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