set up a dummynet pipe to inject IP CE marks:
ipfw pipe 1 config bw 10Mbit/s delay 10 codel target 5 interval 10 ecn
ipfw add 100 pipe 1 ip from any to any out xmit vmx0
start a iperf3 / uperf test.
sample snd_cwnd using convinient tools (dtrace, siftr, bblog) - validate that on the 2nd and subsequent CWR (CONGRECOVERY) the cwnd is shrinking when PRR is enabled, until ssthresh is met (or cwnd is smaller then ssthresh). Also validate that cwnd / ssthresh are "sane" values and not larger then the receive window, or unexpectely large...