[lwip-users] lwip 1.3.0 Duplicate ack flood problem

[B B]

Hi list,
 
I am running lwip 1.3.0 + FreeRTOS on a lpc2468.
 
So i am running i a threaded environment. I use the netconn API. I have 2 threads which are listen on different ports. One of which is serving
a small webpage. And i have one thread sending data on regular intervals. All the connections mentioned is TCP.
 
All is running well and good, if the board is connected through a switch on our internal network.
But if connected to a cable modem router, all TCP communication is shutdown after a while.
Researching this problem gave the following results described below:
 
I have through wireshark observed dup acks send from lwip. Sometimes its just 1 or 2 dup acks, and sometimes more.
The most extreme case observed, 693 dup acks were transmitted.
 
Below is a trace of packets send/received:
 
1135 67.021489 192.168.2.110 192.168.2.106 TCP 52876 > http [SYN] Seq=0 Win=8192 Len=0 MSS=1460
1136 67.027078 192.168.2.106 192.168.2.110 TCP http > 52876 [SYN, ACK] Seq=0 Ack=1 Win=1536 Len=0 MSS=1536
1137 67.027123 192.168.2.110 192.168.2.106 TCP 52876 > http [ACK] Seq=1 Ack=1 Win=64240 Len=0
1138 67.027766 192.168.2.110 192.168.2.106 HTTP GET /index.html HTTP/1.1
1140 67.060687 192.168.2.106 192.168.2.110 TCP http > 52876 [ACK] Seq=1 Ack=571 Win=966 Len=0
1141 67.069519 192.168.2.106 192.168.2.110 TCP [TCP Dup ACK 1140#1] http > 52876 [ACK] Seq=1 Ack=571 Win=966 Len=0
1142 67.077683 192.168.2.106 192.168.2.110 TCP [TCP Dup ACK 1140#2] http > 52876 [ACK] Seq=1 Ack=571 Win=966 Len=0
...
1173 67.323976 192.168.2.106 192.168.2.110 TCP [TCP Dup ACK 1140#33] http > 52876 [ACK] Seq=1 Ack=571 Win=966 Len=0
1174 67.327842 192.168.2.106 192.168.2.110 TCP [TCP segment of a reassembled PDU]
1175 67.375113 192.168.2.106 192.168.2.110 TCP [TCP segment of a reassembled PDU]
1176 67.375172 192.168.2.110 192.168.2.106 TCP 52876 > http [ACK] Seq=571 Ack=868 Win=63373 Len=0
1177 67.376786 192.168.2.106 192.168.2.110 HTTP HTTP/1.0 200 OK  (text/html)
1178 67.376848 192.168.2.110 192.168.2.106 TCP 52876 > http [ACK] Seq=571 Ack=869 Win=63373 Len=0
1179 67.383815 192.168.2.110 192.168.2.106 TCP 52876 > http [FIN, ACK] Seq=571 Ack=869 Win=63373 Len=0
1180 67.411823 192.168.2.106 192.168.2.110 TCP http > 52876 [ACK] Seq=869 Ack=572 Win=1535 Len=0
 
has anyone experienced something similar ?
or has anyone an idea, what the cause of this might be ?
 
best regards,
Martin

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