[STD75]    Internet Standard 75,
              <https://www.rfc-editor.org/info/std75>.
              At the time of writing, this STD comprises the following:

              Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,
              <https://www.rfc-editor.org/info/rfc6891>.