What does a single CR mean ? (telnet)

Question

RFC854 specifies, that a CR NUL means a carriage return to the same line and CR LF means a newline.

What does it mean, if a single CR is transmitted its own? (If it is followed by something not a being \0 or \n?)

Answer 1

From the RFC:

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.

So NUL is the ascii character NULL i.e. code 0.

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.

So here:

• CR LF means new line
• CR NUL means carriage return (same line)
• CR (alone, without LF or NUL) must be avoided

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, nonetheless, 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.