Telnet Protocol specification(4)

　　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　Telnet Protocol Specification　below. These standard representations have standard, but not　required, meanings (with the exception that the 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 local is obliged　to provide the same function as 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 ) 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 . (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 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.

　　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　BS ...

　　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.