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urtwn trouble, only v4 as TCP sender

While testing a memory leak patch from current on up-to-date netbsd-9, I
found strange behavior with a urtwn(4).

On a 2006 MacBook Pro which otherwise works well, I have been using a
urtwn(4) interface.   If I run iperf3 with streams in both directions
and with v4 and v6, I get about 20 Mb/s for three cases and about 500
kb/s for transmitting v4.

From using a homemade blaster to discard, tcpdumping, and visualizing
the TCP behavior with xplot, it looks like packets are lost.  There's a
lot of SACK and packets are retransmitted.  Sometimes that's fast, and
sometimes there are timeouts.   I see a lot of dupacks in the netstat
-s.  I'm guessing that somehow some transmitted packets just aren't
sent.  My wifi network is otherwise working fine; other hosts to/from
all have expected behavior.

I'm trying this on a RPI3 with netbsd-9 from October, with  a different
urtwn(4) device, and not seeing the issue.

So if you have a urtwn(4) device, and are in the mood to test with
iperf, I am curious if "iperf3 -c foo -4" is slow, while -6, or either
with -R is reasonable.  (Run "iperf3 -s" on foo.)

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