[STD28]    Internet Standard 28,
              <https://www.rfc-editor.org/info/std28>.
              At the time of writing, this STD comprises the following:

              Postel, J. and J. Reynolds, "Telnet Echo Option", STD 28,
              RFC 857, DOI 10.17487/RFC0857, May 1983,
              <https://www.rfc-editor.org/info/rfc857>.