Controls Explicit Congestion Notification (ECN) support.
If this parameter is set to 0, TCP does not negotiate with a peer that supports the ECN mechanism.
If this parameter is set to 1 when initiating a connection, TCP does not tell a peer that it supports ECN mechanism.
However, TCP tells a peer that it supports ECN mechanism when accepting a new incoming connection request if the peer indicates that it supports ECN mechanism in the SYN segment.
If this parameter is set to 2, in addition to negotiating with a peer on the ECN mechanism when accepting connections, TCP indicates in the outgoing SYN segment that it supports the ECN mechanism when TCP makes active outgoing connections.
Never, Passive, or Active
ECN can help TCP better handle congestion control. However, there are existing TCP implementations, firewalls, NATs, and other network devices that are confused by this mechanism. These devices do not comply to the IETF standard.
Because of these devices, the default value of this parameter is set to 1. In rare cases, passive enabling can still cause problems. Set the parameter to 0 only if absolutely necessary.