Re: false checksum failure reports

[Rick Jones <rick.jones2 () hp com>]

Harley Stenzel wrote:
> On Nov 6, 2007 2:03 PM, Rick Jones <rick.jones2 () hp com> wrote:
>
> > Any thoughts as to how to deal with false checksum failure reports for outbound
> > traffic being sniffed on a system with ChecKsum Offload (CKO)?  It seems that
> > linux has a flag they can set when capturing the packet that would tell us, not
> > sure what other platforms might have
>
>
> Love it.  It would be very nice to know if a packet's checksum will be
> calculated in a CKO card.

These are some of the specifics courtesy of folks over in the linux netdev 
mailing list:

> The thing to check is "TP_STATUS_CSUMNOTREADY".
>
> When using mmap(), it will be provided in the descriptor.  When using
> recvmsg() it will be provided via a PACKET_AUXDATA control message
> when enabled via the PACKET_AUXDATA socket option.

I have tried to take a quick first look at the pcap code for linux but wasn't in 
the correct frame of mind and so got lost rather quickly.

Without a corresponding change to the binary file format (as in find a spare bit 
somewhere) the change would initially be limited to "live" tracing.  Not a 
complete solution, but a step in the right direction.

Meanwhile, making certain that the docs/manpage etc call-out that tracing on a 
CKO capable system/NIC will result in false checksum failure reports for 
outbound traffic would be goodness.  I suppose I should have checked if that was 
already there before typing the previous sentence, but there you go :)

rick jones

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