Appendix A. Comparison to RFC 1191

RFC 1981 (obsoleted by this document) was based in large part on RFC 1191, which describes Path MTU Discovery for IPv4. Certain portions of RFC 1191 were not needed in RFC 1981:

router specification
Packet Too Big messages and corresponding router behavior are defined in [ICMPv6]

Don't Fragment bit
There is no DF bit in IPv6 packets

TCP MSS discussion
Selecting a value to send in the TCP MSS option is discussed in [RFC8200]

old-style messages
All Packet Too Big messages report the MTU of the constricting link

MTU plateau tables
Not needed because there are no old-style messages