Re: [Xen-devel] networking checksum errors again

[Jason <xen@xxxxxxxxxxxxxxxxx>]

If I understand correctly, the offloading to a physical NIC only works when a 
physical NIC is
present and being used for that traffic. In a purely virtual route setup, that 
traffic should be
subject to this checksum issue.  Is this just a recent development or is routing a fairly uncommon 
configuration in comparison to bridging with xen?
--
Jason
The place where you made your stand never mattered,
only that you were there... and still on your feet

On Sat, 1 Apr 2006, Nivedita Singhvi wrote:

> Jason wrote:
> > w00t!  That fixed it.  I read your email on the logic for not doing the tcp 
> > checksums and, while I
> > am by no means in possesion of the brains the developers have, I am left 
> > wondering why that
> > decision was made.  Would anyone care to comment?
> I think I missed a post somewhere along this thread...
>
> Which decision are you referring to? Avoiding the
> TCP checksum between domains? The rationale for not
> doing the checksum is that it is a significant savings
> in performance. Even for traffic that goes out to
> a public net and must contain a checksum, deferring
> it to dom0 when the OS can offload it to the physical
> NIC (most NICs these days are capable of computing the
> checksums in hardware) is a significant saving.
>
> The native Linux stack offloads the calculation of
> the checksum to the NIC, and not doing so in the virtual
> environment increases the gap when comparing Xen to
> bare metal Linux.
>
> There are a lot of other issues here to resolve, of
> course, and fixing some of the current issues is being
> worked. It's likely going to be an issue for discussion
> when we're up for mainline inclusion.
>
> thanks,
> Nivedita
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