Bush, R. and T. Narten, "Clarifying when Standards Track Documents may Refer Normatively to Documents at a Lower Level", BCP 97, RFC 3967, December 2004. 

Klensin, J. and S. Hartman, "Handling Normative References to Standards-Track Documents", BCP 97, RFC 4897, June 2007.

Leiba, B., "Updating When Standards Track Documents May Refer Normatively to Documents at a Lower Level", BCP 97, RFC 8067, January 2017.

<https://www.rfc-editor.org/info/bcp97>