t congested path. The congestion control algorithm applies only to the increase phase of the reception window. For this, it is necessary to couple the congestion control algorithms operating on the different TCP connections and dynamically control the overall aggressiveness of the MPTCP connection. The Slow Start, Fast Retransmit and Fast Recovery mechanisms are the same as for the standard TCP. When a retransmission over a TCP connection fails, the sender can send the data from the MPTCP connection with the same DSN sequence number to another TCP connection after a timeout. During a retransmission, the host must, however, try to retransmit the original data over the original TCP connection, in order to preserve the integrity of the SSN sequence numbers. 12.3.4. Closing an MPTCP connection When an application requests the closing of a session, it indicates that it no longer has data to send. For a TCP connection, this causes the FIN flag to be issued. For an MPTCP connection, an equivalent mechanism is required, based on the DATA_FIN indication contained in the DSS option (Figure 12.11). Wi-Fi Integration to the 4G Mobile Network MPTCP Proxy Server Wi-Fi access access Option : DSS (DATA_FIN) Closing Option : DSS (DATA ACK) a MPTCP connection Closing a TCP connection Closing a TCP connection Figure 12.11. Closing a MPTCP connection The closure of the MPTCP connection is decoupled from the closures of the TCP connections. When DATA_FIN has been acknowledged, all remaining TCP connections must be closed with exchanges of the FIN flag. The DATA FIN indication is acknowledged when all data has been transferred to the different TCP connections. The TCP connection has the means to close a connection with the RST flag abruptly. The option MP FASTCLOSE allows the same function to be performed for the MPTCP connection (Figure 12.12). MPTCP Proxy Server Wi-Fi access access Option : MP_FASTCLOSE Figure 12.12. Abrupt closure of MPTCP connection Host A (e.g. the proxy server) sends, over a TCP connection, a TCP header containi