0         4         8                16
    +---------+---------+----------------+
    |d|  ver  | flags   |    opcode      |
    +---------+--------------------------+
    |                length              |
    +------------------------------------+
    |                                    |
    .            ...  body ...           .
    .                                    .
    +-------------------------------------

d (1 bit): direction of message
           1: client to server
           0: server to client

ver(3 bit): protocol version
flags(4 bit)
opcode(1 byte)
length(2 bytes): length of message payload following the header
body (variable): message payload