ack retransmission
 

during a tcp communication, when will ack be retransmited?
If never, could some one give me a hint to implement that?

There shouldn't be a need to retransmit an ACK ever. The sender will timeout on the missing ACK and retransmit.

but the sender RTO(64s) make the recver wait too long, that's why I want the recver re-send several dup-acks to trigger the retransmission.

If you send multiple ACKs, you won't get a retransmission, you'll get the next transmission.

according to rfc793:
----------snd.una---------snd.max---------
if the sender get enough dup-ack(snd.una), it will thought that packet been lost and resend it, doesn't it?

Good luck with that. You'll end up breaking state and resetting to return to a synchronized state. Your transmission will essentially come to a stop briefly. Try it with a sniffer running to see the result, but remember that all TCP stacks are not created equal.

Maybe you want to use a UDP-based protocol for your transmission?