The Two Classes of Routing Algorithm

Non-adaptive Algorithms

are characterized by the fact that the choices of routes between each two hosts are computed in advance and loaded into each host in the whole network before the network is 'brought-up'.

Non-adaptive choices are often termed static routing.

Adaptive Algorithms

are characterized by their attempts to adjust their route choices based on their current knowledge of the network topology. There are three type of adaptive algorithms :

1. global algorithms use information periodically collected from the whole network (central routing),
2. local algorithms use only the information that each router knows about itself, such as queue lengths and waiting times, and
3. combined algorithms use some of both global and local information (distributed routing).

Adaptive choices are often termed dynamic routing.

Question: How do we know when a host, router or line has failed?

• Either the host (etc) tells us that it is about to shutdown, or
• We receive too many consecutive Network Layer timeouts for a destination host or router, or too many Data Link Layer timeouts for a link.