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Date: Fri, 6 May 2016 18:57:43 +0300 From: Shmulik Ladkani <shmulik.ladkani@...il.com> To: "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org Cc: Edward Cree <ecree@...arflare.com>, shmulik.ladkani@...il.com Subject: [PATCH] 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". Signed-off-by: Shmulik Ladkani <shmulik.ladkani@...il.com> --- I assume readers interpret the term "ones-complement sum" as the sum using one's complement arithmentic, without the final bitwise complement of sum's result. Hence I added "the complement of" where applicable. Documentation/networking/checksum-offloads.txt | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/Documentation/networking/checksum-offloads.txt b/Documentation/networking/checksum-offloads.txt index de2a327766..9567200e1f 100644 --- a/Documentation/networking/checksum-offloads.txt +++ b/Documentation/networking/checksum-offloads.txt @@ -69,17 +69,17 @@ 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 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 + 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 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 -- 2.7.4
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