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Documentation/networking: more accurate LCO explanation 

In few places the term "ones-complement sum" was used but the actual
meaning is "the complement of the ones-complement sum".

Also, avoid enclosing long statements with underscore, to ease
readability.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Shmulik Ladkani 2016-05-06 20:27:43 +03:00 committed by David S. Miller
parent be0bd31601
commit c81aa79794
1 changed files with 7 additions and 7 deletions
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14
Documentation/networking/checksum-offloads.txt
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@ -69,18 +69,18 @@ LCO: Local Checksum Offload
LCO is a technique for efficiently computing the outer checksum of an
encapsulated datagram when the inner checksum is due to be offloaded.
The ones-complement sum of a correctly checksummed TCP or UDP packet is
equal to the sum of the pseudo header, because everything else gets
'cancelled out' by the checksum field. This is because the sum was
equal to the complement of the sum of the pseudo header, because everything
else gets 'cancelled out' by the checksum field. This is because the sum was
complemented before being written to the checksum field.
More generally, this holds in any case where the 'IP-style' ones complement
checksum is used, and thus any checksum that TX Checksum Offload supports.
That is, if we have set up TX Checksum Offload with a start/offset pair, we
know that _after the device has filled in that checksum_, the ones
know that after the device has filled in that checksum, the ones
complement sum from csum_start to the end of the packet will be equal to
_whatever value we put in the checksum field beforehand_. This allows us
to compute the outer checksum without looking at the payload: we simply
stop summing when we get to csum_start, then add the 16-bit word at
(csum_start + csum_offset).
the complement of whatever value we put in the checksum field beforehand.
This allows us to compute the outer checksum without looking at the payload:
we simply stop summing when we get to csum_start, then add the complement of
the 16-bit word at (csum_start + csum_offset).
Then, when the true inner checksum is filled in (either by hardware or by
skb_checksum_help()), the outer checksum will become correct by virtue of
the arithmetic.