netdev - RE: [PATCH v2 net-next 1/5] net: local checksum offload for encapsulation

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Date:	Thu, 7 Jan 2016 17:22:51 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Edward Cree' <ecree@...arflare.com>,
	David Miller <davem@...emloft.net>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-net-drivers@...arflare.com" <linux-net-drivers@...arflare.com>,
	"Tom Herbert" <tom@...bertland.com>
Subject: RE: [PATCH v2 net-next 1/5] net: local checksum offload for
 encapsulation

From: Edward Cree
> Sent: 07 January 2016 17:12
> The arithmetic properties of the ones-complement checksum mean that a
>  correctly checksummed inner packet, including its checksum, has a ones
>  complement sum depending only on whatever value was used to initialise
>  the checksum field before checksumming (in the case of TCP and UDP,
>  this is the ones complement sum of the pseudo header, complemented).
> Consequently, if we are going to offload the inner checksum with
>  CHECKSUM_PARTIAL, we can compute the outer checksum based only on the
>  packed data not covered by the inner checksum, and the initial value of
>  the inner checksum field.

Isn't it even simpler than that?
The checksum of the inner packet (including its header) is ~0 (ie 0).
So the checksum of the whole packet (for the outer header) is the same
as that of the packet down to the start of the inner header.

	David