Re: Incremental update of TCP Checksum

[Ihar 'Philips' Filipau]

Richard B. Johnson wrote:
> This is all wonderful. This assumes that the stuff being modified
> in the packet is on well-defined boundaries, seldom the case when
> you are re-writing packet data, but certainly the case if you
> are re-writing an IP address.
>
> Also, modern switches do not rewrite checksums using software.
                                                       ^^^^^^^^
> Therefore, they do not use a re-write algorithm as stated by
> others. The checksum gets calculated "for free" during the
> hardware transfer to an output holding FIFO. It is done using
> an ASIC with the appropriate adder and "stumble-carry".

  It depends on definition of software. It is so hard to draw parallel 
especially when what looks like controller runs some flavour of Linux 
from some embedded flash ;-)

  Actually some switches do alter header _before_ they have received 
payload. Just to be able to start rx without waiting for payload to 
arrive completely. Sometimes low latencies (even on narrow bandwidth 
links) are _that_ important...

--
Ihar 'Philips' Filipau  / with best regards from Saarbruecken.
  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -
  * Please avoid sending me Word/PowerPoint/Excel attachments.
  * See http://www.fsf.org/philosophy/no-word-attachments.html
  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -
   There should be some SCO's source code in Linux -
      my servers sometimes are crashing.      -- People

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/