Network Working Group J. Winett
Request for Comments: 466 LL-67
NIC: 14740 27 February 1973
TELNET LOGGER/SERVER For Host LL-67
The attached writeup documents the TELNET LOGGER/SERVER for the
CP/CMS system on the Lincoln Laboratory 360>67 (host 10). The
facility serves both half duplex and full duplex TELNET users with
data in either ASCII or EBCDIC codes.
Use of the hide-your-input and noecho TELNET controls are used for
the EBCDIC print suppress (bypass) and print restore features during
the login procedure. To support half duplex terminals, the TELNET
control break (reverse break) is sent as an input prompt when input
is desired. This code can also be used to indicate that a previous
line sent without an end of line sequence (CR-LF) should be printed.
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.
Operation of the Lincoln Laboratory
CP/CMS TELNET LOGGER/SERVER
The TELNET LOGGER/SERVER follows the ICP protocol for making a pair
of connections. The LOGGER is initially enabled for a connection on
socket X'00000001'. When an RFC is received for this socket a pair
of sockets will be chosen for the TELNET connections. If the maximum
number of TELNET users which can be served are active, the initial
connection is refused. Currently, three TELNET users can be served.
After the ICP connections have been setup, the LOGGER expects a
TELNET data type code, a string of network ASCII characters, or a
null line (just CR-LF) to indicate whether its operation should be in
ASCII or in EBCDIC character codes. ASCII is assumed unless the
first byte received is the TELNET EBCDIC data type code (X'A2). When
something has been received, the message:
Lincoln Laboratory CP/CMS Online
will be transmitted by the LOGGER. For example, if ASCII operation
is desired a null line (just CR-LF) transmitted on the send socket
will cause the welcoming message to be sent in ASCII. The CP login
procedure can then begin. If communications is desired to be carried
on with EBCDIC character codes, the first byte transmitted should be
the TELNET data type code for EBCDIC (X'A2'). Thereafter all
communications will be in the code originally used.
The CP login procedure expects the user to enter:
where the userid specifies the desired virtual machine. CP then
followed by the EBCDIC code for bypass (x'24') which is mapped into
the TELNET code hide-your-input.
The user should then enter a password. Passwords entered from the
network may be different from those entered from a local terminal.
The LOGGER maps network passwords into a corresponding CP password.
Thus, access to an account can only be made from the network if a
network password, together with a CP password and userid, is entered
into a file which is read by the LOGGER. If a userid entered from
the network is not in the LOGGER FILE (or if the network password
does not match the one included in the file for the specified userid)
the LOGGER passes an invalid userid (or password) to CP. The CP
response for an invalid userid or password is then sent to the
After a password is received by CP, CP transmits the EBCDIC code for
restore (X'14') which is mapped into the TELNET control noecho.
Since the CP/CMS system operates with EBCDIC codes, ASCII codes must
be translated into EBCDIC before being sent to a virtual machine.
Figure 1 gives the ASCII codes and their EBCDIC mapping. When the
ASCII sequence CR-LF is received, it is mapped into the EBCDIC code
NL. Whenever the TELNET control NOP is included in an input string,
it is mapped into an EBCDIC idle (X'17') and then removed from the
string. Thus, if TELNET NOP codes are included between a CR and LF,
they are removed before the CR-LF is mapped into the EBCDIC NL.
The TELNET control hide-your-input is mapped into the EBCDIC code for
bypass (X'24') and the TELNET control echo is mapped into the EBCDIC
control for restore (X'14'). If the TELNET control echo is received,
the SERVER should send the control noecho but this feature has not
yet been implemented. Instead, the TELNET control echo is mapped
into the EBCDIC code X'23'. If the TELNET break is received, it is
interpreted as an attention signal and the appropriate action is
taken by CP or CMS.
CP/CMS is a line at a time system and expects all input to consist of
lines ending with a NL code. Characters received are buffered until
the newline code is received.
Since CP/CMS is also a half duplex system, characters are only
examined when the system is expecting input. If the system is not
expecting input, a network interrupt is required to cause the SERVER
to process received characters. On receipt of a network interrupt,
characters received before the TELNET data mark is received are
examined and discarded, except that if a TELNET break code is found,
the appropriate CP/CMS interrupt action is stimulated.
On output, EBCDIC codes are mapped into network ASCII if a mapping
exists; otherwise, the codes are mapped into the TELNET control NOP.
A NL code is mapped into CR-LF. The EBCDIC code for bypass maps into
the TELNET control hide-your-input and the EBCDIC code for restore
maps into the TELNET control noecho. Also, the code X'23' maps into
the TELNET control echo and the code X'38' maps into the TELNET
Since CP/CMS is a line at a time, half duplex system the TELNET
control break is transmitted as an end of message signal and also as
an input prompt code. If characters were output without a NL, the
break, as an end of message code, indicates to the user TELNET
operating on a line at a time mode that the characters previously
transmitted should be printed without waiting for the end of line
sequence. If the user TELNET is also operating in a half duplex
mode, the break as an input prompt indicates that the system is ready
If input had been anticipated and sent by a full duplex user TELNET,
the TELNET SERVER will have that input available for immediate
processing. Thus, in the case of a full duplex user TELNET the break
as a prompt should be ignored.
Though CP/CMS operates in a half duplex mode, it supports half duplex
terminals with the reverse break feature allowing the system to abort
an input mode in order to transmit a priority output message. In
this situation, the TELNET SERVER transmits a TELNET SYNC. A half
duplex user TELNET should interpret this by aborting the input mode,
i.e., revoking a previous TELNET break which was interpreted as an
No codes in the output character stream can cause the TELNET data
mark to be transmitted.
When a user logs out from his virtual machine, CP passes the
equivalent of a line disconnect to the LOGGER. The LOGGER then
closes the TELNET send and receive sockets.