How to programmatically request port forwarding from NAT?

Question

Computer networks

There is a server application that is behind NAT (perhaps even behind several), it is necessary to accept connections from the Internet.
Google pointed to the NAT-PMP protocol and its RFC 6886 specification.
But from the specification, I didn’t really understand how to form a request, and it’s also not clear whether it is possible to request any free port or have to pick them up to success.
Maybe someone will throw links to more human-readable manuals in Russian / English?

Answer 1

Here is an article - everything is described in detail
https://habrahabr.ru/post/279969/
The main idea is that you can feel everything using WireShark
And here is an example of software implementation
https://github.com/mono/Mono.Nat