Re: TCP checksum offloading and non-zero checksums

[Guy Harris <guy () alum mit edu>]

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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