In the current code, NACKing is not implemented.
NACKing is part of the IEEE802.15.4e standard, and should be implemented as per the description in Section 184.108.40.206 of the standard.
The desired behavior is:
When the receiver receives a packet, AND this packet cannot be put on the queue, it should sends back a NACK.
When the transmitter receives a NACK, it should use the timing information to resynchronize, but should consider the transmission attemps failed. The packet should stay in the queue for retransmission.
If the totle number of retries reaches TXRETRIES (which may be a combination of NACKs or failures), the sending component is notified through a sendDone with error code "failed".