Number of individual NAKs sent by the receiver.

This may differ from the tally of lost datagrams (below) due to reasons such as

• Other receiver transports may have already sent a NAK for the same lost datagram, resulting in a retransmitted lost datagram (or an NCF) to arrive at this receiver transport before it has a chance to issue a NAK, or
• During periods of heavy loss, receiver transports may be forced to issue multiple NAKs per lost datagram (controlled by configuration options transport_lbtrm_nak_generation_interval and transport_lbtrm_nak_backoff_interval) until either the retransmission is received or the datagram is declared unrecovered (which may ultimately lead to UM delivering an LBM_MSG_UNRECOVERABLE_LOSS notification to the receiver application).

This method returns a value from the first set of statistics encapsulated within this object.