Hyperoptic: IPv6 and Out-of-Order Packets

Summary

IPv6 Connectivity It's probably about time that I figured out how to enable IPv6 on my RouterPi and network! At first, configuring dhcpcd was fairly straightforward and IPv6 connectivity worked almost right away. However, it later became intermittent after rebooting the router and checking that everything was still working. For some reason my ISP's (Hyperoptic) upstream router (not the one in my home) had decided to stop responding to Router Solicitation (RS) packets sent by my router. Router Solicitations (RS) are part of the IPv6 Neighbour Discovery Protocol (NDP) and are how IPv6-enabled devices locate routers on the link, such as the default gateway. When an RS packet is transmitted, IPv6-enabled routers should respond with a Router Advertisement (RA) packet advertising their presence. Routers also transmit RAs at periodic intervals; these are called unsolicited router advertisements. While Hyperoptic's upstream router did not respond to RS packets, it did send unsolicited RA packets roughly every 15 - 30 minutes. In fact, it would send two identical RA packets at the same time, what's going on there? This meant that after re-plugging the WAN cable or restarting the router, it would: Successfully obtain a DHCPv6 prefix delegation, ...then take up to 30 minutes before receiving an unsolicited RA, ...leaving the network with valid IPv6 addresses but no default route. This resulted in the network seeming slow and strange, as devices would attempt to connect to websites using IPv6 before giving up and sometimes falling back to IPv4. The same thing also happened with the official home router provided by Hyperoptic. After some experimentation I found that changing the MAC address of the WAN interface to any other valid address would trigger the ISP's upstream router into sending an unsolicited RA immediately after a new DHCPv6 prefix delegation had been assigned. This only happened once per MAC address change. I verified this by swapping between two routers - the Rout...