What is Routing ? What is Adaptive and Hierarchical Routing ?

Routing means selecting paths in a computer network along which to send data.
Routing directs forwarding, the passing of logically addressed packets from their source network, toward their ultimate destination through intermediary nodes; typically hardware devices called routers. The routing process usually directs forwarding on the basis of routing tables which maintain a record of the best routes to various network destinations.
Adaptive routing describes the capability of a system, through which routes are characterised by their destination, to alter the path that the route takes through the system in response to a change in conditions. The adaptation is intended to allow as many routes as possible to remain valid (that is, have destinations that can be reached) in response to the change.
People using a transport system can display adaptive routing. For example, if a local railway station is closed, people can alight from a train at a different station and use another method, such as a bus, to reach their destination.
Adaptive routing describes the capability of a system, through which routes are characterised by their destination, to alter the path that the route takes through the system in response to a change in conditions. The adaptation is intended to allow as many routes as possible to remain valid (that is, have destinations that can be reached) in response to the change.
People using a transport system can display adaptive routing. For example, if a local railway station is closed, people can alight from a train at a different station and use another method, such as a bus, to reach their destination.
Hierarchical routing was mainly devised, among other things, to reduce memory requirements of simulations over very large topologies. A topology is broken down into several layers of hierarchy, thus downsizing the routing table. The table size is reduced from n2, for flat routing, to about log n for hierarchical routing. However some overhead costs results as number of hierarchy levels are increased. Optimum results were found for 3 levels of hierarchy and the current ns implementation supports upto a maximum of 3 levels of hierarchical routing.