Mike Hicks

Not Explicit Enough

Okay, so there's this feature of TCP/IP called Explicit Congestion Notification (ECN). It's been part of the standard for a few years now. The problem is, there are still a lot of routers and servers out there (mostly just routers, though, if people know what's good for them) that don't understand it. If you turn on ECN, suddenly some sites on the Internet become inaccessible. This isn't supposed to happen.

TCP ECN uses a field in the TCP header that had previously been defined as "reserved". This means that you should ignore the field. However, many programmers thought this meant the field should always be set to zero. So, many routers just discard ECN-enabled packets.

I was having trouble seeing any pictures hosted from photoisland.com on Kari's LiveJournal page because of this, so I had to execute the command (on Linux):

echo 0 > /proc/sys/net/ipv4/tcp_ecn