To diagnose this, I recommend making a tcpdump of the traffic with tcpdump -w TRACE tcp and then using tcpdump2plot in graphics/xplot-devel. I am not clear if graphics/xplot-devel (or /xplot) has caught up to minor changes in NetBSD tcp These plots are not 100% obvious how to read, but I'm happy to look at them if you can provide the above trace file. Counts are useful, but this lets you see the fine-grained behavior of what got sent when. From the stats, it looks like there is loss in the data stream causing SACK. But I don't see how there would be 237850 pure acks for ~98K data packets; there should be more like 49K.
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