[BCP124]   Best Current Practice 124,
              <https://www.rfc-editor.org/info/bcp124>.
              At the time of writing, this BCP comprises the following:

              Floyd, S., "Specifying Alternate Semantics for the
              Explicit Congestion Notification (ECN) Field", BCP 124,
              RFC 4774, DOI 10.17487/RFC4774, November 2006,
              <https://www.rfc-editor.org/info/rfc4774>.