[BCP223]   Best Current Practice 223,
              <https://www.rfc-editor.org/info/bcp223>.
              At the time of writing, this BCP comprises the following:

              Reilly, D., Stenn, H., and D. Sibold, "Network Time
              Protocol Best Current Practices", BCP 223, RFC 8633,
              DOI 10.17487/RFC8633, July 2019,
              <https://www.rfc-editor.org/info/rfc8633>.