Reactive Routing Protocols

Reactive Routing Protocols: These protocols are called on demand protocols. As opposed to storing a complete routing table having route information to all destinations in the network, reactive protocols finds route between source node and the destination node only when need arises.

This has advantage of reducing the traffic load in the network at the cost of latency occurred during finding out a path only on demand basis.

Some of the well known protocols are:

Dynamic Source Routing Protocols: The nodes get routing information from the source routes which are stored in data packets. A node maintains route caches containing the routes it knows. Protocols span two phases:

Root Discovery

Root Maintenance

Route discovery: When a source node wants to send data, first it checks its own routing cache to see if it already has a route to destination. Routing cache is fast and convenient table used to store recently used routing information. If there is no route to destination in the cache then source node initiates the process of route discovery.

It broadcast packet containing:

• source address,               

• destination address,

• Unique identification number

• path

If a node saw the packet before, it discards it; otherwise, the route looks up its route caches to look for a route to destination, If not find, appends its address into the packet, rebroadcast. If it finds a route in its route cache, sends a route reply packet, which is sent to the source by route cache or the route discovery.

Route reply: Route reply is initiated in two cases. Firstly, if the route request packet has reached its destination; secondly if an intermediate node to the destination has route to the destination. After this the question arises for the path back to the source. The reverse path from destination to source depends on two conditions.

If the destination has the route to the source then follow it.

In case of symmetric links, reverse the route in route record and follow it.

In case of asymmetric links, the destination node is supposed to initiate its own route discovery.

Route Maintenance:  Once the route between source and destination has been established, it must be maintained for the duration of transmission. Route maintenance is done by:

• Route error message

• Acknowledgement

Route Error message: Whenever a node transmits a data packet, a route reply, or a route error, it must verify that the next hop correctly receives the packet. If not, the node must send a route error to the node responsible for generating this route header. The source restarts the route discovery.

Acknowledgement:  Nodes are supposed to notify the correct operation of route links. 

Dynamic Source Routing- Example:

When MH S wants to send a packet to MH D, but does not know a route to D, MH S initiates a route discovery. For this purpose, the source node S floods Route Request (RREQ) to all the neighboring nodes in the network.

Each MH appends own identifier when forwarding RREQ