[BCP172]   Best Current Practice 172,
              <https://www.rfc-editor.org/info/bcp172>.
              At the time of writing, this BCP comprises the following:

              Kumari, W. and K. Sriram, "Recommendation for Not Using
              AS_SET and AS_CONFED_SET in BGP", BCP 172, RFC 6472,
              DOI 10.17487/RFC6472, December 2011,
              <https://www.rfc-editor.org/info/rfc6472>.