Understanding Linux Network Internals

Essential Elements of Routing

In this section, I'll introduce some terms and basic elements of the routing landscape. It's important to have a clear understanding of the meanings of a few key terms that are used extensively in this part of the book, and that appear as part of the variable and function names in the associated kernel code. Fortunately, the routing code uses naming conventions pretty consistently.

Figure 30-4. Examples of network topologies.

A few definitions are simple and are shown in the following list. Other concepts are presented in their own subsections.

Internet Service Provider (ISP)

Company or organization that provides access to the Internet.

Forwarding Information Base (FIB)

This is simply the routing table. See the earlier section "Routers, Routes, and Routing Tables."

Symmetric routes and asymmetric routes

Usually, the route taken from Host A to Host B is the same as the route used to get back from Host B to Host A; the route is then called symmetric . In complex setups, the route back may be different; in this case, it is asymmetric.

Metrics

A metric is an optional parameter that can be configured on a route. Do not confuse these metrics with the ones used by routing protocols: the latter use metrics to quantify how good a route is. Examples of routing protocol metrics are the end-to-end delay, the number of hops, a configuration weight or cost, etc.

When ...