[lwip-devel] [bug #57452] CHECKSUM_ON_COPY leads to random bit error in TCP checksum

[Victor Brzeski]

Follow-up Comment #2, bug #57452 (project lwip):


[comment #1 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.


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