RFC 3967 (BCP 97), 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.
Recent idnits versions added a DOWNREF check.
Following is a list of documents that have had downward references called out during Last Call (so further mentions may be permitted to be waived). Please add to this table whenever a DOWNREF was approved during an IETF Last Call, sorted by RFC number.
The waiver is not automatic, remember, it's meant to be a judgement call. RFC 3967 on the Last Call waiver:
Once a specific down reference to a particular document has been accepted by the community (e.g., has been mentioned in several Last Calls), an Area Director may waive subsequent notices in the Last Call of down references to it. This should only occur when the same document (and version) are being referenced and when the AD believes that the document's use is an accepted part of the community's understanding of the relevant technical area.