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Networking
2.5. ICMP
The Internet Control Messaging Protocol (ICMP) is an extension to the Internet Protocol (IP) as
defined in the Request For Comments (RFC) #792 and supports network packets containing control,
error, and informational messages. ICMP is used by such network applications as the ping utility,
which can determine the availability of a network host or device. Examples of some error messages
returned by ICMP which are useful to both network hosts and devices such as routers, include
Destination Unreachable and Time Exceeded.
2.6. Daemons
Daemons are special system applications which typically execute continuously in the background and
await requests for the functions they provide from other applications. Many daemons are networkcentric; that is, a large number of daemons executing in the background on an Ubuntu system may
provide network-related functionality. Some examples of such network daemons include the Hyper
Text Transport Protocol Daemon (httpd), which provides web server functionality; the Secure SHell
Daemon (sshd), which provides secure remote login shell and file transfer capabilities; and the
Internet Message Access Protocol Daemon (imapd), which provides E-Mail services.
2.7. Resources
• There are man pages for TCP8 and IP9 that contain more useful information.
• Also, see the TCP/IP Tutorial and Technical Overview10 IBM Redbook.
• Another resource is O'Reilly's TCP/IP Network Administration11.
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