On Mon, 31 Dec 2012 18:35:33 +0100
Hannes Frederic Sowa [off-list ref] wrote:

ECN could be more reliable when used with IPv6 (I don't have proofs). For
people who want to try ECN with IPv6 but still have problems connecting
to destinations because of broken IPv4 routers this switch allows one
to enable ECN just for IPv6.

Perhaps ECN could be enabled by default in future.

Signed-off-by: Hannes Frederic Sowa <redacted>

Any new or modified sysctl should be documented in:
  Documentation/networking/ip-sysctl.txt