set to ONE and the ACK bit set to ONE. The server indicates the sequence number Y. The server validates the received segment by indicating the next expected byte X + 1; - the client responds with the SYN bit set to ONE and the ACK bit set to ONE. The client indicates the sequence number X and validates the received segment by indicating the next expected byte Y + 1. An endpoint can close the connection using either the FIN bit or the RST bit. The FIN bit allows the connection to be closed when the data to be transferred have actually been received by the recipient. The RST bit is used to close the session abruptly. MPTCP Aggregation 12.2.3. Data transfer The TCP has the Sequence Number, Acknowledgment Number and Checksum fields to perform error-free data transfer. At the time a segment is transmitted, a retransmission timeout (RTO) is initiated. If the segment is acknowledged before the expiration of the timer, then the latter is stopped. On the other hand, if the timer expires before the acknowledgment arrives, then the segment is retransmitted and the timer is activated again. When a segment is retransmitted, the retransmission timer is not updated, its value being doubled instead. In a standard configuration, retransmission resumes from the segment for which the timer expired, since the sender has no information as to whether or not the next segments were actually received, unless the SACK option is used. The TCP continuously adjusts the value of this retransmission timer. It manages an RTT (Round Trip Time) variable, which is the current estimate of the time elapsed between the transmission of a segment and the reception of the acknowledgment. 12.2.4. Slow Start and Congestion Avoidance mechanisms The source takes advantage of the Slow Start and Congestion Avoidance mechanisms to control the amount of transmitted data. These mechanisms are implemented using two variables: - the congestion window (cwnd) indicates the amount of bytes the source can transmit before it receives an acknowledgment via the Acknowled