Most of the input that a user enters at a terminal takes the form of data and commands that are sent to the application program he or she is using. However, computer systems also provide a means by which a user can instruct the terminal to send certain commands that control how the terminal itself operates, and how it interacts with the computer to which it is connected. The best example of this is the command to interrupt a process, which is usually sent by pressing a special key or key combination on the user terminal.

Telnet needs to have a way to allow such commands to be entered by the user. However, here we run into the same problem that we saw in the last topic, when we talked about the communication of data between terminals and computers: a lack of uniformity in representation. While all terminals and computers support the ability to interrupt a running program, for example, they may each use a different keystroke to represent it. For example, on most UNIX systems, the key combination “<Ctrl>+C” interrupts a program, but typing this on a Windows system will not (it usually represents instead the “copy data” function!)

Since the problem is the same as the one we ran into in representing data flow, it’s not too surprising that the solution is the same: the use of a universal representation for a set of standard commands to be passed between the terminal and host computer. All keystrokes that represent these commands are translated to the standard Telnet codes for transmission, and then translated to the specific needs of the host computer. So to take our example again, if a user presses “<Ctrl>+C” on a UNIX terminal where this is defined as the interrupt function, then instead of sending that exact keystroke, the Telnet client sends the special Telnet Interrupt Process command, which is translated by the Telnet server to the command code appropriate for the connected host.

The Telnet standard includes a number of these special codes to allow a user to control the operation of the remote computer. It also defines a set of commands that are specific to the Telnet protocol itself; these let the Telnet client and Telnet server software communicate. Collectively, these are called Telnet protocol commands.