This allows you to match the ECN bits of the IPv4 and TCP header. ECN is the Explicit Congestion Notification mechanism as specified in RFC3168

--ecn-tcp-cwr
    This matches if the TCP ECN CWR (Congestion Window Received) bit is set.

--ecn-tcp-ece
    This matches if the TCP ECN ECE (ECN Echo) bit is set.

--ecn-ip-ect num
    This matches a particular IPv4 ECT (ECN-Capable Transport). You have to specify a number between `0' and `3'.