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
