Trying to understand the session manager packet payload format

Hi,

I'm trying to understand the client message payload format:

***client***
header_length: 116
message_type: input_stream_data
schema_version: 1
create_date: 1734428130469
sequence_number: 4
flags: 0
message_id: 9378123e-042d-14f1-9f76-c975fdeb4a5b
payload_digest: b'v\xf2\xf2\xf5\xcf\x94\x89C\xa4\xbd\xb2\xf8\x08\xb0O\x1c\xa8\x89\xb5\x7f\xe0\x15=.\x9f\xab\xa0\x9bS\xfd;\x1a'
payload_type: 1
payload_length: 11
payload: b'\x01\x02\x03\x00\x05\x00\x00\x00\x05\x01\x00'

The last 3 bytes are my data - I see the payload often have first 8 bytes of data, which I don't understand. Anyone can tell me what the first 8 bytes mean?

Thanks.