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     |Subtype| (reserved) |F|m|M|a|A|
  +---------------+---------------+-------+----------------------+
  |           Data ACK (4 or 8 octets, depending on flags)       |
  +--------------------------------------------------------------+
  |   Data Sequence Number (4 or 8 octets, depending on flags)   |
  +--------------------------------------------------------------+
  |              Subflow Sequence Number (4 octets)              |
  +-------------------------------+------------------------------+
  |  Data-Level Length (2 octets) |      Checksum (2 octets)     |
  +-------------------------------+------------------------------+