It would be informative to understand what interface MTU the hosts providing DNS use, in the roots.
This is not to suggest there has to be a defined one, but it would help understand how people interact in the UDP/TCP shift under TC=1, because of the lack of ICMP/ICMPv6 signal (ICMP filters) and the effect of the server-side in TCP setting a lower MSS avoiding the problem.
Could the caucus act as a channel to get this data?
-George