[BCP91]    Best Current Practice 91,
              <https://www.rfc-editor.org/info/bcp91>.
              At the time of writing, this BCP comprises the following:

              Durand, A. and J. Ihren, "DNS IPv6 Transport Operational
              Guidelines", BCP 91, RFC 3901, DOI 10.17487/RFC3901,
              September 2004, <https://www.rfc-editor.org/info/rfc3901>.