Table of Contents Application Protocols

4.1 Overview

4.1.1 Characteristics of Applications

All of the higher-level protocols have some characteristics in common:

• They can be user-written applications or applications standardized and shipped with the TCP/IP product. Indeed, the TCP/IP protocol suite includes application protocols such as:
  • TELNET for interactive terminal access to remote internet hosts.
  • FTP (file transfer protocol) for high-speed disk-to-disk file transfers.
  • SMTP (simple mail transfer protocol) as an internet mailing system.
  These are the most widely implemented application protocols, but a lot of others exist. Each particular TCP/IP implementation will include a more or less restricted set of application protocols.
• They use either UDP or TCP as a transport mechanism. Remember that UDP is unreliable and offers no flow-control, so in this case the application has to provide its own error recovery and flow-control routines. It is often easier to build applications on top of TCP, a reliable, connection-oriented protocol. Most application protocols will use TCP, but there are applications built on UDP to provide better performance through reduced protocol overhead.
• Most of them use the client/server model of interaction.

4.1.2 Client/Server Model

TCP is a peer-to-peer, connection-oriented protocol. There are no master/slave relations. The applications, however, use a client/server model for communications.

A server is an application that offers a service to internet users; a client is a requester of a service. An application consists of both a server and a client part, which can run on the same or on different systems.

Users usually invoke the client part of the application, which builds a request for a particular service and sends it to the server part of the application using TCP/IP as a transport vehicle.

The server is a program that receives a request, performs the required service and sends back the results in a reply. A server can usually deal with multiple requests (multiple clients) at the same time.


Figure: The Client/Server Model of Applications

Some servers wait for requests at a well-known port so that their clients know to which IP socket they must direct their requests. The client uses an arbitrary port for its communication. Clients that wish to communicate with a server that does not use a well-known port must have another mechanism for learning to which port they must address their requests. This mechanism might employ a registration service such as Portmap, which uses a well-known port.

The next sections discuss the most widely used application protocols.

Table of Contents TELNET