net.inet.tcp.ecn.enable is a setting allow to incoming TCP connections to negotiate ECN while outgoing connections don't request ECN. Always requesting ECN on outgoing connections can be troublesome because of the number of broken middle-boxes that block ECN connections (though we do have a fallback to turning off ECN if the initial connection request fails).
isi_for_array -s sysctl net.inet.tcp.ecn.enable=0
verify it worked: isi_for_array -s sysctl net.inet.tcp.ecn.enabled
backout:
isi_for_array -s sysctl net.inet.tcp.ecn.enable=1