RFC 856 (rfc856) - Page 2 of 4

Telnet Binary Transmission

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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
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