Telnet Protocol specification(6)

　　NAME CODE MEANING

　　NULL (NUL) 0 A 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 horizontal 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.

　　Note that "CR LF" or "CR NUL" is required in both directions　(in the default ASCII mode), to preserve the symmetry of the　NVT model. Even though it may be known in some situations　(e.g., with remote echo and suppress go ahead options in　effect) that characters are not being sent to an actual　printer, none the less, for the sake of consistency, the　protocol requires that a NUL be inserted following a CR not　followed by a LF in the data stream. The converse of this is　that a NUL received in the data stream after a CR (in the　absence of options negotiations which explicitly specify　otherwise) should be stripped out prior to applying the NVT to　local character set mapping.

　　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.

　　Synch

　　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.

　　Break (BRK)

　　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.