Crocker, D., "Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves", BCP 222, RFC 8552, March 2019. 

Crocker, D., "DNS Attrleaf Changes: Fixing Specifications That Use Underscored Node Names", BCP 222, RFC 8553, March 2019.

<https://www.rfc-editor.org/info/bcp222>