MySQL  8.0.15
Source Code Documentation
Protocol::SSLRequest:

SSL Connection Request Packet.

It is like Protocol::HandshakeResponse: but is truncated right before username field. If server supports CLIENT_SSL capability, client can send this packet to request a secure SSL connection. The CLIENT_SSL capability flag must be set inside the SSL Connection Request Packet.

Payload
TypeNameDescription
if capabilities & CLIENT_PROTOCOL_41 {
int<4> client_flag Capabilities Flags
int<4> max_packet_size maximum packet size
int<1> character_set client charset a_protocol_character_set, only the lower 8-bits
string[23] filler filler to the size of the handhshake response packet. All 0s.
} else {
int<2> client_flag Capabilities Flags, only the lower 16 bits
int<3> max_packet_size maximum packet size, 0xFFFFFF max
}
See also
int2store(), int3store(), int4store(), mysql_fill_packet_header()