TCPNET HTTP server: TCP checksum errors

Hello,

I have implemented a web server using RL-ARM. The problem I am trying to resolve is that, occasionally, the web server will 'hang' for about two seconds while in the middle of serving a HTTP response to the browser. This does not happen very frequently; 95% of the time, complete pages are served almost instantly.

Using Wireshark, I see that what's happening is that TCPNET is sometimes sending out a TCP packet (containing HTTP data) that has an incorrect checksum. Wireshark actually marks the packet as "Continuation or non-HTTP traffic", and the 'bad checksum' flag is 'true'.

About two seconds after the bad packet is issued, I can see that TCPNET is retransmits the packet. Wireshark marks it as "[TCP Retransmission]". Inspection of this retransmitted packet shows that it contains exactly the same HTTP data as the bad packet, except this time the packet is usually a little longer (perhaps just several bytes, or sometimes tens of bytes longer) and has a good checksum value.

So what's happening is that the browser ignores the packet with the bad checksum, and the 'hang' is when it awaits the retransmitted packet.

The retransmitted packet is almost always longer. It's as if the bad packet, with the wrong checksum, has somehow become slightly truncated.

This is the only issue I am experiencing; everything else seems to be running absolutely fine with our web server. It has been going through very extensive testing and I've never seen anything else that would point to data corruption.

The platform is the ST ARM9. The software in use is:

uVision V4.00
MDK-ARM V4.00
RL-ARM V4.00

The problem has been present ever since we started developing using earlier V3.x versions of uVision, MDK and RLARM. This isn't something that has been introduced with any particular release of Keil software.

In our application there are three tasks running: Main application (middle priority), a serial communications task (highest priority), and web server task. The web server is set to lowest priority. As a test, I have tried making it the highest priority task but this didn't eliminate the checksum errors. At the moment, I am in the process of disabling as much of the main application as I can, along with interrupts, etc. to see if I can determine what, if anything, in our code could be upsetting TCPNET.

In the meantime I am just curious as to whether anyone has experienced anything similar to this. It's something that I'm finding very tricky to debug.

Thanks,

Trevor.

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  • Trevor,

    I think I am seeing a very similar situation - And have been trying to narrow it down on and off for the past five months.

    I also use an ST ARM9, started seeing my problem with uVision 3.40, continued to see it with 3.50, and am still seeing it with 4.00.

    What I see is an occasional TCP packet being transmitted (as viewed by WireShark) that looks like it is being truncated and then padded to the Ethernet minimum. The padding always seems to be NULLs and invariably the TCP checksum is corrupt.

    As a consequence of this bad packet, the receiver ignores it and the TCP stack on the STR9 re-transmits it after a few seconds.

    My investigations suggested that there is a problem with the STR9_ENET.C module. I have proven (to my own satisfaction) that the packet is being sent to the module correctly and, after transmission, the packet is still valid; i.e., there is no immediately obvious memory corrution occuring. I strongly suspect there is a DMA transfer issue, but cannot think of a way of proving it.

    Producing a minimalistic example suitable for Keil support to test has been a problem for me. Four months ago I contacted Keil support, sent them the code and only in the past couple of days have been told that I was doing something wrong in the TCP callback.

    This thing I am doing wrong is not documented in any way that I can see and is (apparently) causing corruption of the TCP stack. Regardless of that, I have modified the code accordingly and still I see the problem.

    Unfortunately Keil support are not now seeing the problem.

    In desperation for a resolution, I modified the STR9_ENET.C module. So far, I have seen no corrupt packets when this updated module is used.

    If you like, I can send you my modified module for you to try - If it works for you, then maybe I/you/we can contact Keil to see if they will follow it through.

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  • Trevor,

    I think I am seeing a very similar situation - And have been trying to narrow it down on and off for the past five months.

    I also use an ST ARM9, started seeing my problem with uVision 3.40, continued to see it with 3.50, and am still seeing it with 4.00.

    What I see is an occasional TCP packet being transmitted (as viewed by WireShark) that looks like it is being truncated and then padded to the Ethernet minimum. The padding always seems to be NULLs and invariably the TCP checksum is corrupt.

    As a consequence of this bad packet, the receiver ignores it and the TCP stack on the STR9 re-transmits it after a few seconds.

    My investigations suggested that there is a problem with the STR9_ENET.C module. I have proven (to my own satisfaction) that the packet is being sent to the module correctly and, after transmission, the packet is still valid; i.e., there is no immediately obvious memory corrution occuring. I strongly suspect there is a DMA transfer issue, but cannot think of a way of proving it.

    Producing a minimalistic example suitable for Keil support to test has been a problem for me. Four months ago I contacted Keil support, sent them the code and only in the past couple of days have been told that I was doing something wrong in the TCP callback.

    This thing I am doing wrong is not documented in any way that I can see and is (apparently) causing corruption of the TCP stack. Regardless of that, I have modified the code accordingly and still I see the problem.

    Unfortunately Keil support are not now seeing the problem.

    In desperation for a resolution, I modified the STR9_ENET.C module. So far, I have seen no corrupt packets when this updated module is used.

    If you like, I can send you my modified module for you to try - If it works for you, then maybe I/you/we can contact Keil to see if they will follow it through.

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