[STD51]    Internet Standard 51,
              <https://www.rfc-editor.org/info/std51>.
              At the time of writing, this STD comprises the following:

              Simpson, W., Ed., "The Point-to-Point Protocol (PPP)",
              STD 51, RFC 1661, DOI 10.17487/RFC1661, July 1994,
              <https://www.rfc-editor.org/info/rfc1661>.

              Simpson, W., Ed., "PPP in HDLC-like Framing", STD 51,
              RFC 1662, DOI 10.17487/RFC1662, July 1994,
              <https://www.rfc-editor.org/info/rfc1662>.