What news does Lodovico bring from Venice? How does Desdemona react? How does Othello react to her reaction?

I believe the message is from the Duke which says that Othello must return to Venice and leave Cassio in his place to govern. Desdemona says there has been a falling out between Cassio and Othello but perhaps it can be mended. Othello takes this as further proof of Desdemona's infidelity, he slaps her.