Help Newbie: how to make a handshake between two USRPs in Simulink??

Dear experts, I have been given two USRP devices connected to a single PCs with daughter boards XCVR2450 Half Duplex. My Initial tasks are to

1. send something from sender A and receive it on side B (for that I tested the Simulink built in QPSK example and it works)

2. Send something from Side A for which Side B gives a feedback as ACK or NACK. if it is an ACK, Sender A starts transmitting data or if it is a NACK, Sender A resend the Sync data.

I have the Idea of TCP handshake protocol but as a Simulink beginner I have no Idea how to implement it here using usrp devices and simulink. the Installed package contains seperate SDRtx and SDRrx but no transceiver. it would be nice If some one could be able to help me Initiate these steps.

Best Regards Usman Ali