Comments on Telnet Protocol Changes(1)

　　Network Working Group

　　Request for Comments: 393 Joel M. Winett

　　NIC 11585 Lincoln Laboratory

　　Categories: TELNETLL-67

　　References: RFC109, 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.

　　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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　　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.