I doubt that "not having time to send ACKs" is the problem. If the client is
sending packets and the server is echoing them back to the client, each packet
sent by the server can include a piggyback acknowledge of the previous packet
sent by the client. In your Wireshark capture, observe that the "Ack" number
increases by 1000 in each packet sent by the server. The server will only send
a "plain ACK" packet if it has nothing else to send.
I assume that in your capture, 172.27.42.102 is your server running LWIP with
Echoraw?
Up to packet 238, everything looks normal. Packet 239 is the next data packet
from the client. Packet 240 is the odd one: the server sent a duplicate ack
instead of echoing any data. The duplicate ack repeats the "Ack" value from the
previous data packet sent by the server. The pattern subsequently repeats, with
the client retrying the same data packet and the server asking for it again.
The behaviour suggests the server had a problem such as running out of memory,
which allowed it to receive a packet from the client but it wasn't able to
store the data. It was able to transmit an empty acknowledgement, which shows
that none of the receive data has been accepted by the application (as opposed
to not being able to allocate enough memory to send the data in the echo
response).
The server is still indicating it has a receive window of 1500, which suggests
it has free memory.
I see from an earlier post that you are running LWIP 1.3.0. There were some
bugs fixed in 1.3.1 in areas relating to handling of acknowledgements, so you
might have run into one of these bugs. Is it possible for you to try updating
to 1.3.1 or CVS-head to see if the problem is still there?
I'm not familiar enough with the relevant details, but I'm sure others who can
help will also want to know settings from your lwipopts.h file relating to
memory allocation and pbufs, and it would be useful to get LWIP stats while the
problem is occurring.
----- Original Message -----
From: Francois Bouchard
To: [email protected]
Sent: Tuesday, September 29, 2009 3:27 AM
Subject: [lwip-users] Connection closes with Echoraw demo
HI all,
The server side (embedded) runs the Echoraw demo, which implements a TCP Echo
Server. On the Client Side, my PC an Echo client runs and continuously sends
packest, and measure the Round Trip Time. Client sends Another message as soon
as it receive the echo of the previous one.
That runs fine for a while, but after few time (mostly random I would say,
although it can run for a few hours) the TCP connection terminates by itself !
I think server does'nt have the time to send ACK. Should I pause client apps
once in a while to let the server send acks?
Attached is the wireshark file when the TCP connection stops.
regards,
FB
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