[lwip-users] at ZeroWindowProbe the response is not ZeroWindowProbeAck

[Thomas Richter (TCD - DE/Dresden)]

Hi,

during some tests with lwIP 1.4.0 I could provoke that the WindowSize is
0. This tells the sender to stop sending.
The next sequence by the sender is a TCP_ZeroWindowProbe sequence (some
infos to TCP Analyze Sequence Numbers with Wireshark:
http://wiki.wireshark.org/TCP_Analyze_Sequence_Numbers). But lwIP
responses not with a sequence TCP_ZeroWindowProbeAck, it sends a new
TCP_ZeroWindow sequence.

With another device with another TCP/IP stack the following I can observe:
time 1:    192.168.1.1 ==> 192.168.1.100
seq.no.:        6150
ack.no.:        2532
next seq.no.:   6151

time 2:    192.168.1.1 <== 192.168.1.100
seq.no.:                    2532           (is the expected sequence)
ack.no.:                    6150           (is the expected ACK)

This will be continued until the receiver (192.168.1.100) can handle new
data.

With lwIP the following I can observe:
time 1:    192.168.1.1 ==> 192.168.1.100
seq.no.:         8351
ack.no.:        17202
next seq.no.:    8352

time 2:    192.168.1.1 <== 192.168.1.100
seq.no.:                    17202          (is the expected sequence)
ack.no.:                     8352          (is not the ACK to 8351)

The increase of acknowledge number by 1 is curios.

Why lwIP reacts different?
What can I do that the acknowledge number not increases?

Thanks for any help.

Best regards
Thomas