* Packet Format
    +-------+-----+-----+-----+-----+-----------------+----------------+
    | bytes |  0  |  1  |  2  |  3  |        5        |    6 .. end    |
    +-------+-----+-----+-----+-----+-----------------+----------------+
    |       |    variable length    |  common header  |    payload     |
    +-------+-----------------------+-----------------+----------------+

    The length of the remaining part(common header + payload) is indicated by the Length Header of each packet

* Common Header
    +------+-----+-----+-----+-----+-----+-----+-----+-----+
    | bits |  0  |  1  |  2  |  3  |  4  |  5  |  6  |  7  |
    +------+-----+-----+-----+-----+-----+-----+-----+-----+
    |      |      packet type      |     shared flags      |
    +------+-----------------------+-----------------------+
** Types
    +----------+-----------+
    |   type   |   usage   |
    +----------+-----------+
    |    0     | handshake |
    +----------+-----------+
    |    1     |  forward  |
    +----------+-----------+
    |    2     | heartbeat |
    +----------+-----------+
** Shared Flags
    The usage of each bit is determined by the type of packet
* Handshake Packet
    +-------+---------------+---------------+
    | bytes |       0       |       1       |
    +-------+---------------+---------------+
    |       | common header |    version    |
    +-------+---------------+---------------+
* Forward Packet
    +------+---+---+---+---+---+---+---+-----+-----------+
    | bits | 0 | 1 | 2 | 3 | 4 | 5 | 6 |  7  |    ...    |
    +------+---+---+---+---+---+---+---+-----+-----------+
    |      |               |           | ACK |           |
    |      |  packet type  +-----------+-----+  payload  |
    |      |               |  forward flags  |           |
    +------+---------------+-----------------+-----------+

** Flags
    +------+-------------------------------------------+
    | flag |                   usage                   |
    +------+-------------------------------------------+
    | ACK  |      This packet need a ACK response      |
    +------+-------------------------------------------+

** Payload
    +-------+-----+-------+-----+-----+-----+-----+
    | bytes |  0  |  ..   |  n  | n+1 | ..  |  x  |
    +-------+-----+-------+-----+-----+-----+-----+
    |       |  Content Length   | Message Content |
    +-------+-------------------+-----------------+

    + Content length is a variable length number.
    + Message content is a list in an opaque binary format whose element is a map structure

*** Message Content map structure

#+begin_src json
  {
    id: "0006081CCFF3D48F03C10000058B0000", // unique message id
    qos: 1,
    flags: {dup: false, retain: false},
    from: "clientid",
    topic: "t/1",
    payload: "hello, world",
    timestamp: 1697786555281
  }
#+end_src

* Heartbeat Packet

    +-------+---------------+
    | bytes |       0       |
    +-------+---------------+
    |       | common header |
    +-------+---------------+