Telnet Protocols(4)

　　The representation of the end of a physical line at a terminal is　implemented differently on different network hosts. For example,　some use a return (or new line) key, the terminal hardware both　returns the carriage or printer to start of line and feeds the paper　to the next line. In other implementations, the user hits carriage　return and the hardware returns carriage while the software sends the　terminal a line feed. The network-wide representation is carriage　return followed by line feed. It represents the physical formatting　that is being attempted, and is to be interpreted and appropriately　translated by both using site and serving site.

　　EXAMPLE: A Multics user is working, through the network, on some　serving site host. In the course of the session the user has　numerous occasions to hit New Line on his Model 37 TTY. Each time　the Multics system is awakened by a New Line interrupt, the line　of buffered characters is passed to Telnet where it is scanned for　special characters. If none is found, carriage return followed by　line feed is inserted where New Line was entered, and the line is　turned over the NCP for transmission. Correspondingly, when the　Multics Telnet finds the carriage return line feed sequence in the　data stream coming from the serving site, the two characters are　replaced with the appropriate New Line code which is sent to the　terminal.

　　Telnet defines the end of a line to be indicated by the ASCII　character pair CR LR. Several of the real devices in the world have　only a single new line (NL) function. Several of the computer　systems have in some programs used the CR and LF functions to have　semantic meaning larger than the format effect they provide.

　　Further, several computer systems allow the CR and LF functions to be　used separately (e.g., such that a line may be overprinted). One　problem, for those Telnet (user) programs required to map the NVT　into a device which only has a NL function, is how is the CR LF to be　dealt with. One solution is to examine the character following the　CR. If an LF is found, then perform the NL function; if anything　else is found then back space to the beginning of the line. Another　problem is the case of a computer system which locally uses period,　".", to cause the new line function and which uses, in some programs,　CR and LF for semantically significant operations. Suppose the user　Telnet sends the sequence CR LF. Does this mean "new line" or the　"CR operation" followed by the "LF operation "? A solution to this　problem it to require that Telnet programs send a CR NOT intended to　be part of a CR LF pair as a CR NUL pair. Then the receiving program　can always hold a CR and examine the next character to determine if a　new line function is intended. This solution is strongly　recommended.

　　One other question arises here, "Is it permitted to send the Telnet　signal NOP (code 130) between a CR and a LF when these are intended　to signify new line?" The answer is "yes, the NOP signal may occur　anywhere in the data stream."　　

       Echoing　The decision to have the assumed condition for echo be that the using　site will provide any echo necessary for its terminals was taken　because of the difficulties faced by some installations that cannot　turn off their echo or that have terminals that print locally as a　result of key strokes. Serving sites could take the position "have　user turn echo off," but this seems an unnecessary burden on the　user. In addition, some serving sites may choose not to supply any　echo service, in which case the no echo assumption will supply a　network-wide condition, while other assumptions would give a mixed　starting condition.