RFC 2460 section 5 paragraph 7 allowed a Packet Too Big message
to report a Next-Hop MTU less than 1280 in support of 6-to-4 routers.
A node receiving such a message was required to add a Fragment
Header to outgoing packets, even though they were not fragmented.
Almost 20 years later, RFC 8200 was published. It obsoletes RFC 2460
and removes that paragraph. UNH IOL Intact was updated to test for
compliance with the new standard.