Note how the AccECN flags of each final 3WHS ACK adjust to the IP ECN and
inbound <SYN,ACK> ECN flags change.
```
`
sysctl -q net.inet.tcp.ecn.enable=3
`
0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 4
+.002 ... 0.052 connect(4, ..., ...) = 0
+.002 > [noecn] SEWA 0:0(0) <mss 1460,nop,wscale 6,sackOK,TS val 100 ecr 0>
+0.05 < [noecn] SW. 0:0(0) ack 1 win 32767 <mss 1460,sackOK,nop,wscale 8,eol,eol>
+.002 > [noecn] W. 1:1(0) ack 1
+0.05 < [noecn] SW. 0:0(0) ack 1 win 32767 <mss 1460,sackOK,nop,wscale 8,eol,eol>
+.002 > [noecn] W. 1:1(0) ack 1
+0.05 < [ect0] SW. 0:0(0) ack 1 win 32767 <mss 1460,sackOK,nop,wscale 8,eol,eol>
+.002 > [noecn] A. 1:1(0) ack 1
+0.05 < [ect1] SW. 0:0(0) ack 1 win 32767 <mss 1460,sackOK,nop,wscale 8,eol,eol>
+.002 > [noecn] EW. 1:1(0) ack 1
+0.05 < [ce] SW. 0:0(0) ack 1 win 32767 <mss 1460,sackOK,nop,wscale 8,eol,eol>
+.002 > [noecn] WA. 1:1(0) ack 1
+0.05 < [noecn] SE. 0:0(0) ack 1 win 32767 <mss 1460,sackOK,nop,wscale 8,eol,eol>
+.002 > [noecn] . 1:1(0) ack 1
+0.05 < [noecn] S. 0:0(0) ack 1 win 32767 <mss 1460,sackOK,nop,wscale 8,eol,eol>
+.002 > [noecn] . 1:1(0) ack 1
```