STANDARD REPRESENTATION OF CONTROL FUNCTIONS

As stated in the Introduction to this document, the primary goal of the TELNET protocol is the provision of a standard interfacing of terminal devices and terminal-oriented processes through the network. Early experiences with this type of interconnection have shown that certain functions are implemented by most servers, but that the methods of invoking these functions differ widely. For a human user who interacts with several server systems, these differences are highly frustrating. TELNET, therefore, defines a standard representation for five of these functions, as described below. These standard representations have standard, but not required, meanings (with the exception that the Interrupt Process (IP) function may be required by other protocols which use TELNET); that is, a system which does not provide the function to local users need not provide it to network users and may treat the standard representation for the function as a No-operation. On the other hand, a system which does provide the function to a local user is obliged to provide the same function to a network user who transmits the standard representation for the function.

Interrupt Process (IP)

Many systems provide a function which suspends, interrupts, aborts, or terminates the operation of a user process. This function is frequently used when a user believes his process is in an unending loop, or when an unwanted process has been inadvertently activated. IP is the standard representation for invoking this function. It should be noted by implementers that IP may be required by other protocols which use TELNET, and therefore should be implemented if these other protocols are to be supported.

Abort Output (AO)

Many systems provide a function which allows a process, which is generating output, to run to completion (or to reach the same stopping point it would reach if running to completion) but without sending the output to the user's terminal. Further, this function typically clears any output already produced but not yet actually printed (or displayed) on the user's terminal. AO is the standard representation for invoking this function. For example, some subsystem might normally accept a user's command, send a long text string to the user's terminal in response, and finally signal readiness to accept the next command by sending a "prompt" character (preceded by <CR><LF>) to the user's terminal. If the AO were received during the transmission of the text string, a reasonable implementation would be to suppress the remainder of the text string, but transmit the prompt character and the preceding <CR><LF>. (This is possibly in distinction to the action which might be taken if an IP were received; the IP might cause suppression of the text string and an exit from the subsystem.)

It should be noted, by server systems which provide this function, that there may be buffers external to the system (in the network and the user's local host) which should be cleared; the appropriate way to do this is to transmit the "Synch" signal (described below) to the user system.

Are You There (AYT)

Many systems provide a function which provides the user with some visible (e.g., printable) evidence that the system is still up and running. This function may be invoked by the user when the system is unexpectedly "silent" for a long time, because of the unanticipated (by the user) length of a computation, an unusually heavy system load, etc. AYT is the standard representation for invoking this function.

Erase Character (EC)

Many systems provide a function which deletes the last preceding undeleted character or "print position"* from the stream of data being supplied by the user. This function is typically used to edit keyboard input when typing mistakes are made. EC is the standard representation for invoking this function.

*NOTE: A "print position" may contain several characters which are the result of overstrikes, or of sequences such as <char1> BS <char2>...

Erase Line (EL)

Many systems provide a function which deletes all the data in the current "line" of input. This function is typically used to edit keyboard input. EL is the standard representation for invoking this function.