Closing the IPv6 First-Packet Gap with Grand

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.

A device joins an IPv6 network, receives a Router Advertisement, configures a new IPv6 address, and immediately starts communicating with the Internet.

From the host's perspective, everything is ready. It knows the link-layer address of its default router, so it can immediately send packets towards the Internet.

However, from the router's perspective, things may look different. The router may not yet know how to reach the host's newly configured global IPv6 address.