On a mtu=1500 interface, such as "ifconfig lo0 mtu 1500":
before change:
command `iperf3 -c localhost4 -n1 -V` over IPv4 reports: TCP MSS: 1460 (default)
command `iperf3 -c localhost6 -n1 -V` over IPv6 reports: TCP MSS: 1440 (default)
after change:
command `iperf3 -c localhost4 -n1 -V` over IPv4 reports: TCP MSS: 1448 (default)
command `iperf3 -c localhost6 -n1 -V` over IPv6 reports: TCP MSS: 1428 (default)