MySQL  8.0.12
Source Code Documentation
Command Phase

In the command phase, the client sends a command packet with the sequence-id [00]:

13 00 00 00 03 53 ...
01 00 00 00 01
^^---- sequence-id == 0

The first byte of the payload describes the command-type. See enum_server_command for the list of commands supported.

The commands belong to one of the following sub-protocols

See also
dispatch_command