ng the MP_FASTCLOSE option, containing the key of host B (for example the mobile) declared during the establishment of the MPTCP connection. For all TCP connections, host A sends the RST flag. MPTCP Aggregation On receipt of the MP_FASTCLOSE option containing the valid key, host B responds on the same TCP connection with the RST flag and closes the other TCP connections. 12.3.5. Adding and removing an address An MPTCP connection is initially configured between the A1 pair (address/port for LTE access) of host A (the mobile) and the B1 pair (address/port) of host B (the proxy server). Host A can start an additional TCP connection from its A2 pair (Wi-Fi access) by sending the SYN flag and the MP_JOIN option. At the end of the first TCP connection, host A uses the ADD_ADDR option, informing the recipient of the A2 pair (address/port). Each IP address is associated with a unique address identifier (address ID) that can be used for address removal or for the MP_JOIN option. Due to the proliferation of NAT devices, it is likely that a host might attempt to publish private IP addresses. The MP_JOIN procedure for creating a new TCP connection provides mechanisms to minimize security risks. Similarly, host B can use the ADD_ADDR option to inform host A of the availability of the B2 pair (address/port). From the point of view of host A (the mobile), the following four TCP connections can thus be opened: address/port A1 and address/port B1, on LTE access; address/port A1 and address/port B2, on LTE access; address/port A2 and address/port B1, on Wi-Fi access; address/port A2 and address/port B2, on Wi-Fi access. If during an MPTCP connection, a previously advertised address becomes invalid, then the affected host must advertise it in the REMOVE_ADDR option SO that the other end can delete the TCP connections bound to that address. Wi-Fi Integration to the 4G Mobile Network 12.3.6. Return to the TCP connection At the beginning of an MPTCP connection, it is important to ensure that the path is fully compatible with the MPTCP