[Mailman-Users] Restrict Mass Subscription

[enyc at sheer.us]

> > The easiest way to demonstrate is turn on ECN yourself on a linux-2.4 
> >  or linux-2.6 host. 
> > Just "echo 1 > /proc/sys/net/ipv4/tcp_ecn" as root.... 
> > Then you will find list.org doesn't work, until you... 
> >  "echo 0 > /proc/sys/net/ipv4/tcp_ecn" as root....
> I don't know what the tcp_ecn stuff does, but I can repro that easily. 
> Just followed your steps and then telnet www.list.org 25. No go. At the 
> same time I could still reach other servers.
Well, indeed....

This is because "list.org" server has a broken firewall of some form.
Major firewall manafacturers had code that exhibited this problem and
  have fixed it basically.
But, this problem is "all-but" gone away. I.e. much better than it
  used to be, but some places are still broken!  [like "list.org"].

What ECN [Explicit Congestion Notification] does, is allow intermediate
  internet routers to 'notify' a TCP connection of congestion BEFORE a
  state of high-latency/packet_loss occurs.
This is generally a good-thing-for-everyone etc.

In order for this to work, the TCP-client needs to USE TCP_ECN
  (by setting the relevant flags in TCP header initially), and
  the TCP-server needs to 'support' ECN, and the congested router
  needs to support marking TCP packets when busy....

> Kai
--"enyc" <enyc at sheer.us>