User Telnet implementations SHOULD be able to send or receive any 7-bit ASCII character. Where possible, any special character interpretations by the user host's operating system SHOULD be bypassed so that these characters can conveniently be sent and received on the connection.
Some character value MUST be reserved as "escape to command mode"; conventionally, doubling this character allows it to be entered as data. The specific character used SHOULD be user selectable. On binary-mode connections, a User Telnet program MAY provide an escape mechanism for entering arbitrary 8-bit values, if the host operating system doesn't allow them to be entered directly from the keyboard.
The transparency issues are less pressing on servers, but implementors should take care in dealing with issues like: masking off parity bits (sent by an older, non-conforming client) before they reach programs that expect only NVT ASCII, and properly handling programs that request 8-bit data streams.