comment #1:

> Have you tried TCP_CHECKSUM_ON_COPY_SANITY_CHECK? That should run TCP_CHECKSUM_ON_COPY and traditional checksum in parallel and report errors at the same time. If you put a breakpoint on the error-reporting lines or insert code to dump state there, you might be able to debug what's wrong.

Yes I have,

I have even went as far as to write a sanity check for each call to my own LWIP_CHKSUM (compared to algorithm 2), and LWIP_CHKSUM_COPY (compared to algorithm 2, and memcmp).

I have done breakpoints, but the issue seems random. I did have a bug in my netif driver which proved unrelated. There just seems to be a random bit error, where the normal checksum routine doesn't have it, nor my own LWIP_CHKSUM.

When using TCP_CHECKSUM_ON_COPY_SANITY_CHECK the checksum that was incorrect is corrected in the packet itself, and not reported via Wireshark, but only via the DEBUGF utility.

I have attached a capture, as well as a log. To find an example of the error, ctrl+F for "tcp_output_segment: calculated checksum is FF56 instead of FF55"

Every single one of the checksum errors is off by 1. I will dedicate more time to debugging in a week or two.

Hello,

I am having problems after developing a LWIP_CHKSUM_COPY() assembly routine. Implementations of a superior MEMCPY to the standard library, as well as a faster LWIP_CHKSUM by utilizing ADDC instructions were both successful. Unfortunately this improvement has caused me much frustration.

Naturally I assumed my assembly code was erroneous so I spent time I disabled all of these macros and found the error to still exist. Considering the bug was consistent (checksum off by 1 in MSB of checksum) I attempted to squish it. Unfortunately after a few days I was unsuccessful.

I have done:
 - Sanity checks of my MEMCPY(), LWIP_CHKSUM_COPY() and LWIP_CHKSUM() - data copied correctly and checksum correct
 - Turned off these features
 - Turned off my asm byteswapping functions
 - Turned off compiler optimizations (only constant folding)

Chasing the bug it seems random, and I cannot define the behavior that reproduces it, however, it is very common. I have attached logs of debug messages as well as a netcap with the sanity check override disabled (commenting line 1595 in tcp_out.c).

Target: Infineon XC22xxM with ASIX Ax88796b netif

Thanks & regards,
-Victor