* \param[in] rlcP RLC AM protocol instance pointer.
* \param[in] snP Sequence number of the PDU that is acknowledged.
* \param[in] free_pdu Boolean indicating that the PDU can be freed because smaller than new vtA.
* \note Depending on the state of the retransmission buffer, positive confirmation can be sent to higher layers about the receiving by the peer RLC AM instance of a particular SDU.