Telnet Protocol Specification(7)

The NVT keyboard has keys, or key combinations, or key sequences,for generating all 128 USASCII codes. Note that although manyhave no effect on the NVT printer, the NVT keyboard is capable ofgenerating them.

In addition to these codes, the NVT keyboard shall be capable ofgenerating the following additional codes which, except as noted,have defined, but not reguired, meanings. The actual codeassignments for these "characters" are in the TELNET Commandsection, because they are viewed as being, in some sense, genericand should be available even when the data stream is interpretedas being some other character set.

Synch

This key allows the user to clear his data path to the otherparty. The activation of this key causes a DM (see commandsection) to be sent in the data stream and a TCP Urgentnotification is associated with it. The pair DM-Urgent is tohave required meaning as defined previously.

Break (BRK)

This code is provided because it is a signal outside theUSASCII set which is currently given local meaning within manysystems. It is intended to indicate that the Break Key or theAttention Key was hit. Note, however, that this is intended toprovide a 129th code for systems which require it, not as asynonym for the IP standard representation.

Interrupt Process (IP)

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

RFC854 May 1983

Abort Output (AO)

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

Are You There (AYT)

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

Erase Character (EC)

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

Erase Line (EL)

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

The spirit of these "extra" keys, and also the printer formateffectors, is that they should represent a natural extension ofthe mapping that already must be done from "NVT" into "local".
Just as the NVT data byte 68 (104 octal) should be mapped intowhatever the local code for "uppercase D" is, so the EC charactershould be mapped into whatever the local "Erase Character"function is. Further, just as the mapping for 124 (174 octal) issomewhat 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 doeshave a "Vertical Tab", then the mapping for VT is obvious, andonly 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 codefor the command. The commands dealing with option negotiation arethree byte sequences, the third byte being the code for the optionreferenced. This format was chosen so that as more comprehensive useof the "data space" is made -- by negotiations from the basic NVT, ofcourse -- collisions of data bytes with reserved command values willbe minimized, all such collisions requiring the inconvenience, andRFC854 May 1983

inefficiency, of "escaping" the data bytes into the stream. With thecurrent set-up, only the IAC need be doubled to be sent as data, andthe other 255 codes may be passed transparently.

The following are the defined TELNET commands. Note that these codesand code sequences have the indicated meaning only when immediatelypreceded 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 accompaniedby 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 issubnegotiation of the indicatedoption.

WILL (option code) 251 Indicates the desire to beginperforming, or confirmation thatyou are now performing, theindicated 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 theother party perform, orconfirmation that you are expectingthe other party to perform, theindicated option.

DON'T (option code) 254 Indicates the demand that theother party stop performing,or confirmation that you are nolonger expecting the other partyto perform, the indicated option.

IAC 255 Data Byte 255.

RFC854 May 1983

CONNECTION ESTABLISHMENT

The TELNET TCP connection is established between the user's port Uand the server's port L. The server listens on its well known port Lfor such connections. Since a TCP connection is full duplex andidentified by the pair of ports, the server can engage in manysimultaneous connections involving its port L and different userports U.

Port Assignment

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