Thanks to Birslip & ElHobito, I've successfully developped a really really really simple chat between 2 NDS. First, connection settings are retrieved from the firmware. So, WFC data must be correct. Then, used port is 8888, so you would have to route this port to your NDS? on your router config. Then, you must know the IP of your counterpart. That's a lot of bad things, I know.
To use the chat, first enter the dest IP with the keyboard, then press enter. Then you can start to send messages. The chat just displays the last received message, and the last sent. This chat won't be improved !!! It's really just a demo.