he missing segment. It should be noted that the reception of duplicate acknowledgments may also be caused by network-induced desequencing. When three duplicate acknowledgments are received, the sender resends the missing segment. The Fast Recovery mechanism operates as follows: - when three duplicate acknowledgments are received, the congestion avoidance threshold (ssthresh) is set to half the current congestion window; - the congestion window (cwnd) takes on a value greater than the congestion avoidance threshold by three MSS values; - for every additional duplicate acknowledgment received, the congestion window is incremented by one MSS value; 224 Wi-Fi Integration to the 4G Mobile Network - at the first acknowledgment corresponding to the retransmitted segments, the congestion window is positioned at the congestion avoidance threshold. 12.2.6. ECN mechanism WRED (Weighed Random Early Discard) queue management is a method of giving the recipient an indication of congestion. It is based on the anticipated and random destruction of packets. The ECN (Explicit Congestion Notification) mechanism constitutes another method to provide such a congestion indication without destroying packets. It is based on the use of fields in the header of the IP and TCP to warn terminal stations about the beginning of congestion. The ECN mechanism uses two bits in the IP header (Figure 12.5). These two bits complement the six bits in the DSCP (DiffServ Code Point) field. Version DiffServ ECN Total Length Figure 12.5. ECN field in IP header When these two bits equal 10 or 01, they indicate that the source and the destination are able to treat the ECN mechanism. These two bits are called ECT (ECN-Capable Transport, ECT(0) and ECT(1) respectively). Both endpoints are made aware of each other's ability to treat the ECN mechanism during the TCP connection procedure (Table 12.1). ECN field Designation Non-ECT ECT(1) ECT(0) Table 12.1. ECN field in IP header MPTCP Aggregation When these two bits equal 00, they indicate that the source and t