[BCP148]   Best Current Practice 148,
              <https://www.rfc-editor.org/info/bcp148>.
              At the time of writing, this BCP comprises the following:

              Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
              Behavioral Requirements for ICMP", BCP 148, RFC 5508,
              DOI 10.17487/RFC5508, April 2009,
              <https://www.rfc-editor.org/info/rfc5508>.