Explicit Congestion Notification

[Mark]

Hello,

Someone on the freebsd-questions list suggested I ask my question on the
hackers list. So here goes. If this is the wrong place, then you have my
apologies.

I am looking for a way to enable Explicit Congestion Notification for TCP on
FreeBSD 4.7R (IPv4). And google is really not very helpful in that regard.
Nor the FreeBSD site, for that matter. I found "option TCP_ECN" for the
kernel. Does that option alone suffice? I still rather hope this can be
controlled with a sysctl maneouvre. But if a kernel recompile is required, I
rather know what to do exactly.

Thank you for your time.

- Mark