Overview

We said in Chapter 4 that one of the main aims of the IP Address was to provide for the easy routing of traffic between networks, and briefly introduced how this is achieved. In this chapter we shall study the actual mechanics of routing, the maintenance of routing tables, and define the operation of routers generally.

All stations are responsible for the delivery of data, either directly to another station, or to a router for eventual delivery to a distant host. Routers are concerned with the reachability of networks and maintain tables ( Routing Tables) that list those networks that are reachable. These tables also include a measure of the quality [the metric] of the route used, and the address of the next relay system ( hop) used to onward route the data. Routers also use a series of specialized protocols in order to learnabout routes to new networks as they become available, and to delete routes that become unusable due to failures. In this way, meshed networks can be created that require an absolute minimum of operator maintenance.

In Chapter 5 we saw how IP incorporates the concept of Type of Service. This allows applications to request that particular routes are chosen over those that may not otherwise offer the best cost in terms of Throughput, Reliability, or Delay. Through the routing protocol, routers are able to make routing decisions based upon the best and most up to date information available,...