First, you have a bug in your code (which is present in the original version http://www.digitalbond.com/scadapedia/security-controls/scapy-modbus-extensions/), you need to add .seq
there: tcp.ack = data[TCP].seq + 17
.
As said in the comment, you could write tcp.ack = data[TCP].seq + len(data[TCP].payload)
.
Anyway it's generally useless to do the TCP stack's work for the kind of things you try to do.
I would do something like that:
from scapy import *
from modLib import *
import socket
sock = socket.socket()
sock.connect(("192.168.10.131", 502))
s = StreamSocket(sock, basecls=ModbusADU)
ans, unans = s.sr(ModbusADU()/ModbusPDU03())
ans.show()
Does this work better?