Re: Incremental update of TCP Checksum

Ihar 'Philips' Filipau
2003-09-17T14:05:35Z

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.
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