[BCP48]    Best Current Practice 48,
              <https://www.rfc-editor.org/info/bcp48>.
              At the time of writing, this BCP comprises the following:

              Dawkins, S., Montenegro, G., Kojo, M., and V. Magret,
              "End-to-end Performance Implications of Slow Links",
              BCP 48, RFC 3150, DOI 10.17487/RFC3150, July 2001,
              <https://www.rfc-editor.org/info/rfc3150>.