A copy of the Wireshark/Ethereal capture would be helpful so that others can
see everything in detail.
There is probably no need to be concerned about duplicate acks. They happen
occasionally in any TCP connection.
The repeat of the data frame is more interesting. What was the timing of that
repeated frame? It sounds like your board's original data frame wasn't
acknowledged by the PC, so it was sent again after a delay. This might indicate
a problem such as a bad checksum in the TCP or IP header of your data frame,
causing the PC to ignore most or all your data-carrying frames.
----- Original Message -----
From: shiyou yu
To: [email protected]
Sent: Tuesday, May 19, 2009 12:55 AM
Subject: [lwip-users] TCP duplicate ACK
Hello, everyone!
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.
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