netdev - Re: Checksum offload queries

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Date:	Thu, 10 Dec 2015 21:02:18 +0000
From:	"Rustad, Mark D" <mark.d.rustad@...el.com>
To:	Edward Cree <ecree@...arflare.com>
CC:	Tom Herbert <tom@...bertland.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: Checksum offload queries

Edward Cree <ecree@...arflare.com> wrote:

> I have just realised something startling.  Assuming the inner protocol uses the ones complement checksum in the way IP, UDP and TCP do, the outer checksum can be computed *without looking at the payload*.  Why?  Because the ones complement sum of (say) a correctly checksummed UDP datagram is simply the complement of the ones complement sum of the pseudo header.  Similarly, the ones complement sum of a correctly checksummed IP header is zero.
> Therefore, the outer checksum depends _only_ on the inner and outer pseudo headers and the encapsulation headers.  For example, with UDP encapsulated in VXLAN, we have the following packet structure:
> ETH IP UDP VXLAN inner-ETH inner-IP inner-UDP PAYLOAD
> and the outer checksum equals
> ~([outer_pseudo] + [UDP] + [VXLAN] + [inner-ETH] + ~[inner_pseudo])
> where [] denotes summation, and all addition is ones complement.
> This can easily be computed in software, especially as the stack already has ~[inner_pseudo]: it's stored in the inner checksum field to help inner checksum offload.
> 
> Have I made a mistake in my ones-complement maths, or is outer checksum offload as unnecessary as IP header checksum offload?

I agree with the overall observation, in that the outer checksum can be derived from the inner one. I think that the inner-ip header needs to be added (after subtracting out the inner_pseudo as you indicate above), because the entire raw inner IP header needs to be included in the outer checksum. I haven't thought this all through in detail yet. It would be really nice to have a function that implemented something like this. Could one be structured to handle most encapsulations?

--
Mark Rustad, Networking Division, Intel Corporation

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