I read that IPSec is mandatory for IPv6 implementations. Does this mean that it should be handled by the OS and that IPSec configuration should be mandatory for IPv6 to work? If so, why is this not the case? I have a working 6-in-4 tunnel set up between two Ubuntu machines, and IPv6 seems to be perfectly happy without IPSec configured/working.
Also, I read that for Ubuntu, IPSec for IPv6 can be configured using external software (racoon?). Why is IPSec not mandatory to use with Ubuntu's IPv6 implementation?