DNS/RFC/4472について、ここに記述してください。

https://tools.ietf.org/html/rfc4472

Operational Considerations and Issues with IPv6 DNS

In the terminology of RFC 4472, in-bailiwick glue is "critical" additional data, which must either be included in a response or the TC bit must be set. There's a really good discussion of the issues in appendix B.