[BCP88]    Best Current Practice 88,
              <https://www.rfc-editor.org/info/bcp88>.
              At the time of writing, this BCP comprises the following:

              Schryver, V., "IANA Considerations for the Point-to-Point
              Protocol (PPP)", BCP 88, RFC 3818, DOI 10.17487/RFC3818,
              June 2004, <https://www.rfc-editor.org/info/rfc3818>.