General problems with checksums (txcsum/rxcsum) on FreeBSD 8.0?

[Pyun YongHyeon]

On Fri, Jan 15, 2010 at 11:12:43AM -0800, alan bryan wrote:
> I just read a different thread about problems with checksums on vge (and nfe in the replies).
> 
> I'll just chime in here with some more information - I have a couple other message threads going about some weird high packet volumes on my new FreeBSD 8.0-Release NFS server.  I thought it might be an issue with the igb driver so I put in a new card using em instead and got the exact same behavior.  I'm currently sifting through a tcpdump in wireshark and there are all sorts of messages in there about checksums being incorrect - both TCP and UDP.  This is for communications between this client machine (FreeBSD 7.0-Release) and any of the 8.0 machines I have.  The packets going to non-8.0 machines (at least so far) appear to be fine.
> 
> I'll defer to those who know more than I about the networking code, but is there perhaps an issue in general with the checksuming and not specific to one card or driver - is that even possible?  That's now 4 different drivers all with various checksum problem reports.
> 
> I'm going to be working on this all day today (and likely over the weekend) so if I can help by supplying information please let me know what you need.
> 

If you are seeing bad checksum reported by tcpdump/wireshark for TX
frames on checksum capable controller, it's normal. bpf(4) just
sees TX frames before inserting checksum computed by hardware so
tcpdump/wireshark reports invalid checksum. You can safely ignore
that. If you want to verify whether sending host generated correct
checksum, you should capture the frame on receive side. If tcpdump/
wireshark reports bad checksummed frame on received frames it's
real bad checksummed frame.