Please note that the uni-directional TCP data would work very well since it requires the receiving side to ACK packets that are received. So packets in both directions are always going to be present with TCP. It's possible to customize a DMF to only send data packets one direction, but there will still be SYN ACK, ACK and FIN ACK packets sent from the other side. Attached is the example of downstream only from the network host, please take a look.