These flags, Congestion Window Reduced and Explicit Congestion Notification are used to handle congestion. To put it very simply, the ECN flag is set by the receiver, so that the sender knows that congestion is occurring. The sender sets the CWR flag in response to this so that the receiver knows that the receiver has reduced its congestion window to compensate for congestion and the sender is sending data at a slower rate. Originally, CWR and ECN were not part of the TCP header. In RFC 3168, these two additional flags were introduced by borrowing 2-bit space from the Reserved field.