MTP Packet format.

    0           7 8           15 16         23 24         31
   ----------------------------------------------------------    -----
   |  protocol    |    packet   |    type     |    client   |      |
   |  version     |    type     |    modifier |    channel  |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              source connection identifier              |      |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              destination connection identifier         |
   ---------------------------------------------------------- transport
   |                                                        |    header
   |              message acceptance criteria               |
   ----------------------------------------------------------      |
   |                                                        |      |
   |              heartbeat                                 |      |
   ----------------------------------------------------------      |
   |                            |                           |      |
   |        window              |        retention          |      |
   ----------------------------------------------------------    -----
   |                                                        |      |
   |                                                        |      |
   |                                                        |      |
   |                   (data content and format             |
   |                   dependent on packet type             |    data
   |                   and modifier)                        |    fields
   |                                                        |
   |                                                        |      |
   |                                                        |      |
   |                                                        |      |
   ----------------------------------------------------------    -----