I went to enable ECN for TCP on a 2.0_RC4 box, and didn't find the
sysctl that I remember using on FreeBSD in 1999.  From a quick grep of
the code, I conclude that there is no support for ECN in tcp, but that
the ALTQ code will do ECN marking.

Can anyone point me to an TCP/ECN implementation for 2.0, or point me
in the right direction if it is there?

Also, I wonder if there is some objection to adding ECN support, or if
it's just that no one has gotten around to it.