impossible packet length ...

[Danny Braniss]

> On Sun, 8 Feb 2009, Peter Jeremy wrote:
> 
> > On 2009-Feb-08 11:31:45 +0200, Danny Braniss <danny at cs.huji.ac.il> wrote:
> >> Q: with rxcsum on, and a bad checksum packet is received, is it
> >>   dropped by the NIC? if not, then it somewhat explains the behaviour
> >
> > If checksum offloading is working correctly then a bad packet should be 
> > dropped by the NIC.  If checksum offloading isn't working correctly then you 
> > can wind up in the situation where both the NIC and the driver think the 
> > other party has verified the checksum.  It's also possible that you may be 
> > running into corruption during DMA transfer from the NIC to RAM.  ISTR there 
> > have been some issues reported recently with checksum offloading on some 
> > NICs - though I don't have details to hand - you might like to search the 
> > lists.
> >
> >> changing the nic is tough, but if needed will be done.
> >
> > If disabling checksum offloading fixes the problem and the additional CPU 
> > load is acceptable (at least until you find a real fix) then there's no need 
> > to change NICs.
> 
> Actually, my understanding was that packets with bad checksums are delivered 
> to software, and flag the descriptor ring header for each packet tells us 
> whether the checksum was (a) checked and (b) validated by the hardware.  We 
> then propagate these to mbuf flags so that higher stack layers know whether or 
> not to calculate the checksum themselves.  Regardless of the specifics, 
> though, packets with checked but bad checksums shouldn't make it to the socket 
> layer where they would be visible to NFS.  If the NIC is marking apparently 
> bad packets as good, there are a number of possible sources -- be it bad 
> checksum handling in the card, corruption between the card and higher levels 
> of the stack (a DMA problem, as you point out, would have this symptom).

looking at the bce source, it's not clear (to me :-). If errors are detected in
bce_rx_intr(), the packet gets dropped, which I would expect to be the 
treatment
of an offloded chekcum error, but it seems that is not the case. 

danny