—if a node that receives a RREQ
for a path to destination that is already known (because it already has it in its cache), it can reply with a
RREP, using the local routing cache. Secondly, the transmission of RREQ is reduced, because if the path to
the destination is already known because a node has it stored in its route cache, there is no need to continue
transmitting RREQ messages. If a route breaks, the source node can consider another route that it has in
its cache for the same address. If it does not have a route stored, it starts route discovery again. Figure 4.4
shows an example of the route cache stored in the nodes. However, using the route cache can cause two
problems. When two nodes receive a RREQ at the same time and both of them reply based on the routes
stored in their caches, sending their responses at the same time, this can cause message collisions and
network congestion. However, simultaneous replies can be avoided by adding a delay before replying with
the information in the cache. The second problem is that formation of a loop may occur in a route sent in
a RREP message. Therefore, to solve this problem, if a node receives a RREQ and is not the objective of the
search but can reply with information in its cache, the node dismisses the reply if the route contains a loop.
The node will reply only with its cache with a route in which it appears at the end of the route stored in the
RREQ message and at the beginning of the path obtained in the route cache.
© 2011 by Taylor and Francis Group, LLC