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From: rpw3@fortune.UUCP
Newsgroups: net.crypt
Subject: Re: Error Detecting Codes - (nf)
Message-ID: <2947@fortune.UUCP>
Date: Wed, 4-Apr-84 04:04:31 EST
Article-I.D.: fortune.2947
Posted: Wed Apr  4 04:04:31 1984
Date-Received: Sat, 7-Apr-84 02:39:33 EST
Sender: notes@fortune.UUCP
Organization: Fortune Systems, Redwood City, CA
Lines: 34

#R:allegra:-237000:fortune:26600005:000:1525
fortune!rpw3    Apr  4 00:42:00 1984

NOT FAIR! (to either PUP or TCP/IP)

Look, the PUP and TCP/IP (and the XNS/"IP") software checksums are NOT
intended for catching data transmission errors. They expect to be used
inside a link-level protocol which has a "reasonable" CRC.  The stated
purpose of the software checksum is to detect errors WITHIN a system
(like a stuck bit on a bus), not between systems (although it helps
there too, see below).

Experimental Ethernet (3 Mb/s) used a 16-bit CRC (not particularly swuft).
But what is the probability of BOTH a CRC-16 and a ones-complement-and-rotate
checksum agreeing that a bad packet is good? (And while having an error which
is missed by both the CRC and the soft-check, oh yes, it must have a good
destination network, host, socket, and protocol number. Right.)

Standard Ethernet (10 Mbit/s, DEC/Intel/Xerox) uses a CRC-32 at the link-level,
plus the usual 16-bit 1's-comp-add+rotate software checksum at the IP level.
What is the probability of an undetected error THERE? (Same rules about
address fields) The CRC-32 they use is supposed to be a very well studied
polynomial developed for Autodin-II. It is initialized to all 1's so it
doesn't have the old blanking (large bursts of zeros) problem.

Even if TCP/IP is running on top of plain old HDLC, you still have two very
different nested error checks (like 3Mb/s Ethernet).

Rob Warnock

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