Background

Path MTU (PMTU) discovery is mandatory in IPv6.
Routers always discard too big packets.

rfc2292bis(-02) socket options and ancillary data items
IPV6_USE_MIN_MTU
requires the kernel to send packets at the minimum MTU.
for applications that do not want packet drop during the PMTU discovery phase.
e.g. DNS, IKE, DHCP(?)
IPV6_RECVPATHMTU
requires the kernel to tell applications an appropriate MTU.
for applications that can divide a single transaction into multiple packets.
in order to avoid fragmentation.