[BCP64]    Best Current Practice 64,
              <https://www.rfc-editor.org/info/bcp64>.
              At the time of writing, this BCP comprises the following:

              Zeilenga, K., "Internet Assigned Numbers Authority (IANA)
              Considerations for the Lightweight Directory Access
              Protocol (LDAP)", BCP 64, RFC 4520, DOI 10.17487/RFC4520,
              June 2006, <https://www.rfc-editor.org/info/rfc4520>.