code: def _read_mptcp_fail(self, bits, size):
          
          ____ = self._read_fileng(1)
          dsn_ = self._read_unpack(8)
  
          data = dict(
              subtype='MP_FAIL',
              fail=dict(
                  dsn=dsn_,
              ),
          )
  
          return data
docs: Read Fallback option.
  
          Positional arguments:
              * bits - str, 4-bit data
              * size - int, length of option
  
          Returns:
              * dict -- extracted Fallback (MP_FAIL) option
  
          Structure of MP_FAIL [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=12   |Subtype|      (reserved)      |
              +---------------+---------------+-------+----------------------+
              |                                                              |
              |                 Data Sequence Number (8 octets)              |
              |                                                              |
              +--------------------------------------------------------------+
  
              Octets      Bits        Name                    Description
                0           0     tcp.opt.kind            Kind (30)
                1           8     tcp.opt.length          Length (12)
                2          16     tcp.opt.mp.subtype      Subtype (6)
                2          23     -                       Reserved (must be zero)
                4          32     tcp.opt.mp.fail.dsn     Data Sequence Number