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<FONT SIZE=5 COLOR="#FF0000"><B>A Quick Overview of TCP/IP Components</B></FONT></CENTER></H3>

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<P>To understand the roles of the many components of the TCP/IP protocol family, it is useful to know what you can do over a TCP/IP network. Then, once the applications are understood, the protocols that make it possible are a little easier to comprehend. The following list is not exhaustive but mentions the primary user applications that TCP/IP provides.

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<FONT SIZE=4 COLOR="#FF0000"><B>Telnet</B></FONT></CENTER></H4>

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<P>The Telnet program provides a remote login capability. This lets a user on one machine log onto another machine and act as though he or she were directly in front of the second machine. The connection can be anywhere on the local network or on another network anywhere in the world, as long as the user has permission to log onto the remote system.

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<P>You can use Telnet when you need to perform actions on a machine across the country. This isn't often done except in a LAN or WAN context, but a few systems accessible through the Internet allow Telnet sessions while users play around with a new application or operating system.

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<FONT SIZE=4 COLOR="#FF0000"><B>File Transfer Protocol</B></FONT></CENTER></H4>

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<P>File Transfer Protocol (FTP) enables a file on one system to be copied to another system. The user doesn't actually log in as a full user to the machine he or she wants to access, as with Telnet, but instead uses the FTP program to enable access. Again, the correct permissions are necessary to provide access to the files.

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<P>Once the connection to a remote machine has been established, FTP enables you to copy one or more files to your machine. (The term <I>transfer</I> implies that the file is moved from one system to another but the original is not affected. Files are copied.) FTP is a widely used service on the Internet, as well as on many large LANs and WANs.

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<FONT SIZE=4 COLOR="#FF0000"><B>Simple Mail Transfer Protocol</B></FONT></CENTER></H4>

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<P>Simple Mail Transfer Protocol (SMTP) is used for transferring electronic mail. SMTP is completely transparent to the user. Behind the scenes, SMTP connects to remote machines and transfers mail messages much like FTP transfers files. Users are almost never aware of SMTP working, and few system administrators have to bother with it. SMTP is a mostly trouble-free protocol and is in very wide use.

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<FONT SIZE=4 COLOR="#FF0000"><B>Kerberos</B></FONT></CENTER></H4>

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<P>Kerberos is a widely supported security protocol. Kerberos uses a special application called an <I>authentication server</I> to validate passwords and encryption schemes. Kerberos is one of the more secure encryption systems used in communications and is quite common in UNIX.

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<FONT SIZE=4 COLOR="#FF0000"><B>Domain Name System</B></FONT></CENTER></H4>

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<P>Domain Name System (DNS) enables a computer with a common name to be converted to a special network address. For example, a PC called Darkstar cannot be accessed by another machine on the same network (or any other connected network) unless some method of checking the local machine name and replacing the name with the machine's hardware address is available. DNS provides a conversion from the common local name to the unique physical address of the device's network connection.

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<FONT SIZE=4 COLOR="#FF0000"><B>Simple Network Management Protocol</B></FONT></CENTER></H4>

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<P>Simple Network Management Protocol (SNMP) provides status messages and problem reports across a network to an administrator. SNMP uses User Datagram Protocol (UDP) as a transport mechanism. SNMP employs slightly different terms from TCP/IP, working with managers and agents instead of clients and servers (although they mean essentially the same thing). An agent provides information about a device, whereas a manager communicates across a network with agents.

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<FONT SIZE=4 COLOR="#FF0000"><B>Network File System</B></FONT></CENTER></H4>

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<P>Network File System (NFS) is a set of protocols developed by Sun Microsystems to enable multiple machines to access each other's directories transparently. They accomplish this by using a distributed file system scheme. NFS systems are common in large corporate environments, especially those that use UNIX workstations.

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<FONT SIZE=4 COLOR="#FF0000"><B>Remote Procedure Call</B></FONT></CENTER></H4>

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<P>The Remote Procedure Call (RPC) protocol is a set of functions that enable an application to communicate with another machine (the server). It provides for programming functions, return codes, and predefined variables to support distributed computing.

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<FONT SIZE=4 COLOR="#FF0000"><B>Trivial File Transfer Protocol</B></FONT></CENTER></H4>

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<P>Trivial File Transfer Protocol (TFTP) is a very simple, unsophisticated file transfer protocol that lacks security. It uses UDP as a transport. TFTP performs the same task as FTP, but uses a different transport protocol.

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<FONT SIZE=4 COLOR="#FF0000"><B>Transmission Control Protocol</B></FONT></CENTER></H4>

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<P>Transmission Control Protocol (the TCP part of TCP/IP) is a communications protocol that provides reliable transfer of data. It is responsible for assembling data passed from higher-layer applications into standard packets and ensuring that the data is transferred correctly.

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<FONT SIZE=4 COLOR="#FF0000"><B>User Datagram Protocol</B></FONT></CENTER></H4>

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<P>User Datagram Protocol (UDP) is a connectionless-oriented protocol, meaning that it does not provide for the retransmission of datagrams (unlike TCP, which is connection-oriented). UDP is not very reliable, but it does have specialized purposes. If the applications that use UDP have reliability checking built into them, the shortcomings of UDP are overcome.

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<FONT SIZE=4 COLOR="#FF0000"><B>Internet Protocol</B></FONT></CENTER></H4>

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<P>Internet Protocol (IP) is responsible for moving the packets of data assembled by either TCP or UDP across networks. It uses a set of unique addresses for every device on the network to determine routing and destinations.

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<FONT SIZE=4 COLOR="#FF0000"><B>Internet Control Message Protocol</B></FONT></CENTER></H4>

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<P>Internet Control Message Protocol (ICMP) is responsible for checking and generating messages on the status of devices on a network. It can be used to inform other devices of a failure in one particular machine. ICMP and IP usually work together.

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<FONT SIZE=5 COLOR="#FF0000"><B>TCP/IP History</B></FONT></CENTER></H3>

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<P>The architecture of TCP/IP is often called the Internet architecture because TCP/IP and the Internet as so closely interwoven. In the last chapter, you saw how the Internet standards were developed by the Defense Advanced Research Projects Agency (DARPA) and eventually passed on to the Internet Society.

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<P>The Internet was originally proposed by the precursor of DARPA, called the Advanced Research Projects Agency (ARPA), as a method of testing the viability of packet-switching networks. (When ARPA's focus became military in nature, the name was changed.) During its tenure with the project, ARPA foresaw a network of leased lines connected by switching nodes. The network was called ARPANET, and the switching nodes were called Internet Message Processors, or IMPs.

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<P>The ARPANET was initially to be comprised of four IMPs located at the University of California at Los Angeles, the University of California at Santa Barbara, the Stanford Research Institute, and the University of Utah. The original IMPs were to be Honeywell 316 minicomputers.