RFC 393 (rfc393) - Page 2 of 4

Comments on Telnet Protocol Changes

    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