Solution for 2-way communcation between public network server and servers behind NATs

Question

I have below scenario and would like to find a solution for it:

Private Server A is behind NAT, it has no fixed public IP address.

Public Server B has public IP.

Public Server B will need to send messages to private server A consequently upon business requirements.

I am wonder whether there is any Java solution for this kind of communication?

Since private server A has no fixed IP address, it will has to first has a way to tell public server B its IP/Port, etc, or start a connection like websocket first. Then keep the connection long alive, then public server B can send data back to server A.

This scenario is similar but not exactly the same as UDP hole punching. Since I am new to Java, I am wondering whether there is any existing Java lib for such case?

The restrictions are:

1. We are not allowed to configure the NAT network, which means port forwarding are not applicable.

2. There will be many NATs. Inside each NAT there will be a server(Or a cluster) communicating to the public server. So websocket solution may not a best solution as it will keep the public server maintaining many connections in memory.

Thanks

Answer 1

The NAT configuration basically forces you to make connections from server A, which is located behind a NAT, to server B. If you do not want to keep a connection open, then server A needs to poll periodically for information, then disconnect. This obviously introduces latency; server A is disconnected for most of the period and only receives updates after it connects.

I would be surprised if you have so many servers that you cannot keep a socket connection open.

Java is a side issue. Any kind of socket or websocket you would open in Java has direct analogues in other languages.