A violation regarding the specifications for the protocol identifier.

[ProtocolSpecCheck]

Hi,

I noticed a violation regarding the specifications for the protocol identifier.

In the MODBUS MESSAGING ON TCP/IP IMPLEMENTATION GUIDE V1.0b there is one specification of the protocol identifier in the Page 24 and Page 29 :

Precondition - !(Protocol Identifier == 0x0000)

Postcondition - the request should be discarded(rejected)

Page 24

Page 29

However, there is no verification of the protocol identifier."

Steps to reproduce(after installation):

1）Terminal-1 (Server: bandwidth-server-many-up)
./libmodbus/tests/bandwidth-server-many-up

2. Terminal-2 (Client: Send one invalid inputs with the protocol identifier set to 0xffff)
   b"\x00\x01\xff\xff\x00\x06\xff\x01\x00\x01\x00\x01"

3. Observer the received response (still valid with no error):
   b"\x00\x01\x00\x00\x00\x04\xff\x01\x01\x00"

Thank you for your attention.