Oddly, one of the oldest Internet protocols goes misunderstood. The Internet took a different approach to specifying session settings. IAC (Interpret As Command) is a 3 byte pattern from Client to Server and/or Server to Client. This allows a form of handshake, usually established at the start of a new connection ~ but it is not limited to the start, changes can be implemented on the fly.

Terms

NVT (Network Virtual Terminal) is an imaginary device which provides a standard, network-wide, intermediate representation of a canonical terminal. This eliminates the need for “server” and “user” hosts to keep information about the characteristics of each other's terminals and terminal handling conventions. All hosts, both user and server, map their local device characteristics and conventions so as to appear to be dealing with an NVT over the network, and each can assume a similar mapping by the other party. The NVT is intended to strike a balance between being overly restricted (not providing hosts a rich enough vocabulary for mapping into their local character sets), and being overly inclusive (penalizing users with modest terminals).

TCP (Transmission Control Protocol) is the type of connection used to transmit data with interspersed TELNET control information.

IAC has a lead byte of 255 aka 0xFF in C hex notation. The second byte of the 3 byte pattern denotes WILL, DO, WON'T and DON'T.

For example, if the server (or BBS in this case) wants the client to go to CHAR-BY-CHAR mode, it would send 0xFF 0xFB 0x03 (see below for 0x03) and expects the client (Terminal) to response 0xFF 0xFC 0x03.

So, if you understand the commands, now you need to know the expected responses.

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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.

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.

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.

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>...

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.