 sequence number; - each TCP connection must end with FIN or RST flag. The MPTCP uses the Kind (=30) option of the TCP header, with each MPTCP message being identified by the Subtype field (Figure 12.7). 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 Length Subtype Subtype-specific data (variable length) Figure 12.7. Format of MPTCP option Table 12.2 summarizes the different MPTCP options corresponding to the Subtype field. Value Designation (in hexadecimal) MP_CAPABLE MP_JOIN ADD_ADDR REMOVE_ADDR MP PRIO MP_FAIL MP_FASTCLOSE Table 12.2. MPTCP options MPTCP Aggregation 12.3.1. Establishment of MPTCP connection The establishment of MPTCP connection begins with an exchange SYN, SYN/ACK, ACK of the first TCP connection. Each TCP header contains the MP_CAPABLE option. This option declares that the sender is capable of rolling MPTCP over the first TCP connection (Figure 12.8). MPTCP Proxy Server Wi-Fi Access Access SYN = 1 Option : MP_CAPABLE (UE key) SYN = 1 ACK = 1 Option : MP_CAPABLE (Proxy Server key) ACK = 1 Option : MP CAPABLE (UE key, Proxy Server key) Figure 12.8. Establishment of an MPTCP connection This option is used to declare the 64-bit key that each entity (the mobile, the Proxy Server MPTCP) has generated for this MPTCP connection. This key will later authenticate the addition of new TCP connections to this MPTCP connection. This is the only time that the key will be sent clearly on the first TCP connection, with the exception of the MP_FASTCLOSE option. All future TCP connections will identify the connection using a 32-bit token. This token is a cryptographic hash of this key. 12.3.2. Adding a TCP connection When the MPTCP connection is established with the MP_CAPABLE exchange on the LTE access, a new TCP connection can be added for the Wi-Fi access. The new TCP connection starts with a normal exchange SYN, SYN/ACK, ACK. The MP_JOIN option is used to identify the new TCP connection (Figure 12.9). Wi-Fi Integration to the 4G Mobile Network MPTCP Proxy Server Wi-Fi access access SYN = 1 Op