[lwip-users] lwip and/or general tcp problems

[Paul Butler]

Hello all,

 

I am working on a data acquisition system using an Analog Devices' Blackfin BF537, which has a 100Mb/s MAC and utilizes a port of lwip.  The lwip port appears to be derived from STABLE-0_6_3.  My application requires high throughput on the ethernet interface (~20Mb/s), so I have been creating very simple applications to run on the embedded processor with lwip to test the throughput and reliability of the setup.  The sample application on the BF537 simply creates, binds, and listens on a socket, and then in an infinite loop accepts a single connection and then while that connection is open sends large packets (1460 bytes) on the connection.  I have a simple LabVIEW application that receives the data, and I have also been using the Wireshark analyzer to look at the transfers.  In this configuration, I am experiencing the following that I would really appreciate some insight on:

 

1) When lwip is configured to use DHCP, it is very difficult to maintain a high throughput.  In fact, the connection very frequently times out after transferring just a few packets.  I don't see much other traffic related to having the DHCP server on the LAN, and I use a switch to isolate the transmitting device and the receiving PC.

 

2) When not using DHCP, in general the connection is more reliable.  However, there appears to be a "cold start" issue, where when the devices on the LAN (transmitter, switch, and receiving PC) are powered on for the first time the connection has trouble establishing itself.  A few packets will transfer successfully, followed by a dropped packet with no successful retransmissions over 30 seconds.

 

3) Again without DHCP, I can observe stalls in the transmitted data stream.  Normally, packets are transmitted more than once a millisecond (up to 8 or ten per millisecond), but occasionally there are periods of ~150ms where no data is transmitted.  The receive window has not closed, and there is not indication of dropped packets or retransmission in the log file.

 

4) Still without DHCP, I observe ~2s stalls.  These appear to be caused by >1 dropped packet, which results in the first dropped packet being resent by fast retransmission, and all other packets being resent by the retransmission timers.

 

Can anyone confirm that any or all of these behaviors is unexpected in a LAN environment (RTT normally <1ms)?  Although I'm new to this, it seems surprising that my little LAN with <15' CAT5 cable segments is so likely to have corrupted or lost packets.  Can anyone give me some guidance on what to expect regarding lost packets?  Are the recovery processes I've observed correct behavior?  Should only a single packet be resent usign fast retransmission?  Is there anything inherent in the stack that could cause brief pauses in the data stream?  Why does using DHCP apparently make it so difficult to establish and maintain a high-throughput connection, particularly since there doesn't seem to be any other traffic on the LAN?

 

Apologies for the multiple questions, but I needed to start somewhere, and I've already reached the limit of what the Analog Devices' support engineers can help with.  I can provide the log files from Wireshark if that would be helpful, but some are very large (tens of megabytes).  I'd also be interested if anyone can suggest other resources to further my understanding of networking and TCP/IP issues.

 

Thanks,

 

Paul Butler