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RFC 3967, titled "Clarifying when Standards Track Documents may Refer Normatively to Documents at a Lower Level", specifies the rules for downward references (DOWNREFs):

"IETF procedures generally require that a standards track RFC may not have a normative reference to another standards track document at a lower maturity level or to a non standards track specification (other than specifications from other standards bodies). For example, a standards track document may not have a normative reference to an informational RFC. Exceptions to this rule are sometimes needed as the IETF uses informational RFCs to describe non-IETF standards or IETF-specific modes of use of such standards."