Network Working Group
Request for Comments: 393 Joel M. Winett
NIC 11585 Lincoln Laboratory
Categories: TELNET LL-67
References: RFC 109, 139, 158,318, and 328 3 October 1972 Comments on TELNET Protocol Changes
Through this RFC, I am registering my objection to two of the
three suggestions for changing the TELNET protocol as described in RFC
328 and am adding my suggestion for the interpretation of the TELNET
Reverse Break Control Code.
This code was originally put in the TELNET protocol to give the
virtual terminal the ability to simulate a real terminal which has the
print suppress capability. If the terminals being used at some
installations do not have the ability to disable the printing
mechanism, the TELNET being used can either ignore this code or
attempt to simulate the function using other means (e. g., blacking
out a number of character positions and returning to the first
character position). Every attempt should be made to allow a network
user of a time-sharing system to have the same facilities as a local
user of the time-sharing system. The specification of TELNET protocol
should not limit the function of users if a function cannot be
supported by all users.
The "Hide-your-input" and "Echo" TELNET control codes provide for the
support of two functions available in some time-sharing systems. The
"Hide-your-input" function is really a special case of the "Echo" mode
of operation where the server tells the user that the server will echo
but the server does not. A separate code is used for this func- tion
since some servers may support this function but may not support the
full "Echo" mode of operation.
] 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. [
The "Hide-your-input" and "Echo" modes of operation are disabled with
the "No-echo" control. ASCII control codes could have been chosen for
these functions but it was decided that the NVT ASCII control codes
should only be specified for commonly used functions.
To indicate the number of characters for which the printing should be
suppressed, the "Hide-your-input" TELNET control could be rede- fined
to include a byte following the "Hide-your-input" control to indicate
the number of characters for which the printing should be concealed.
The "No-echo" control would still be sent so that systems with the
print suppress feature would not have to count characters.
2. Data Types
The protocol should allow a server to support users with character
codes other than ASCII, e. g., EBCDIC. The definition of an alter-
nate character code should include the definition of the TELNET
control codes. An EBCDIC code has been proposed in RFC # 109 and has
been implemented on the Lincoln Laboratory 360/67. If it is desired to
allow one to return to the network standard ASCII code, the non-ASCII
code should contain a code to indicate return to ASCII.
3. Reverse Break
The code for Break is defined as a 129th ASCII data code. It is
usually transmitted from a user's network virtual terminal to a server
when a corresponding key (break key or attention key) is typed on the
TELNET terminal and is interpreted by serving systems as that special
key. Since a common function of this key is to interrupt a running
process the server must be alerted to the fact that this code has been
transmitted no matter when it is sent. Thus, the TELNET SYNC (TELNET
data mark together with a network interrupt on the TELNET send socket)
must also be trans- mitted to cause the serving process to examine the
received charac- ters. The ASCII control code EOT (Octal 4) could have
been chosen for the break function but his code is not interpreted by
all systems. Thus, it was decided that an NVT TELNET control code
should be used for this purpose.
The use of the Break Code from server to user TELNET has not
previously been defined and, thus, could be used to solve the
following problems which occur with line at a time and half duplex
systems. Line at a time systems do not output characters to the
terminal a character at a time but, instead, wait until a line is ready
for output. If a CR-LF sequence (TELNET protocol for end of line)
is received it is interpreted as an end of line and the characters
received are output. If characters are received which do not end
with CR-LF the user TELNET does not know whether or not other
characters will follow which are part of the current line. Thus, the
characters received thus far must be output, without a CR-LF (new
line). If an end of message code were transmitted, the user TELNET
would know whether or not other characters would be received for
output. The user TELNET would then print characters either when
the TELNET Break control is received or when the CR-LF newline
sequence is received.
If the user TELNET is being run from a half duplex terminal, the
terminal cannot receive input and type output at the same time. Thus,
if output is received while the terminal is being used for input the
TELNET program must either buffer the received characters or abort the
input mode of operation to write out the received charac- ters. If
characters received are written out as they are received, the terminal
operation would be very similar to a full duplex terminal. This mode
of operation requires that the terminal have a reverse break
capability to allow the input mode to be aborted by program control.
In some systems it is only desirable to abort the input mode of
operation when a complete line is ready for output. If a string of
characters received does not end with an end of line code, the
characters received will not be output until after the input line is
entered, i. e., the mode of operation changed from input to output.
If an end of message code were transmitted, the user TELNET could
abort the input mode of operation even though the end of line code was
In systems which do not support the reverse break feature or if the
terminal does not have this feature it is not possible to abort an
input mode of operation in order to output received characters. In
this case, the systems can operate in either of two modes, a) un-
locked keyboard, or b) locked keyboard mode.
In an unlocked keyboard system, received characters are not output
until the user completes an input line. An input line is completed
when the end of line code is entered. This might be a CR, a LF, or
a NL code. After received characters, if any, are output, the input
modes is re-entered. To receive output the user must enter an input
line (possibly a null line). If the user is waiting for output, he must
repeatedly enter a line until the output has been received and typed.
Since an input line must be entered just to receive output, it is
desirable to define an input line which does not result in anything
being sent to the serving system. If a null line (a line consisting of
just the end of line code) is chosen for this purpose, some other input
line must be defined to cause a null line to be transmitted.
In a locked keyboard system, the input mode is not immediately
re-entered after an input line is entered. It is re-entered only after
a defined prompt is received. The prompt can be defined to be the
reception of any character or can be defined to be a specific charac-
ter code. If a specific code is chosen the serving site must send this
code whenever the terminal should be put into input mode. If an end of
message code were transmitted this code could be inter- preted to be
the input prompt code.
In summary, three situations have been described where an end of
message code would be desirable.
a) To indicate when a line which does not end with CR-LF should
be output for line at a time systems
b) To indicate that the input mode in half duplex operation should
be aborted so that received characters can be output
c) As a prompt character to cause the input mode to be entered
for locked keyboard half duplex systems
The ASCII TELNET control code for Break (Reverse Break) could be
interpreted as an end of message code when sent from server to user.
[ This RFC was put into machine readable form for entry ]
[ into the online RFC archives by BBN Corp. under the ]
[ direction of Alex McKenzie. 1/97 ]