Original text:
retrieve the full response.  This document clarifies that expectation. <del> While not common, real life examples of servers that fail to set TC=1 when glue records are available exist and they do cause resolution failures.  The example below from June 2020 shows a case where none of the glue records, present in the zone, fitted into the available space and TC=1 was not set in the response.  While this example shows an DNSSEC RFC4033, RFC4034, RFC4035 referral response, this behaviour has also been seen with plain DNS responses as well.  The records have been truncated for display purposes.  Note that at the time of this writing, this configuration has been corrected and the response correctly sets the TC=1 flag. 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. </del> 1.1. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 3. <del> This document clarifies correct DNS server behaviour expectations and does not introduce new security considerations. </del>  4.  There are no actions for IANA.

Reviewer comments:
Clarify glue, slightly reorder.
Merging - as mentioned in email, I will also add a more typical case of sibling glue (that doesn't involve cross zone and circular dependency). PR forthcoming ...

Revised text:
retrieve the full response.  This document clarifies that expectation.  1.1. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 3.  <ins> While not common, real life examples of servers that fail to set TC=1 when glue records are available exist and they do cause resolution failures. 3.1. The example below from June 2020 shows a case where none of the glue records, present in the zone, fitted into the available space and TC=1 was not set in the response.  While this example shows an DNSSEC RFC4033, RFC4034, RFC4035 referral response, this behaviour has also been seen with plain DNS responses as well.  The records have been truncated for display purposes.  Note that at the time of this writing, this configuration has been corrected and the response correctly sets the TC=1 flag. 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. 3.2. A zone's sibling glue is glue in another zone that is required for proper resolving.  While the example below is obvious, real life examples can be more complex and not obvious. This situation is harder to detect if the sibling zones are nog hosted on the same nameservers. 3.3. When a zone is deleted but the parent notices that its NS glue records are required for other zones, it MAY opt to take these (now orphaned) glue records into its own zone to ensure that other zones depending on this glue are not broken.  Technically, these NS records are no longer glue records, but authorative data of the parent zone, and should be added to the DNS response similarly to regular glue records. </ins> 4. <ins> This document clarifies correct DNS server behaviour and does not introduce any changes or new security considerations. 5. </ins> There are no actions for IANA.