Re: [uml-devel] uml "ip header error" for large (fragmented) udp packets

[Andi Kleen <ak@suse.de>]

On Wed, Oct 06, 2004 at 08:48:23PM +0200, Lars Ellenberg wrote:
> > So this could help you for now, while we try to find a clue about this. Quite 
> > frankly, I must say that I'm not seeing any network kernel hacker here 
> > (correct me if I'm wrong), so it will take some time to debug it. Maybe Gerd 
> > Knorr is an exception, actually.
> 
> Well, then I take Andi Kleen and Lars Marowsky-Br?e into CC for now.
> Lars, because I expect him to be interessted in having UML as full
> featured cluster simulation tool available, and Andi because I hope he
> might know the network code much better than me...
> 
> FYI, full thread can be found for example at
> http://thread.gmane.org/gmane.linux.uml.devel/4607

Paolo's analysis is basically correct. loopback sets this flag
for better performance.  Actually in 2.6 it probably doesn't help
very much anymore because TCP can do checksum copy RX now, and that
would get the checksum basically for free. But it's still there
and may still make things slightly faster.

If UML taps the packets from lo it will see incorrect checksums.

Using a tun or ethertap device would avoid this. In the worst
case you could also just delete the flag from the loopback
interface, it's only an optimization.

-Andi



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