9.1 Introduction

9.1 Introduction

Routing is one of the most important functions of IP. Figure 9.1 shows a simplified view of the processing done at the IP layer. Datagrams to be routed can be generated either on the local host or on some other host. In the latter case this host must be configured to act as a router, or datagrams received through the network interfaces that are not ours are dropped (i.e., silently discarded).

Figure 9.1. Processing done at the IP layer.

In Figure 9.1 we also show a routing daemon, which is normally a user process. The most common daemons used on Unix systems are the programs routed and gated. (The term daemon means the process is running "in the background," carrying out operations on behalf of the whole system. Daemons are normally started when the system is bootstrapped and run as long as the system is up.) The topics of which routing protocol to use on a given host, how to exchange routing information with adjacent routers, and how the routing protocols work are complex and can fill an entire book of their own. (Interested readers are referred to [Perlman 1992] for many of the details.) We'll look briefly at dynamic routing and the Routing Information Protocol (RIP) in Chapter 10. Our main interest in the current chapter is how a single IP layer makes its routing decisions.

The routing table that we show in Figure 9.1 is accessed frequently by IP (on a busy host this could mean hundreds of times a second) but is updated much less frequently by a routing daemon (possibly about once every 30 seconds). The routing table can also be updated when ICMP "redirect" messages are received, something we'll look at in Section 9.5, and by the route command. This command is often executed when the system is bootstrapped, to install some initial routes. We'll also use the netstat command in this chapter to display the routing table.