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+
+Network Working Group                                          J. Postel
+Request for Comments: 856                                    J. Reynolds
+                                                                     ISI
+Obsoletes: NIC 15389                                            May 1983
+
+                       TELNET BINARY TRANSMISSION
+
+
+This RFC specifies a standard for the ARPA Internet community.  Hosts on
+the ARPA Internet are expected to adopt and implement this standard.
+
+1.  Command Name and Code
+
+   TRANSMIT-BINARY      0
+
+2.  Command Meanings
+
+   IAC WILL TRANSMIT-BINARY
+
+      The sender of this command REQUESTS permission to begin
+      transmitting, or confirms that it will now begin transmitting
+      characters which are to be interpreted as 8 bits of binary data by
+      the receiver of the data.
+
+   IAC WON'T TRANSMIT-BINARY
+
+      If the connection is already being operated in binary transmission
+      mode, the sender of this command DEMANDS to begin transmitting
+      data characters which are to be interpreted as standard NVT ASCII
+      characters by the receiver of the data.  If the connection is not
+      already being operated in binary transmission mode, the sender of
+      this command REFUSES to begin transmitting characters which are to
+      be interpreted as binary characters by the receiver of the data
+      (i.e., the sender of the data demands to continue transmitting
+      characters in its present mode).
+
+      A connection is being operated in binary transmission mode only
+      when one party has requested it and the other has acknowledged it.
+
+   IAC DO TRANSMIT-BINARY
+
+      The sender of this command REQUESTS that the sender of the data
+      start transmitting, or confirms that the sender of data is
+      expected to transmit, characters which are to be interpreted as 8
+      bits of binary data (i.e., by the party sending this command).
+
+   IAC DON'T TRANSMIT-BINARY
+
+      If the connection is already being operated in binary transmission
+      mode, the sender of this command DEMANDS that the sender of the
+      data start transmitting characters which are to be interpreted as
+
+
+Postel & Reynolds                                               [Page 1]
+
+
+
+RFC 856                                                         May 1983
+
+
+      standard NVT ASCII characters by the receiver of the data (i.e.,
+      the party sending this command).  If the connection is not already
+      being operated in binary transmission mode, the sender of this
+      command DEMANDS that the sender of data continue transmitting
+      characters which are to be interpreted in the present mode.
+
+      A connection is being operated in binary transmission mode only
+      when one party has requested it and the other has acknowledged it.
+
+3.  Default
+
+   WON'T TRANSMIT-BINARY
+
+   DON'T TRANSMIT-BINARY
+
+      The connection is not operated in binary mode.
+
+4.  Motivation for the Option
+
+   It is sometimes useful to have available a binary transmission path
+   within TELNET without having to utilize one of the more efficient,
+   higher level protocols providing binary transmission (such as the
+   File Transfer Protocol).  The use of the IAC prefix within the basic
+   TELNET protocol provides the option of binary transmission in a
+   natural way, requiring only the addition of a mechanism by which the
+   parties involved can agree to INTERPRET the characters transmitted
+   over a TELNET connection as binary data.
+
+5.  Description of the Option
+
+   With the binary transmission option in effect, the receiver should
+   interpret characters received from the transmitter which are not
+   preceded with IAC as 8 bit binary data, with the exception of IAC
+   followed by IAC which stands for the 8 bit binary data with the
+   decimal value 255.  IAC followed by an effective TELNET command (plus
+   any additional characters required to complete the command) is still
+   the command even with the binary transmission option in effect.  IAC
+   followed by a character which is not a defined TELNET command has the
+   same meaning as IAC followed by NOP, although an IAC followed by an
+   undefined command should not normally be sent in this mode.
+
+6.  Implementation Suggestions
+
+   It is foreseen that implementations of the binary transmission option
+   will choose to refuse some other options (such as the EBCDIC
+   transmission option) while the binary transmission option is in
+
+
+
+
+Postel & Reynolds                                               [Page 2]
+
+
+
+RFC 856                                                         May 1983
+
+
+   effect.  However, if a pair of hosts can understand being in binary
+   transmission mode simultaneous with being in, for example, echo mode,
+   then it is all right if they negotiate that combination.
+
+   It should be mentioned that the meanings of WON'T and DON'T are
+   dependent upon whether the connection is presently being operated in
+   binary mode or not.  Consider a connection operating in, say, EBCDIC
+   mode which involves a system which has chosen not to implement any
+   knowledge of the binary command.  If this system were to receive a DO
+   TRANSMIT-BINARY, it would not recognize the TRANSMIT-BINARY option
+   and therefore would return a WON'T TRANSMIT-BINARY.  If the default
+   for the WON'T TRANSMIT-BINARY were always NVT ASCII, the sender of
+   the DO TRANSMIT-BINARY would expect the recipient to have switched to
+   NVT ASCII, whereas the receiver of the DO TRANSMIT-BINARY would not
+   make this interpretation.
+
+   Thus, we have the rule that when a connection is not presently
+   operating in binary mode, the default (i.e., the interpretation of
+   WON'T and DON'T) is to continue operating in the current mode,
+   whether that is NVT ASCII, EBCDIC, or some other mode.  This rule,
+   however, is not applied once a connection is operating in a binary
+   mode (as agreed to by both ends); this would require each end of the
+   connection to maintain a stack, containing all of the encoding-method
+   transitions which had previously occurred on the connection, in order
+   to properly interpret a WON'T or DON'T.  Thus, a WON'T or DON'T
+   received after the connection is operating in binary mode causes the
+   encoding method to revert to NVT ASCII.
+
+   It should be remembered that a TELNET connection is a two way
+   communication channel.  The binary transmission mode must be
+   negotiated separately for each direction of data flow, if that is
+   desired.
+
+   Implementation of the binary transmission option, as is the case with
+   implementations of all other TELNET options, must follow the loop
+   preventing rules given in the General Considerations section of the
+   TELNET Protocol Specification.
+
+   Consider now some issues of binary transmission both to and from
+   both a process and a terminal:
+
+      a. Binary transmission from a terminal.
+
+         The implementer of the binary transmission option should
+         consider how (or whether) a terminal transmitting over a TELNET
+         connection with binary transmission in effect is allowed to
+         generate all eight bit characters, ignoring parity
+         considerations, etc., on input from the terminal.
+
+
+Postel & Reynolds                                               [Page 3]
+
+
+
+RFC 856                                                         May 1983
+
+
+      b. Binary transmission to a process.
+
+         The implementer of the binary transmission option should
+         consider how (or whether) all characters are passed to a
+         process receiving over a connection with binary transmission in
+         effect.  As an example of the possible problem, TOPS-20
+         intercepts certain characters (e.g., ETX, the terminal
+         control-C) at monitor level and does not pass them to the
+         process.
+
+      c. Binary transmission from a process.
+
+         The implementer of the binary transmission option should
+         consider how (or whether) a process transmitting over a
+         connection with binary transmission in effect is allowed to
+         send all eight bit characters with no characters intercepted by
+         the monitor and changed to other characters.  An example of
+         such a conversion may be found in the TOPS-20 system where
+         certain non-printing characters are normally converted to a
+         Circumflex (up-arrow) followed by a printing character.
+
+      d. Binary transmission to a terminal.
+
+         The implementer of the binary transmission option should
+         consider how (or whether) all characters received over a
+         connection with binary transmission in effect are sent to a
+         local terminal.  At issue may be the addition of timing
+         characters normally inserted locally, parity calculations, and
+         any normal code conversion.
+
+
+
+
+
+
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+Postel & Reynolds                                               [Page 4]
+