Re: Incremental update of TCP Checksum

[Lincoln Dale]

At 06:35 AM 17/09/2003, Jesper Juhl wrote:
> Personally I can't see that you have any other option. The way the
> checksum is calculated information is lost, so it's impossible to
> determine exactely what input generated the current output (the checksum).
> Just as it is impossible to tell if the number 6 was generated from 2+2+2,
> from 3*2 or from 3+3 or some other...  So I don't see what else you can do
> except just recalculate the checksum from scratch. To try and determine
> how your modification would affect the checksum would probably take far
> longer than just re-calculating it.

of course you can do an incremental checksum update.
you know that if you're changing a field from (say) 0x22 to 0x11 then you 
can 'back out' the 0x22 and recalculate the checksum with 0x11.

this is the whole rationale behind why its a _checksum_ and not a _CRC_.

router(s) have taken advantage of incremental since day one.



cheers,

lincoln.

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