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In a company network I noticed that all examined Windows Server 2012 machines had ECN turned on:

> netsh interface tcp show global

TCP Global Parameters


ECN Capability                      : enabled

Is it turned on by default on Windows Server 2012? On all older Windows systems I saw it turned off by default which is confirmed in the Wikipedia article.1 I could not find the information about Windows Server 2012 anywhere.

Though the ECN RFC was published in 2001 there are still many devices and internet services which do not work correctly with ECN. Such a change in Windows would be somewhat courageous.

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up vote 4 down vote accepted

Yes, ECN (which is used by DCTCP) is enabled by default in Server 2012.

Oh, and here's the article about it, exclusively for Server 2012. It's pretty neat:

Here's a very nice PDF on the subject from Microsoft Research:

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Answer is useful, but perhaps a misleading: ECN (2001) predates DCTCP (2010). Its a flag in the IP packet which provides a means for devices on a route to say "I'm busy - slow down before I stop dropping packets". DCTCP is explicitly intended for use on small (LAN sized) busy networks where the ratio of ECN flagged / non-ECN flagged packets informs nodes about available network capacity. i.e. ECN behaves differently for DCTCP compared with non-DCTCP devices. – symcbean Sep 3 '15 at 11:58

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