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 = 24  |Subtype|      (reserved)       |
  +---------------+---------------+-------+-----------------------+
  |                                                               |
  |                                                               |
  |              Sender's Truncated HMAC (160 bits)               |
  |                                                               |
  |                                                               |
  +---------------------------------------------------------------+