I'd only send the following in the most basic setup:
Player starts moving bar up.
Player starts moving bar down.
Player stops moving bar.
Point is made.
That of course doesn't include a lot like setting up a game, ending a game or general communication to keep everything working as well as fancy stuff like changing speeds and alike. The idea would be that the clients and the server calculate the movement of the ball simultaniously (using exactly the same algorithm, and only input (moving stopping) and made points are being sent. You'd get practically no traffic and the game stays smooth.
//Game starts: Ball direction is sent to both players.
//Player 1 moves his bar by pressing UP: Only one single packet is sent to the server containing that the player moves his bar. Gets sent to Player 2 by the server.
//Player 1 stops his bar by releasing UP: Only one single packet is sent to the server containing that. Gets sent to Player 2 by the server.
//Ball hits Player 1s bar: No communication. Both clients know where the ball is and where it will move to, and the server knows as well.
//Player 2 dosen't do anything and therefore ball hits border. Server detects that and tells both clients that point was made. Clients may detect it as well, but only to stop ball-movement or any other visual changes.
I've never done anything like that, so it's just a basic idea. It doesn't take varying latency into account as well as lag on any of the machines which would make the movement of anything out of sync, or anything else you'd have to think of.