Number of datagrams that were unrecovered (LBM_MSG_UNRECOVERABLE_LOSS message was delivered) because a retransmission was not successfully received within the NAK generation interval.

This is controlled by the transport's nak_generation_interval, with a default of 10000ms. This field also includes messages declared unrecoverable due to a burst loss event (LBM_MSG_UNRECOVERABLE_LOSS_BURST). This method returns a value from the first set of statistics encapsulated within this object and would normally be used when the object has been created using a specific source to instantiate the class.