Telnet Protocol Specification(6)

"Interesting" signals are defined to be: the TELNET standardrepresentations of IP, AO, and AYT (but not EC or EL); the localanalogs of these standard representations (if any); all otherTELNET commands; other site-defined signals which can be acted onwithout delaying the scan of the data stream.

Since one effect of the SYNCH mechanism is the discarding ofessentially all characters (except TELNET commands) between thesender of the Synch and its recipient, this mechanism is specifiedas the standard way to clear the data path when that is desired.For example, if a user at a terminal causes an AO to betransmitted, the server which receives the AO (if it provides thatfunction at all) should return a Synch to the user.

Finally, just as the TCP Urgent notification is needed at theTELNET level as an out-of-band signal, so other protocols whichmake use of TELNET may require a TELNET command which can be
viewed as an out-of-band signal at a different level.

RFC854 May 1983

By convention the sequence [IP, Synch] is to be used as such asignal. For example, suppose that some other protocol, which usesTELNET, defines the character string STOP analogously to theTELNET command AO. Imagine that a user of this protocol wishes aserver to process the STOP string, but the connection is blockedbecause the server is processing other commands. The user shouldinstruct his system to:

1. Send the TELNET IP character;

2. Send the TELNET SYNC sequence, that is:

Send the Data Mark (DM) as the only characterin a TCP urgent mode send operation.

3. Send the character string STOP; and

4. Send the other protocol's analog of the TELNET DM, if any.

The user (or process acting on his behalf) must transmit theTELNET SYNCH sequence of step 2 above to ensure that the TELNET IPgets through to the server's TELNET interpreter.

The Urgent should wake up the TELNET process; the IP shouldwake up the next higher level process.

THE NVT PRINTER AND KEYBOARD

The NVT printer has an unspecified carriage width and page lengthand can produce representations of all 95 USASCII graphics (codes32 through 126). Of the 33 USASCII control codes (0 through 31and 127), and the 128 uncovered codes (128 through 255), thefollowing have specified meaning to the NVT printer:

NAME CODE MEANING

NULL (NUL) 0 No OperationLine Feed (LF) 10 Moves the printer to thenext print line, keeping thesame horizontal position.Carriage Return (CR) 13 Moves the printer to the leftmargin of the current line.

RFC854 May 1983

In addition, the following codes shall have defined, but notrequired, effects on the NVT printer. Neither end of a TELNETconnection may assume that the other party will take, or willhave taken, any particular action upon receipt or transmissionof these:

BELL (BEL) 7 Produces an audible orvisible signal (which doesNOT move the print head).Back Space (BS) 8 Moves the print head onecharacter position towardsthe left margin.Horizontal Tab (HT) 9 Moves the printer to thenext horizontal tab stop.It remains unspecified how
either party determines orestablishes where such tabstops are located.Vertical Tab (VT) 11 Moves the printer to thenext vertical tab stop. Itremains unspecified howeither party determines orestablishes where such tabstops are located.
Form Feed (FF) 12 Moves the printer to the topof the next page, keepingthe same horizontal position.

All remaining codes do not cause the NVT printer to take anyaction.

The sequence "CR LF", as defined, will cause the NVT to bepositioned 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 togo to some effort to simulate their effect. (For example, someterminals do not have a CR independent of the LF, but on suchterminals it may be possible to simulate a CR by backspacing.)Therefore, the sequence "CR LF" must be treated as a single "newline" character and used whenever their combined action isintended; the sequence "CR NUL" must be used where a carriagereturn alone is actually desired; and the CR character must beavoided in other contexts. This rule gives assurance to systemswhich must decide whether to perform a "new line" function or amultiple-backspace that the TELNET stream contains a characterfollowing a CR that will allow a ational decision.

Note that "CR LF" or "CR NUL" is required in both directions

RFC854 May 1983

(in the default ASCII mode), to preserve the symmetry of theNVT model. Even though it may be known in some situations(e.g., with remote echo and suppress go ahead options ineffect) that characters are not being sent to an actualprinter, nonetheless, for the sake of consistency, the protocolrequires that a NUL be inserted following a CR not followed bya LF in the data stream. The converse of this is that a NULeceived in the data stream after a CR (in the absence ofoptions negotiations which explicitly specify otherwise) shouldbe stripped out prior to applying the NVT to local characterset mapping.