Discussion of Telnet Protocol(2)

　　The understanding that ASCII be used as a network-wide code has been　established for some time. Its use in TELNET provided a problem with　respect to the limitation of a maximum character set of 128. Some　systems provide for more than this number in their operation, and　therefor, as serving sites cannot map on a one for one basis.

　　Each such serving site could probably provide a reasonably useful　character set, including all system control signals, by mapping 128　of its codes and just not provide a network user accessto the other　codes. However, any character left out might later be used in a　major application at that site as a special control signal. This　could result in denying network users the facility offered by that　application. Serving sites are, therefor, encouraged to provide a　full mapping between the ASCII code and the code used on the serving　system.

　　The ASCII code for ESC (known to some as ALT MODE) has been selected　as an escape [1]. For each serving site character not mapped on a　one for one basis, the serving site can specify an escape character　or string of escape characters (preferably a printable graphic) to　represent it. Thus, the user could enter the full set of serving　site code from any network terminal operating through the Network　Virtual Terminal (NVT) ASCII convention. The serving site, in　generating output directed at the user's terminal, would be expected　to map out such a character and transmit the appropriate ESC　character or string of ESC characters.

　　Example: A serving site, whose normal code is EBCDIC, has　specified that cent ([5]) has not been mapped on a one for one　basis and that to transmit the character, users must enter ESC　followed by C. At a using site, the TELNET implementers have　decided to try to print out all ESC characters using \ to indicate　ESC. On receipt of the representation for cent, the user would　see \C on his print-out.

　　The representation of the end of a physical line at a terminal is　implemented differently on 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 returns to the　terminal a line feed. The network-wide representation will be　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 Mod 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 to the NCP for transmission. When the TELNET finds　the carriage return line feed sequence in the data stream coming　from the serving site, the two characters are replaced with New　Line code and sent to the terminal.

　　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 "let　the user turn my 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 connection. [2]