Explicit Congestion Notification (ECN, RFC 3168) has been around for quite a while now, but there are still lots of devices and hosts out there that don’t support it properly. For instance, http://www.npr.org, and apparently all HP’s ILO and ILO2 baseboard management systems.
If you’re on a GNU/Linux system, just try it:
echo 0×1 > /proc/sys/net/ipv4/tcp_ecn
Now you can’t go to NPR’s website.
echo 0×0 > /proc/sys/net/ipv4/tcp_ecn
And now you can. If you compile your own kernels, you might have noticed that ECN is on by default these days. I can’t seem to find the configuration option to disable it anymore in the latest 2.6.20rc kernels, but the proc interface is still there.
I’ve left a message on NPR’s tech support page, let’s see if they fix this. As for HP’s ILO’s, maybe I can ask an acquaintance to forward this bug report to the right group/person…