Scott Bradner writes: > it's not "reserved" -- the ECN bits are assigned by RFC 3168 In that case, the only issue is one of backward compatibility. Some sites may not be conformant with recent changes to the TCP/IP standards. The smart TCP/IP implementation allows for this. Which in turn means that a host implementing ECN must be prepared to deal with other hosts that don't--and forcing the ECN bits upon the latter is not the way to do this.