Network Working Group J. Winett Request for Comments: 452 JMW NIC: 14136 8 February 1973
TELNET Command at Host LL
The attached writeup documents the use of the TELNET command at Host LL for uses under the CP/CMS time-sharing system. This command provides for use in a HALF DUPLEX or FULL DUPLEX mode and can be used with ASCII codes or with EBCDIC codes (i.e., transparent mode). The keyboard conventions allow for entering all ASCII codes using a CONTROL character followed by another character for the codes not present on an IBM 2741 terminal. In addition a number of local TELNET control commands have been defined which allows the mode of operation to be changed, the redefinition of special characters, the sending of TELNET control codes, as well as for obtaining input from a file instead of from the terminal. These later features provide for a rudimentary file transfer facility.
This material has not been reviewed for public release and is intended only for use with the ARPA network. It should not be quoted or cited in any publication not related to the ARPA network.
TELNET
Purpose:
To access another terminal oriented system on the ARPA network.
Format:
TELNET host <tag> RESUME EBCDIC HALFDUP 1 OPEN ASCII FULLDUP
host - either the hexadecimal code for a foreign network service site or a standard mnemonic for a foreign site. See Figure 1.
tag - the identifier for the local connections to the network. The tag is used together with the address of the virtual machine descriptor table (UTABLE) to form local socket numbers which are used in the network protocol.
RESUME - used to reactivate communications with a foreign site after having previously left the TELNET command leaving the connections open.
Winett [Page 1]
RFC 452 TELNET Command at Host LL February 1973
EBCDIC - to communicate with EBCDIC codes. The default is network ASCII.
HALFDUP - to operate under a half duplex protocol, i.e. with a locked keyboard.
The EBCDIC HALFDUP the protocol assumes that the TELNET break code (circle C) will be received to indicate when the keyboard should be locked for input.
In ASCII HALFDUP the keyboard will lock after a line of input and will unlock after one or more lines have been received for output. An external interrupt will also unlock a locked keyboard.
The default is full duplex where the keyboard is always unlocked for input. A null line is required to temporarily lock the keyboard in order to receive output.
Usage:
A number of hosts on the ARPA network provide TELNET service. A Network Virtual Terminal (NVT) has been specified so that using sites can write one TELNET program which maps a local terminal into the NVT to access any serving site on the network. Once communication has been established between a using site and a serving site, keyed input is sent to the serving system and output from the serving site, when received, is typed on the local terminal.
The NVT protocol requires that the keyboard be capable of entering all of the 128 ASCII codes together with a number of the TELNET control codes. To support an NVT with an IBM 2741 terminal, it is necessary to adapt a control convention for entering codes which are not associated with single keys on the keyboard. In addition, since CP/CMS processes input from a 2741 on a line at a time terminated with a newline, a means must be establish for entering a sequence of characters for transmission which is not terminated with a newline code.
When TELNET is initiated the message
ENTER CONTROL CHARACTER
is typed. A non-blank character should then be entered which defines the character which, in combination with another character, will be used to enter codes not associated with single
Winett [Page 2]
RFC 452 TELNET Command at Host LL February 1973
keys. The control character is also used for other special control functions as described below.
Codes:
The NVT usually requires that characters be transmitted in an eight bit ASCII code. Since the TELNET command is written to process EBCDIC codes ASCII codes received are translated into ASCII before being sent to a serving site. Figure 2 gives the complete definition of EBCDIC indicating the EBCDIC controls and EBCDIC graphics. Figure 3 gives the codes for the ASCII controls and graphics. The complete mapping between 8-bit EBCDIC codes and 8-bit network ASCII codes is shown in Figure 4. The EBCDIC newline code (NL) is mapped into the ASCII codes for the pair of characters CR-IF.
The following ASCII/EBCDIC mapping is used for the non-EBCDIC graphics:
ASCII EBCDIC
TILDE (7E) = (A1) NOT BAR (7C) = (6A) OR BACK SLASH (5C) = (EO) CARAT (5E) = (71) GRAVE (6O) = (79) LEFT BRACE (7B) = (8B) RIGHT BRACE (7D) = (9B) LEFT BRACKET (5B) = (AD) RIGHT BRACKET (5D) = (BD)
The ASCII control DC3 (X' 13') maps to the EBCDIC control TM (X'13'). The ASCII control NUL (X' oo') is sent to the terminal as the EBCDIC code for NULL (X' oo') and is not mapped into an IDLE (X'17').
The TELNET control hide-your-input is mapped into the EBCDIC code for bypass (print suppress) and the TELNET control noecho is mapped into the EBCDIC code for restore (print restore). If the TELNET control for echo is received, a message is printed and it is mapped into an IDLE. Similarly, if the TELNET control for break is received, a message is printed and it is mapped into an IDLE unless operation is in EBCDIC HALFDUP mode in which case the break is used to indicate that any received characters should be printed and the keyboard unlocked for input. If a data mark or an interrupt is received, no action is taken except to print a message to notify the user of this occurrence.
Winett [Page 3]
RFC 452 TELNET Command at Host LL February 1973
Input:
When the control character is entered, the following character is mapped into a different code than that which it is normally mapped into, except when the following character is a space or a character not defined to have a meaning when preceded by the control character. Figure 5 gives the mapping of the characters on a 2741 keyboard when preceded by a control character. The following 2741 keyboard characters do not have a different meaning when preceded by the control character.
When a character is mapped into its control code, the control character is mapped into the code for IDLE. If the control character is entered as the last character before the newline key is entered, the sequence of characters entered is transmitted without the newline code. That is, the newline code is not transmitted when it is preceded by the control character.
When the 2741 keyboard is unlocked for input, characters received cannot be typed until the keyboard is locked again. After a line is entered, received characters can then be typed. When operating in full duplex or ASCII half duplex, a null line entered will allow received characters to be typed but will not cause the new line code to be transmitted. To cause a null line, i.e., just the new line code to be transmitted, the control character should be entered as the only character in the input line. In EBCDIC HALFDUP a null line entered will cause a null line to be transmitted.
Output:
ASCII output received from the NVT is converted into EBCDIC with the sequences CR-LF converted into IDLE-NL. The EBCDIC characters are then sent to the terminal. Note that not all 128 ASCII codes when converted to EBCDIC will print on a 2741. Of the 95 ASCII graphics and the 8 ASCII controls which are defined for the NVT printer, the following are not visible or audible:
Winett [Page 4]
RFC 452 TELNET Command at Host LL February 1973
CARAT GRAVE BACK SLASH LEFT BRACE RIGHT BRACE LEFT BRACKET RIGHT BRACKET ASCII CONTROL BELL (BEL) ASCII CONTROL VERTICAL TAB (HT) ASCII CONTROL FORM FEED (FF) ASCII CONTROL CARRIAGE RETURN (CR)
Figure 6 shows how the EBCDIC codes from X' 40' through X' FF' will appear on a 2741 terminal. Figure 7 shows how the EBCDIC codes will appear when printed with a PN train on the offline printer and Figure 8 shows how these codes appear when printed with a TN train.
Controls:
If the first character in an input line is the control character and the next character is a space, the rest of the line is interpreted as a TELNET control command. A control command consists of a control word and parameters separated by spaces. Controls are defined which permit TELNET controls to be transmitted to the serving site, allow input to come from a file or output to go to a file, allow CMS functions or transient commands to be issued, redefine the control character or TELNET mode, close connections or leave the TELNET command with connections still open, as well as controls to support a reader, punch, and printer with RJS operation. The controls are described below.
CONTROL x
Where x is the new control character
CLOSE
To close all connections and quit
QUIT
To leave TELNET
EBCDIC
To go into transparent mode, i.e., no translation
Winett [Page 5]
RFC 452 TELNET Command at Host LL February 1973
ASCII
To translate input and output to network ASCII
Break
To send the TELNET break code
SYNC
To send the TELNET data mark code and an interrupt
AATN
To send a TELNET break and a SYNC
HIDE-YOUR-INPUT
To send the TELNET hide you input code
NOECHO
To send the TELNET noecho code
ECHO
To send the TELNET echo code
CMS command arg1...argN
To issue CMS core resident function or transient command.
INPUT fn ft * TERMIN * *
To get input from a file If fn is defaulted, input is reset to come from the terminal. If fn is * file input resumes after the last line read. After an EOF, the next line read will be the first line of the file.
An external interrupt while input is coming from a file will cause the line number of the next line to be read from the file to be typed and input to be reset to come from the terminal.
Winett [Page 6]
RFC 452 TELNET Command at Host LL February 1973
OUTPUT fn OFF TERM INPUT INOUT * ON NOTERM NOINPUT OUTPUT
To write output to the file "fn TERMOUT". If fn is defaulted, output is reset to go to the terminal. If fn is *, file OUTPUT is resumed with the same options as were last used.
For Output to the Terminal:
If the last character is a CR, a line with just the control character is typed on the next line (with a NL)
If the last character is not a NL or a CR, the line is typed without a NL (i.e., with TYPE).
For Output to a File:
If just a NL is in the line, just the control character is sent to the file.
If the last CHAR is not NL or CR, the control character is added after the last character, except if 130 characters must be sent to the file.
If the last CHAR is a CR, it is included in the file.
OFF causes all output to be discarded.
ON is the default, and causes output to the terminal.
TERM causes output to also go to the terminal.
NOTERM is the default, and causes output to go the file but not to the terminal.
OUTPUT is the default and causes just terminal output to be put to the file "FN termout".
INPUT causes both terminal input but not output to be put to the output file.
NOINPUT is defaulted and causes input to not go to the file.
PURGE
To purge all output currently received by the NCP. *****NOT YET IMPLEMENTED*****
Winett [Page 7]
RFC 452 TELNET Command at Host LL February 1973
READER fn ft * READER
To send a job to the RJS system at UCLA's CCN.
If fn and ft are defaulted, input will come from the card reader.
PRINTER fn ft * PRINTER
To receive printer output from the RJS system at UCLA's CCN.
To receive punch output from the RJS system at UCLA's CCN.
If fn and ft are defaulted, output goes to the printer.
PUNCH fn ft * PUNCH
If fn and ft are defaulted, output goes to the punch.