As far as I understand, the replacement for IPv4 broadcastas are IPv6 multicasts to ff02::1 . On my linux box, sending an UDP multicast to this group results in other, connected hosts receiving a packet with link-local sender address like fe80::5a38:a519:8ff4:1f1f – although the sending interface has a global unicast address. How can a client a resolv the global unicast address of the sender from the link-local address encoded in the packet?
IPv6 link-local and global unicast addresses have no real relationship to each other. In fact, you could have multiple IPv6 global unicast addresses on an interface, and then which one would you want to resolve?
You may also not have any IPv6 global unicast addresses on an interface; only a link-local address, or one or more ULA addresses along with the link-local address. You could also have a combination of global unicast and ULA addresses (including single or multiple addresses of each type).