Enable or disable ECN (Explicit Congestion Notification) on Windows

The TCP protocol can detect network congestion with different mechanisms:

• packets loss
• timeouts
• duplicate acknowledgments 

In order to avoid this behavior on a saturated link, TCP ECN  can be enable (on by default on Windows 2012 server). TCP ECN are generated by the network in order to signal to the receiver that the network component is close to drop packets. The receiver can notify the sender to slow down the traffic rate. This mechanisms can react faster than the traditional TCP timeout or DUP ack.

You can use the following command to enable it on Windows:
C:\Windows\system32>netsh int tcp set global ecncapability=enabled
Ok.
Verify:
C:\Windows\system32>netsh int tcp show global
Querying active state...

TCP Global Parameters
----------------------------------------------
Receive-Side Scaling State          : enabled
Chimney Offload State               : automatic
NetDMA State                        : enabled
Direct Cache Acess (DCA)            : disabled
Receive Window Auto-Tuning Level    : normal
Add-On Congestion Control Provider  : none
ECN Capability                      : enabled
RFC 1323 Timestamps                 : disabled
Disable:
C:\Windows\system32>netsh int tcp set global ecncapability=disabled
Ok.

Windows: find the MTU for a specific IP

The following command, give us the MTU for a specific destination:
U:\>netsh interface ipv4 show destinationcache address='IP_addresss'

• Example on a classic LAN:

U:\>netsh interface ipv4 show destinationcache address=192.168.95.216
Destination              : 192.168.95.216
Next Hop Address         : 192.168.211.254
Source                   : 192.168.211.1
Interface                : Local Area Connection
Path MTU                 : 1500
Upper-layer MTU          : 1480
RTT mean                 : 3000
RTT deviation            : 0
Path transmit speed (Bps): 0
Path receive speed (Bps) : 0
Link transmit Speed (bps): 1000000000
Link receive Speed (bps) : 1000000000

• Example with an IPsec Tunnel:

U:\>netsh interface ipv4 show destinationcache address=192.168.95.216
Destination              : 192.168.95.216
Next Hop Address         : 192.168.95.216
Source                   : 192.168.243.197
Interface                : Local Area Connection
Path MTU                 : 1400
Upper-layer MTU          : 1380
RTT mean                 : 40
RTT deviation            : 20
Path transmit speed (Bps): 0
Path receive speed (Bps) : 0
Link transmit Speed (bps): 2000000000
Link receive Speed (bps) : 2000000000