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Unformatted text preview: e., silently discarded).
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 [Periman 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. file:///D|/Documents%20and%20Settings/bigini/Docum...i/homenet2run/tcpip/tcp-ip-illustrated/ip_rout.htm (1 of 17) [12/09/2001 14.46.50] Chapter 9. IP Routing Figure 9.1 Processing done at the IP layer. 9.2 Routing Principles
The place to start our discussion of IP routing is to understand what is maintained by the
kernel in its routing table. The information contained in the routing table drives all the
routing decisions made by IP.
In Section 3.3 we listed the steps that IP performs when it searches its routing table.
1. Search for a matching host address.
2. Search for a matching network address.
3. Search for a d...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12