Earlier quoted context omitted.
Why did you choose to write your backend application using IPv6 APIs only instead of higher abstraction network libraries that support both v4 and v6?
Good question - It's not the APIs that are the issue, it's the actual IPv6 addresses in the packets on the wire that we need. All of our remote devices (typically on the order of 5 million+ for larger customers) have IPv6 addresses. We can use tunnel routers to get the traffic over IPv4 networks to the destination, but the applications themselves are addressing the devices with IPv6 addresses - which means, at least…
Medical devices of some kind? Just guessing.