Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!pasteur!ucbvax!ANDREW.CMU.EDU!ddp+
From: ddp+@ANDREW.CMU.EDU (Drew Daniel Perkins)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: SLIP complexity
Message-ID: <8WK10ty00UoJQ1H-MI@andrew.cmu.edu>
Date: 4 Apr 88 22:37:45 GMT
References: <8803302201.AA00162@wb6rqn.UUCP>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 14

In fact, there is one very nasty error which the IP checksum does not catch.
Because carries are added in, a word of 0's is interchangable with a word of
1's.  For example:
x + 0 = x
x + 0xffff = x !!!!

This problem has caused us no end of trouble.  Early Ungermann Bass ethernet
boards for IBM RTPC's had a problem such that a packet would be corrupted AFTER
it had passed the ethernet CRC check.  However it also passed the IP/UDP
checksum test.  This caused us many problems with corrupted files and
directories.  The point is that this type of error may not be that uncommon on
async lines.  Noise could easily insert nulls.  If those nulls were in the
place of valid 0xff's you may be in trouble.

Drew