I use lwip-1.3.0 and freemodbus to implement a modbus/TCP application. I use modbuspoll to test it and I found my board always closed after several query for data. Capture frame data using ethereal, I found that after PC send a query frame, the board will send a ACK frame and then a response data frame, but then a TCP duplicate ACK frame. After that, the board send another response data frame same as the last data frame, and then a ACK frame. I'm new to TCP/IP and I don't know what's the problem.