def _read_join_synack(self, bits, size, kind):
adid = self._read_unpack(<NUM_LIT:1>)<EOL>hmac = self._read_fileng(<NUM_LIT:8>)<EOL>srno = self._read_unpack(<NUM_LIT:4>)<EOL>data = dict(<EOL>kind=kind,<EOL>length=size + <NUM_LIT:1>,<EOL>subtype='<STR_LIT>',<EOL>join=dict(<EOL>synack=dict(<EOL>backup=True if int(bits[<NUM_LIT:3>]) else False,<EOL>addrid=adid,<EOL>hmac=hmac,<EOL>randnum=srno,<EOL>),<EOL>),<EOL>)<EOL>return data<EOL>
Read Join Connection option for Responding SYN/ACK.

        Positional arguments:
            * bits - str, 4-bit data
            * size - int, length of option
            * kind - int, 30 (Multipath TCP)

        Returns:
            * dict -- extracted Join Connection (MP_JOIN-SYN/ACK) option for Responding SYN/ACK

        Structure of MP_JOIN-SYN/ACK [RFC 6824]:
                                 1                   2                   3
             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 1
            +---------------+---------------+-------+-----+-+---------------+
            |     Kind      |  Length = 16  |Subtype|     |B|   Address ID  |
            +---------------+---------------+-------+-----+-+---------------+
            |                                                               |
            |                Sender's Truncated HMAC (64 bits)              |
            |                                                               |
            +---------------------------------------------------------------+
            |                Sender's Random Number (32 bits)               |
            +---------------------------------------------------------------+

            Octets      Bits        Name                        Description
              0           0     tcp.mp.kind                 Kind (30)
              1           8     tcp.mp.length               Length (16)
              2          16     tcp.mp.subtype              Subtype (1|SYN/ACK)
              2          20     -                           Reserved (must be zero)
              2          23     tcp.mp.join.synack.backup   Backup Path (B)
              3          24     tcp.mp.join.synack.addrid   Address ID
              4          32     tcp.mp.join.synack.hmac     Sender's Truncated HMAC
              12         96     tcp.mp.join.synack.randnum  Sender's Random Number