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|Version|A|B|C|D|E|F|G|H|
  +---------------+---------------+-------+-------+---------------+
  |                   Option Sender's Key (64 bits)               |
  |                      (if option Length > 4)                   |
  |                                                               |
  +---------------------------------------------------------------+
  |                  Option Receiver's Key (64 bits)              |
  |                      (if option Length > 12)                  |
  |                                                               |
  +-------------------------------+-------------------------------+
  |  Data-Level Length (16 bits)  |  Checksum (16 bits, optional) |
  +-------------------------------+-------------------------------+