Change:
The DNS uses glue records to allow iterative clients to find the addresses of nameservers that are contained within a delegated zone. <del> Servers are expected to return available glue records in referrals. If message size constraints prevent the inclusion of all glue records in a UDP response, the server MUST set the TC flag to inform the client that the response is incomplete, and that the client SHOULD use TCP to retrieve the full response. </del> <ins> Authoritative Servers are expected to return all available glue records in referrals.  If message size constraints prevent the inclusion of all glue records in a UDP response, the server MUST set the TC flag to inform the client that the response is incomplete, and that the client SHOULD use TCP to retrieve the full response. </ins> 1. The Domain Name System (DNS) RFC1034, RFC1035 uses glue records to allow iterative clients to find the addresses of nameservers that are contained within a delegated zone.  Glue records are added to the <del> parent zone as part of the delegation process.  Servers are expected to return all available glue records in referrals.  If message size constraints prevent the inclusion of all glue records in a UDP response, the server MUST set the TC flag to inform the client that the response is incomplete, and that the client SHOULD use TCP to retrieve the full response.  This document clarifies that expectation. </del> <ins> parent zone as part of the delegation process and returned in referral responses, otherwise a resolver following the referral has no way of finding these addresses.  Authoritative servers are expected to return all available glue records in referrals.  If message size constraints prevent the inclusion of all glue records in a UDP response, the server MUST set the TC (Truncated) flag to inform the client that the response is incomplete, and that the client SHOULD use TCP to retrieve the full response.  This document clarifies that expectation. DNS responses sometimes contain optional data in the additional section.  Glue records however are not optional.  Several other protocol extensions, when used, are also not optional.  This includes TSIG RFC2845, OPT RFC6891, and SIG(0) RFC2931. </ins> 1.1.

Discussion:
Misc updates .. Rework Sibling Glue section, and clarify that they MUST be included, and also subject to TC=1 if they don't fit. Remove the cross parent zone cyclic dependency example, which doesn't work (I think we agreed to that, but I can be persuaded to add it back, if we want to point out a misconfiguration as a warning). Move the paragraph about non-optionality of types of additional section data into the tail end of the Intro. It deserves to be more prominently mentioned than buried at the end of the example of missing glue. Addressed Duane's comment that the "Why is Glue Required?" section did not actually say why, but instead just gave examples. Instead, I added a one phrase explaining why to the Intro section that defines glue records. I don't think it requires a separate section. Removed Duane's comment about "I think the document should say that some servers might experience an increase in TCP if implementations require all glue or set TC=1. If not in this section then somewhere else._". I agree we should address this, so I've made an "issue" for it. And we can add the relevant text in the next update, once we figure out where it goes (an "Operational Considerations" section perhaps?).
NAME - added your tweaks. Please review/approve!