Re: TCP checksum offloading and non-zero checksums

[Vic Chester <vcsubscriptions () gmail com>]

That could be the case. It would be great if someone could confirm that is
how it works.

As for the VM, it was communicating with hosts across the internet. Not VM
to VM.

Thanks

On Fri, Nov 23, 2018, 7:03 PM Guy Harris <guy () alum mit edu wrote:

> On Nov 23, 2018, at 3:57 PM, Vic Chester <vcsubscriptions () gmail com>
> wrote:
>
> > I was under the impression that if TCP checksum offloading occurs, the
> actual TCP checksum would be zero. Can anyone confirm?
>
> I would not assume that; the networking stack might just not bother
> initializing the checksum field to zero before handing a packet to a NIC
> doing checksumming.
>
> > The reason I ask is because I have a capture from a Centos 7 VM where
> several packets (73) are reported as having incorrect TCP checksums. The
> actual checksums are not zero however, and some of them when have replies
> to them. If the checksum was really incorrect the ip stack of the receiver
> should have discarded the packet.
>
> And if this is a "virtual NIC" that just moves packets from one VM to
> another, it might not even bother generating a checksum - or checking it on
> the other side.  If *memory* isn't reliable, you have bigger problems than
> just "TCP connections between VMs might get corrupted data"....
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