[BCP40]    Best Current Practice 40,
              <https://www.rfc-editor.org/info/bcp40>.
              At the time of writing, this BCP comprises the following:

              Blanchet, M. and L. Liman, "DNS Root Name Service Protocol
              and Deployment Requirements", BCP 40, RFC 7720,
              DOI 10.17487/RFC7720, December 2015,
              <https://www.rfc-editor.org/info/rfc7720>.