Modbus TCP/IP is a very, very simple protocol. It does not require an explicit, app-level connection - only a socket level connection. This means you can send it a message and it responds to that message and that is really all you have to worry about. Of course, if the connection is dropped you have to reconnect but that is the same with every socket.
There is also quite a bit of sample code available, some of which is here too. Just search the articles for the keyword "modbus" and here is what it finds : Search