[Discuss-gnuradio] How to pass time tag into usrp sink as a parameter ?[hurry...]

[Rui Zou]

Hello,
I would like to implement the process as follows at the Rx side: Record 
the time when the first sample arrived, and then wait for a certain 
interval and then send ack back to Tx. Meanwhile, finish the cfo 
calculating and compensating.
Here is my whole flow graph design. (Each means a block)

                           pkt_dtc-> cfo_calc -> file_sink
                                            cfo_calc -> cfo_cpst
                                       file_source -> cfo_cpst -> usrp_sink

where pkt_dtc, grab_time_tag and cfo_calc was done myself with C++. 
However, My question are:
1. Is it correct of my whole desgin (I want to implement them in one top 
block) or are there any easier solutions ?
2. After the block grab_time_tag, how should I pass the time information 
(directly got from the method get_tags_in_range) into the usrp_sink as a 
parameter so as to send ack at the intended time? Should I write in 
Python(but they are not the same level) or C++(but I can't rewrite a 
usrp_sink) ?

Any doubts or suggestions are highly appreciated !