Re: Incremental update of TCP Checksum

Lincoln Dale
2003-09-17T01:37:38Z

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.