Okay. It is up to the developers who are working on the client to get it to synchronize with your server. Kuphryn

I solved the problem, but I got a new problem. ;) I'm able to send the return packet and I'm able to call WSARecv() on the socket, but I always receive the data, I sent. ;) What's this???

I don't know how I solved the problem. I didn't use malloc and I had no static variables in the class. It was a quite weird problem.

Hi, well, you have two possibilities - first is to close the socket before you use the new AcceptEx using the closesocket(). But then you simply destroyed everything and there's no reuse. Second is to re-use the socket - for this you can use this trick: Instead of calling closesocket() you can use this one: TransmitFile( sock, NULL, 0, 0, NULL, NULL, TF_DISCONNECT | TF_REUSE_SOCKET ) This will close the socket, but makes it ready for AcceptEx reuse. Works only on NT, but as you wrote you use IOCP, so should not be a problem here ;)