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Network Working Group                                          J. PostelRequest for Comments: 856                                    J. Reynolds                                                                     ISIObsoletes: NIC 15389                                            May 1983TELNET BINARY TRANSMISSIONThis RFC specifies a standard for the ARPA Internet community.  Hosts onthe ARPA Internet are expected to adopt and implement this standard.1.  Command Name and Code   TRANSMIT-BINARY      02.  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 asPostel & 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 inPostel & 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.Postel & Reynolds                                               [Page 4]