NAME CODE MEANING
NULL (NUL) 0 No Operation
Line Feed (LF) 10 Moves the printer to the
next print line, keeping the
same horizontal position.
Carriage Return (CR) 13 Moves the printer to the left
margin of the current line.
In addition, the following codes shall have defined, but not required, effects on the NVT printer. Neither end of a TELNET connection may assume that the other party will take, or will have taken, any particular action upon receipt or transmission of these:
BELL (BEL) 7 Produces an audible or
visible signal (which does
NOT move the print head).
Back Space (BS) 8 Moves the print head one
character position towards
the left margin.
Horizontal Tab (HT) 9 Moves the printer to the
next horizontal tab stop.
It remains unspecified how
either party determines or
establishes where such tab
stops are located.
Vertical Tab (VT) 11 Moves the printer to the
next vertical tab stop. It
remains unspecified how
either party determines or
establishes where such tab
stops are located.
Form Feed (FF) 12 Moves the printer to the top
of the next page, keeping
the same horizontal position.
All remaining codes do not cause the NVT printer to take any action.
The sequence "CR LF", as defined, will cause the NVT to be positioned at the left margin of the next print line (as would, for example, the sequence "LF CR"). However, many systems and terminals do not treat CR and LF independently, and will have to go to some effort to simulate their effect. (For example, some terminals do not have a CR independent of the LF, but on such terminals it may be possible to simulate a CR by backspacing.) Therefore, the sequence "CR LF" must be treated as a single "new line" character and used whenever their combined action is intended; the sequence "CR NUL" must be used where a carriage return alone is actually desired; and the CR character must be avoided in other contexts. This rule gives assurance to systems which must decide whether to perform a "new line" function or a multiple-backspace that the TELNET stream contains a character following a CR that will allow a rational decision.
The NVT keyboard has keys, or key combinations, or key sequences, for generating all 128 USASCII codes. Note that although many have no effect on the NVT printer, the NVT keyboard is capable of generating them.
In addition to these codes, the NVT keyboard shall be capable of generating the following additional codes which, except as noted, have defined, but not reguired, meanings. The actual code assignments for these "characters" are in the TELNET Command section, because they are viewed as being, in some sense, generic and should be available even when the data stream is interpreted as being some other character set.
This key allows the user to clear his data path to the other party. The activation of this key causes a DM (see command section) to be sent in the data stream and a TCP Urgent notification is associated with it. The pair DM-Urgent is to have required meaning as defined previously.
This code is provided because it is a signal outside the USASCII set which is currently given local meaning within many systems. It is intended to indicate that the Break Key or the Attention Key was hit. Note, however, that this is intended to provide a 129th code for systems which require it, not as a synonym for the IP standard representation.
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.
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.
Send back to the NVT some visible (i.e., printable) evidence that the AYT was received.
The recipient should delete the last preceding undeleted character or "print position" from the data stream.
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 68 (104 octal) 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 124 (174 octal) 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.