There is a subtle asymmetry in IPv6 Neighbour Discovery: a host knows how to reach its router before the router necessarily knows how to reach the host. GRAND fixes that asymmetry by making the host proactively advertise its address, and I implemented that mechanism in FreeBSD.
Read more here: https://labs.ripe.net/author/pouria/closing-the-ipv6-first-packet-gap-with-grand/
