[Gllug] Barclays routers don't support ECN

[Ian Northeast]

Dylan wrote:

> Just out of interest, what is ECN...?

I'm surprised you've managed to avoid this. There was a lot of fuss
about it when 2.4 came out.

Explicit Congestion Notification. A new-ish feature in TCP/IP which
allows routers which support it to send a notification that they are
getting over utilised and suggesting that other routers find an
alternate path. It uses a bit in the IP header previously classed as
"unused". An ECN capable router will send its "I am busy" notification
only if this bit is on in a packet it is routing.

It has always been the case that if a bit is turned on in the IP header
and a router does not understand it it is supposed to ignore it. But
when ECN came out it was discovered that there are a lot of broken
routers out there which drop the packet in such a case.

ECN appeared in Linux at 2.4, and many of the early 2.4 based distros
shipped with it turned on. This led to huge problems and nowadays it is
almost always off by default, which is why we don't hear about it so
much any more.

Regards, Ian

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