Messages in this thread | | | Date | Wed, 17 Sep 2003 03:39:17 +0200 (CEST) | From | Jesper Juhl <> | Subject | Re: Incremental update of TCP Checksum |
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On Wed, 17 Sep 2003, Lincoln Dale wrote:
> 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. >
I stand corrected. Thank you for pointing that out.
Jesper Juhl <jju@dif.dk>
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