Assume I have two networks, A and B, on sites either side of the globe. Both have native IPv6 and are using (hypothetically) just IPv6. IPv4 no longer exists. They can both access the internet natively as IPv6 and use IPv6 hosts. Their IPv6 addresses are all globally routable. So, notwithstanding firewall rules, each host in network A can reach each host in network B and vice versa.
How would I set up IPSec to secure traffic between those two networks?
In an IPv4 world I would set up an IPSec tunnel between two firewalls at each site or use GRE tunnelling or similar.
But how would I do this in an all-IPv6 world? Is there something intrinsic in IPv6 that makes this easier? Ideally I want to configure this just on the firewalls/routers at each site. I don't want to have to set up transport mode IPSec on each and every host within each network.
FWIW, I use OpenBSD for my routing/firewalling/ipsec needs, so any specifics with that would be helpful, but a general answer to how IPv6 / IPSec deal with this scenario would also be good.