UDP Socket with Acknowlegment

Hi everybody, my problem is with UDP Socket , I have two methods broadcastDatagram(send message to server) and ProcessPendingDatagram (receive message from server) the problem is that the server must send an acknowlegment when he receive a message sent by a sender. I send message in a loop instruction(for) because I have so many message to be sent. Server can’t respond to all my messages, he send me an acknowlegment for only the last one. I must prevent Sender from sending a message until he got acknowlegement from server. I tried Sleep(ms) but it seems to block receiving message as well.