Commit c81aa797 authored by Shmulik Ladkani's avatar Shmulik Ladkani Committed by David S. Miller
Browse files

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: default avatarShmulik Ladkani <shmulik.ladkani@gmail.com>
Acked-by: default avatarEdward Cree <ecree@solarflare.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent be0bd316

Documentation/networking/checksum-offloads.txt

+7 −7
Original line number Original line Diff line number Diff line
@@ -69,18 +69,18 @@ LCO: Local Checksum Offload 
LCO is a technique for efficiently computing the outer checksum of an

LCO is a technique for efficiently computing the outer checksum of an

 encapsulated datagram when the inner checksum is due to be offloaded.

 encapsulated datagram when the inner checksum is due to be offloaded.

The ones-complement sum of a correctly checksummed TCP or UDP packet is

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

 equal to the complement of the sum of the pseudo header, because everything

 'cancelled out' by the checksum field.  This is because the sum was

 else gets 'cancelled out' by the checksum field.  This is because the sum was

 complemented before being written to the checksum field.

 complemented before being written to the checksum field.

More generally, this holds in any case where the 'IP-style' ones complement

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.

 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

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

 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

 the complement of whatever value we put in the checksum field beforehand.

 to compute the outer checksum without looking at the payload: we simply

 This allows us to compute the outer checksum without looking at the payload:

 stop summing when we get to csum_start, then add the 16-bit word at

 we simply stop summing when we get to csum_start, then add the complement of

 (csum_start + csum_offset).

 the 16-bit word at (csum_start + csum_offset).

Then, when the true inner checksum is filled in (either by hardware or by

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

 skb_checksum_help()), the outer checksum will become correct by virtue of

 the arithmetic.

 the arithmetic.