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.