This isn't technically a bug in any Linux software, but the interoperability problems are going to confuse the hell out of users anyway. Basically, 2.6 kernels have TCP ECN enabled by default. Unfortunately, everybody else craps themselves when they see packets with the ECN bit set, resulting in "myserious web sites that I can't connect to"-type problems. So, until the rest of the networking world is dragged kicking and screaming into, well, 1999 or so, /etc/sysctl.conf should have "net.ipv4.tcp_ecn = 0" in it.
Created attachment 98345 [details] A handy-dandy patch to the three flavors of sysctl.conf New! and Improved! Bug: Now with Minimal Effort Required!
Nothing gets fixed until something breaks. So it's good to break things in order to get things fixed. Minor break is not fixed, major is. Better break more when you breaking things. Keep the default.
That doesn't help upgrades though. Arjan: what's the plan on the kernel-level default?
*** Bug 117745 has been marked as a duplicate of this bug. ***
I don't think this is going to be changed in sysctl.conf. I believe the kernel defaults to off these days.