MySQL  8.0.27
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
^^- command-byte
^^---- sequence-id == 0
long long sequence(UDF_INIT *initid, UDF_ARGS *args, unsigned char *, unsigned char *)
Definition: udf_example.cc:567
command
Definition: version_token.cc:279

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