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