RFC 856 (rfc856) - Page 2 of 4
Telnet Binary Transmission
Alternative Format: Original Text Document
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
