TCP/IP Tutorial and Technical Overview - IBM Redbooks

11.1 Characteristics of applications
Each of the application protocols share come common characteristics:
► They can be user-written applications or applications standardized and
shipped with the TCP/IP product. Examples of applications native to the
TCP/IP protocol suite include:
– Telnet, which provides interactive terminal access to remote hosts
– The File Transfer Protocol (FTP), which provides the ability to transfer files
between remote hosts
– The Simple Mail Transfer Protocol (SMTP), which provides an Internet
mailing system
While these are widely implemented application protocols, many others exist.
► They use either UDP or TCP as a transport mechanism. Remember that UDP
(see 4.2, “User Datagram Protocol (UDP)” on page 146) is unreliable and
offers no flow control. In this case, the application must provide its own error
recovery and flow control routines. For this reason, it is often easier to build
applications that use TCP (see 4.3, “Transmission Control Protocol (TCP)” on
page 149), a reliable, connection-oriented protocol.
► Most applications implement the client/server model of interaction.
11.1.1 The client/server model
TCP is a peer-to-peer, connection-oriented protocol. There are no
master/subordinate relationships, in which one instance of the application
protocol controls or is controlled by another instance. Instead, the applications
use a client/server model for communications. In such a model, the server offers
a service to users. The client is the interface by which the user accesses the
offered service. Both a client instance and a server instance must be active for
the application protocol to operate. Note that the both instances can reside on
the same host or on different hosts (see Figure 11-1 on page 409).
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