gment Number field; - the receive window (rwnd) indicates the amount of bytes the receiver is ready to receive, a value indicated via the Window field. On establishing the connection, the sender initializes the congestion window (cwnd) with a value equal to two segments of maximum length (MSS) (Figure 12.3). If the segments are acknowledged before the expiration Wi-Fi Integration to the 4G Mobile Network of the timer, the size of the congestion window is doubled. Each successful transmission thus enables the size of the congestion window to be doubled. The Congestion Avoidance mechanism introduces a new variable: the congestion avoidance threshold (ssthresh) (Figure 12.3). When the congestion avoidance threshold is reached, the congestion window grows linearly by an increment corresponding to the maximum size of one segment. Size of the congestion window (in kilo byte) Timer expiration Congestion avoidance threshold Figure 12.3. Slow Start and Congestion Avoidance mechanisms In the beginning, the value of the congestion avoidance threshold is equal to half that of the receive window (rwnd). If the timer expires before receiving the acknowledgment for the transmitted data, then the congestion avoidance threshold is then set to half the current congestion window and the Slow Start mechanism restarts. 12.2.5. Fast Retransmit and Fast Recovery mechanisms The Fast Retransmit mechanism enables the sender to retransmit a segment without waiting for the timer to expire, thus avoiding the start-up of the Slow Start mechanism (Figure 12.4). MPTCP Aggregation Size of the congestion window (in kilo byte) Reception of duplicate acknowledgments Acknowledgment reception of retransmitted segments Congestion avoidance threshold Figure 12.4. Fast Retransmit and Fast Recovery mechanisms If a segment is missing at the receiver, then the latter will emit segments with duplicate acknowledgments, having the same value in the Acknowledgment Number field. At the other end, the reception of duplicate acknowledgments is used to determine t