Telnet Protocol specification(7)

　　Interrupt Process (IP)

　　Suspend, interrupt, abort or terminate the process to which the　NVT is connected. Also, part of the out-of-band signal for　other protocols which use TELNET.

　　Abort Output (AO)

　　Allow the current process to (appear to) run to completion, but　do not send its output to the user. Also, send a Synch to the　user.

　　Are You There (AYT)

　　Send back to the NVT some visible (i.e., printable) evidence　that the AYT was received.

　　Erase Character (EC)

　　The recipient should delete the last preceding undeleted　character or "print position" from the data stream.

　　Erase Line (EL)

　　The recipient should delete characters from the data stream　back to, but not including, the last "CR LF" sequence sent over　the TELNET connection.

　　The spirit of these "extra" keys, and also the printer format　effectors, is that they should represent a natural extension of　the mapping that already must be done from "NVT" into "local".

　　Just as the NVT data byte 104 should be mapped into whatever the　local code for "uppercase D" is, so the EC character should be　mapped into whatever the local "Erase Character" function is.

　　Further, just as the mapping for 174 is somewhat arbitrary in an　environment that has no "vertical bar" character, the EL character　may have a somewhat arbitrary mapping (or none at all) if there is　no local "Erase Line" facility. Similarly for format effectors:

　　if the terminal actually does have a "Vertical tab", then the　mapping for VT is obvious, and only when the terminal does not　have a vertical tab should the effect of VT be unpredictable.

　　TELNET COMMAND STRUCTURE

　　All TELNET commands consist of at least a two byte sequence: the　"Interpret as Command" (IAC) escape character followed by the code　for the command. The commands dealing with option negotiation are　three byte sequences, the third byte being the code for the option　referenced. This format was chosen so that as more comprehensive use　of the "data space" is made -- by negotiations from the basic NVT, of　course -- collisions of data bytes with reserved command values will　be minimized, all such collisions requiring the inconvenience, and　inefficiency, of "escaping" the data bytes into the stream. With the　current set-up, only the IAC need be doubled to be sent as data, and　the other 255 codes may be passed transparently.

　　The following are the defined TELNET commands. Note that these codes　and code sequences have the indicated meaning only when immediately　preceded by an IAC.

　　NAME CODE MEANING

　　SE 240 End of subnegotiation parameters

　　NOP 241 No operation

　　Data Mark 242 The data stream portion of a Synch

　　This should always be accompanied　by a TCP Urgent notification.

　　Break 243 NVT character BRK

　　Interrupt Process 244 The function IP

　　Abort output 245 The function AO

　　Are You There 246 The function AYT

　　Erase character 247 The function EC

　　Erase Line 248 The function EL

　　Go ahead 249 The GA signal

　　SB 250 Indicates that what follows is　subnegotiation of the indicated　option

　　WILL (option code) 251 Indicates the desire to begin　performing, or confirmation that　you are now performing, the　indicated option

　　WON't (option code) 252 Indicates the refusal to perform,　or continue performing, the　indicated option.

　　DO (option code) 253 Indicates the request that the　other party perform, or　confirmation that you are expecting　the other party to perform, the　indicated option.

　　DON'T (option code) 254 Indicates the demand that the　other party stop performing,　or confirmation that you are no　longer expecting the other party　to perform, the indicated option.

　　IAC 255 Data Byte 255.

　　CONNECTION ESTABLISHMENT

　　The TELNET TCP connection is established between the user's port U　and the server's port L. The server listens on its well known port L　for such connections. Since a TCP connection is full duplex and　identified by the pair of ports, the server can engage in many　simultaneous connections involving it's port L and different user　ports U.

　　Port Assignment

　　When used for remote user access to service hosts (i.e., remote　terminal access) this protocol is assigned server port 23 (27　octal). That is L=23.