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- 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
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- Comments on TELNET Protocol Changes
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- 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.
-
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- 1. Hide-your-input
-
- 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. [
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- [Page 1]
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- 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.
-
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- 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.
-
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- 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
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- [Page 2]
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- 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
- not received.
-
- 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
-
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- [Page 3]
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- 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.
-
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- [ 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 ]
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