[BCP57]    Best Current Practice 57,
              <https://www.rfc-editor.org/info/bcp57>.
              At the time of writing, this BCP comprises the following:

              Fenner, B., "IANA Considerations for IPv4 Internet Group
              Management Protocol (IGMP)", BCP 57, RFC 3228,
              DOI 10.17487/RFC3228, February 2002,
              <https://www.rfc-editor.org/info/rfc3228>.