logic improvements and docs

Author: robamu

src/cfdppy/handler/dest.py

@@ -567,8 +567,6 @@ def __non_idle_fsm(self, packet: GenericPduPacket | None) -> None:
         if self.states.step == TransactionStep.WAITING_FOR_MISSING_DATA:
             if packet is not None and pdu_holder.pdu_type == PduType.FILE_DATA:
                 self._handle_fd_pdu(pdu_holder.to_file_data_pdu())
-                if self._params.acked_params.deferred_lost_segment_detection_active:
-                    self._reset_nak_activity_parameters()
             if self._params.acked_params.deferred_lost_segment_detection_active:
                 self._deferred_lost_segment_handling()
         if self.states.step == TransactionStep.TRANSFER_COMPLETION:
@@ -779,10 +777,12 @@ def _handle_waiting_for_missing_metadata(self, packet_holder: PduHolder) -> None
             self._handle_file_data_without_previous_metadata(packet_holder.to_file_data_pdu())
         elif packet_holder.pdu_directive_type == DirectiveType.METADATA_PDU:
             self._handle_metadata_packet(packet_holder.to_metadata_pdu())
+            # Reception of missing segments resets the NAK activity parameters. See CFDP 4.6.4.7.
             if self._params.acked_params.deferred_lost_segment_detection_active:
                 self._reset_nak_activity_parameters()
         elif packet_holder.pdu_directive_type == DirectiveType.EOF_PDU:  # type: ignore
             self._handle_eof_without_previous_metadata(packet_holder.to_eof_pdu())
+            # Reception of missing segments resets the NAK activity parameters. See CFDP 4.6.4.7.
             if self._params.acked_params.deferred_lost_segment_detection_active:
                 self._reset_nak_activity_parameters()
 
@@ -908,9 +908,13 @@ def _lost_segment_handling(self, offset: int, data_len: int) -> None:
             self._params.acked_params.last_end_offset = offset + data_len
         if offset + data_len <= self._params.acked_params.last_start_offset:
             # Might be a re-requested FD PDU.
-            _ = self._params.acked_params.lost_seg_tracker.remove_lost_segment(
+            removed = self._params.acked_params.lost_seg_tracker.remove_lost_segment(
                 (offset, offset + data_len)
             )
+            # Reception of missing segments resets the NAK activity parameters.
+            # See CFDP 4.6.4.7.
+            if removed and self._params.acked_params.deferred_lost_segment_detection_active:
+                self._reset_nak_activity_parameters()
 
     @staticmethod
     def _iter_segment_requests(