This read-write Gauge reports the maximum length ever obtained by this port's inbound packet queue, in packets, based on periodic polling rather than continuous recalculation, so it is an approximation. An administrator uses this high-water mark to judge whether the port's receive buffering is adequate for its traffic bursts. For example, the admin sees sifInQLen consistently near its configured maximum and concludes the port needs more receive buffer headroom to avoid drops during bursts.