http://www.rfc-editor.org/rfc/rfc0857.txt
RFC 0857: Telnet Echo Option, J. Postel, J. Reynolds1983-05-01T23:00:00-00:00This Telnet Option enables remote echoing by the other Telnet module. This RFC specifies a standard for the ARPA Internet community. Hosts on the ARPA Internet are expected to adopt and implement this standard. Obsoletes NIC 15390.