Application Protocols And Software Flexibility

The example above shows how a single piece of software, in this instance the telnet program, can be used to access more than one service. The design of the TELNET protocol and its use to access the Knowbot service illustrate two important points. First, the goal of all protocol design is to find fundamental abstractions that can be reused in multiple applications. In practice, TELNET suffices for a wide variety of services because it provides a basic interactive communication facility. Conceptually, the protocol used to access a service remains separate from the service itself. Second, when architects specify application services, they use standard application protocols whenever possible. The Knowbot service described above can be accessed easily because it uses the standard TELNET protocol for communication. Furthermore, because most TCP/IP software includes an application program that users can invoke to run TELNET, no additional client software is needed to access the Knowbot service. Designers who invent new interactive applications can reuse software if they choose TELNET for their access protocol. The point can be summarized:

The TELNET protocol provides incredible flexibility because it only defines interactive communication and not the details of the service accessed. TELNET can be used as the communication mechanism for many interactive services besides remote login.