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)       |
  +---------------+---------------+-------+-----------------------+
  |                      Option Receiver's Key                    |
  |                            (64 bits)                          |
  |                                                               |
  +---------------------------------------------------------------+