The layered protocol concept is finally clicking. I read some introductory stuff on it years ago, now its coming back to me :)

Step 4 of earlier post gives me an issue

4. if on windows 10, go to control panel>>systems and security>>windows firewall

click on "Advanced settings" (on the left)

on the panel that shows up, scroll down and click on "inbound rules"

on the right of the main panel (not the panel itself), click on "New rules"

another panel shows up. Here the default check is "program".

click on port. And start to enter details as it is on your "port-forwarding" details. click "next" ... and after the last entry you click "finish"

One option ask if i want "Allow connection if secure" (IPsec). (see images) But if I check yes it places a lock on the "new inbound connection rule", and the connection doesn't work anymore. It only works if I select non secure connection. I see a customise dialog popup, i reckon i should configure here and write my own algorithm. But there is no information how to connect such authentication algorithm back to Windows Firewall with Advanced security so as to unlock the new inbound rule

So for now i use non secure connection, but in future I would (obviously) like to use "Allow connection if secure" option. So how do I connect my security algorithm to "inbound rule" so as to unlock and connect for connections that pass the security checks? All these would be done programmatically

Or how do I programmatically make Windows Firewall with Advanced security allow authenticated connections through?

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Yeah, "secure" in this control panel means something totally different than what you want. It basically means that you only allow connections that run inside a "virtual" network that you've set up for your enterprise business, and nobody outside the "virtual" network will be able to connect to it. That's not what you want for a game on the internet :-)