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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 -----
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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