tion : MP CAPABLE (UE key) SYN = 1 ACK = 1 Option : MP_CAPABLE (Proxy Server key) Establishment ACK = 1 of TCP connection Option : MP CAPABLE (UE key, Proxy Server key) SYN = 1 Option : MP_JOIN (UE token, UE nonce) SYN = 1 ACK = 1 Option : MP JOIN (Proxy Server HMAC, Proxy Server nonce) Adding ACK = 1 a TCP connection Option : MP_JOIN (UE HMAC) ACK = 1 Figure 12.9. Adding a TCP connection The mobile transmits the SYN flag in the first TCP header whose MP_JOIN option contains a token and a random number (nonce). The token is a cryptographic hash generated from the key of the MPTCP Proxy Server. It is used by the MPTCP Proxy Server to identify the MPTCP connection. The random number (nonce) is used to calculate the HMAC authentication of the following headers: The MPTCP Proxy Server responds with the SYN/ACK flags in the TCP header whose MP_JOIN option contains HMAC (Proxy Server) authentication and a random number (nonce): HMAC (Proxy Server) = HMAC (Key, Message) Key = Proxy Server key+ mobile key Message = nonce (Proxy Server) + nonce (mobile) MPTCP Aggregation The mobile performs the third exchange with the ACK flag in the TCP header whose MP_JOIN option contains HMAC (mobile) authentication: HMAC (mobile) = HMAC (Key, Message) Key = mobile key + Proxy Server key Message = nonce (mobile) + nonce (Proxy Server) The Proxy Server MPTCP terminates the procedure with the ACK flag in the TCP header to acknowledge the TCP header received during the third exchange. Each MP_JOIN option contains an address identifier. The address identifier has only a meaning in a single connection, where it identifies the source IP address of this packet, even if this IP address has been modified by an intermediate device (e.g. by the NAT function). Hosts can specify in the MP JOIN option when configuring a TCP connection whether they want the TCP connection to be used as a normal path or as a backup path. The host may request a change in the priority of the connection via the MP_PRIO option. 12.3.3. Data transfer The DSS (Data Sequence