Answer 1

Just look into existing chat protocols and implementations of briar and jami. That should give a good idea how it can be done and if you want to do one.

Answer 2

Nasif Rahman was right. since you gonna need a server to process and send the data. so where the data will come and from without internet conncetion?

Answer 3

My first program was an intraoffice chat program that worked without internet. It simply opened the receiver's console as a file and wrote whatever you typed out to the file channel. That is how it worked from both ends. I wrote it in DEC BASIC-11. I was 10 years old. Internet did not exist yet.

Answer 4

If you do not use a wireless connection it will not work.