ROUTING ALGORITHMS

              -:Routing algorithms: -

The Routing algorithm  is that part of the network layer software responsible for deciding which output line ans incoming packet should be transmitted on. If the network uses datagrams internally, this decision must be made a new for every arriving data packet since the best route may have changed since last time. If the network uses virtual circuit internally, routing decision are made only when a new virtual circuit is being set up. Therefore, data packet just follow the already established route. The later case sometimes called session routing because a route remains in force for an entire session (e.g, while logged in over a VPN). 

It is sometimes useful to make a distinction between routing, which is making the decision which routes to use, and forwarding, which is what happens when a packet arrives. One can think of a router as having two processes inside it. One of them handles each packets as it arrives, looking up the outgoing line to use for it in the routing tables. This process is forwarding. The other process is  for filling in and updating the routing tables. That is where the routing algorithm comes into play.