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Unformatted text preview: plication layer can ignore all these
UDP, on the other hand, provides a much simpler service to the application layer. It just
sends packets of data called datagrams from one host to the other, but there is no
guarantee that the datagrams reach the other end. Any desired reliability must be added
by the application layer.
There is a use for each type of transport protocol, which we'll see when we look at the
different applications that use TCP and UDP.
4. The application layer handles the details of the particular application. There are many
common TCP/IP applications that almost every implementation provides:
r Telnet for remote login,
r FTP, the File Transfer Protocol,
r SMTP, the Simple Mail Transfer protocol, for electronic mail,
r SNMP, the Simple Network Management Protocol,
and many more, some of which we cover in later chapters.
If we have two hosts on a local area network (LAN) such as an Ethernet, both running FTP,
Figure 1.2 shows the protocols involved. Figure 1.2 Two hosts on a LAN running FTP.
We have labeled one application box the FTP client and the other the FTP server. Most network file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/introduc.htm (2 of 20) [12/09/2001 14.46.31] Chapter 1. Introduction applications are designed so that one end is the client and the other side the server. The server
provides some type of service to clients, in this case access to files on the server host. In the
remote login application, Telnet, the service provided to the client is the ability to login to the
Each layer has one or more protocols for communicating with its peer at the same layer. One
protocol, for example, allows the two TCP layers to communicate, and another protocol lets the
two IP layers communicate.
On the right side of Figure 1.2 we have noted that normally the application layer is a user
process while the lower three layers are usually implemented in the kernel (the operating
system). Although this isn't a requirement, it's typical and this is the way it's done und...
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