[BCP97]    Best Current Practice 97,
              <https://www.rfc-editor.org/info/bcp97>.
              At the time of writing, this BCP comprises the following:

              Bush, R. and T. Narten, "Clarifying when Standards Track
              Documents may Refer Normatively to Documents at a Lower
              Level", BCP 97, RFC 3967, DOI 10.17487/RFC3967, December
              2004, <https://www.rfc-editor.org/info/rfc3967>.

              Klensin, J. and S. Hartman, "Handling Normative References
              to Standards-Track Documents", BCP 97, RFC 4897,
              DOI 10.17487/RFC4897, June 2007,
              <https://www.rfc-editor.org/info/rfc4897>.

              Leiba, B., "Updating When Standards Track Documents May
              Refer Normatively to Documents at a Lower Level", BCP 97,
              RFC 8067, DOI 10.17487/RFC8067, January 2017,
              <https://www.rfc-editor.org/info/rfc8067>.