Re: RFC 1812, section

Tony Li <> Fri, 15 September 1995 03:44 UTC

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From: Tony Li <>
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In-Reply-To: <v02130500ac7e9ade669a@[]> (
Subject: Re: RFC 1812, section

   >    1.  You must verify the IP checksum even if you use incremental checksum
   >    updating.

   It seems to me that it says

     - when you verify the checksum, you must add up all five words and
       verify that the result is a negetive zero

     - when you bump the TTL, you need not recalculate the entire checksum;
       you may bump the existing (checked) checksum by the negative of the
       amount you change the TTL.

   which is to say, interpretation 1. My recollection was that there was major
   debate here, with my current employer among those whose implementations
   were of version 2 ("the end system is going to check it, and routing has
   developed a spanning tree, so why should the router check it?") and that
   interpretation lost.

   Does anyone have other recollections or opinions?

I agree with your interpretation.  However, I would like to make it
quite clear that your current employer has NEVER, EVER used or
supported interpretation 2.

I'll also point out that interpretation 1 is inconsistent with the
design of IPv6.