http://www.rfc-editor.org/rfc/rfc0860.txt
RFC 0860: Telnet Timing Mark Option, J. Postel, J. Reynolds1983-05-01T23:00:00-00:00This Telnet Option provides a way to check the roundtrip path between two Telnet modules. 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 16238.