Path: utzoo!attcan!uunet!snorkelwacker!bu.edu!orc!decwrl!shlump.nac.dec.com!carafe.enet.dec.com!goldstein
From: goldstein@carafe.enet.dec.com (Fred R. Goldstein)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: SLIP reliability
Message-ID: <12435@shlump.nac.dec.com>
Date: 13 Jun 90 14:48:55 GMT
Sender: newsdaemon@shlump.nac.dec.com
Organization: Digital Equipment Corp., Littleton MA USA
Lines: 38


In article <9006130114.AA05996@vax.ftp.com>, jbvb@VAX.FTP.COM (James B. Van Bokkelen) writes...
>What Ethernet has that SLIP doesn't is a link-layer CRC-16, which can
>do a pretty good job of detecting damage while on the cable.  SLIP
>has no error detection at all at the link layer.  Of course, the
>Ethernet CRC-16 may not detect packets damaged in the interface, or
>in the host before transmission or after reception.  

Slight correction.  Ethernet, like 802.{3,4,5}, has a 32-bit CRC, not a 
16-bit CRC.  (BTW, "CRC-16" is the name of the bisync/DDCMP 16-bit 
checksum; "CRC-CCITT" is the 16-bit HDLC checksum.  CRC-32 is the 
LAN checksum, also optional in HDLC, and historically developed for 
Autodin-II.)

The 32-bit Ethernet checksum maintains a Hamming distance of 4 up to the 
maximum Ethernet packet, or for that matter up to about 12K bytes.  Thus 
ANY 3 bit errors will be caught, or any one error of under 32 bits.
A 16-bit CRC maintains a Hamming distance of 3 up to 4K bytes.  Exceed
that error limit and you're subject to a "crap shoot" of 2**n for n bits 
of CRC.  With 32 bits, that's a very low probability of undetected error 
no matter how you slice it.

The TCP and IP checksums are straight arithmetic.  Frog two 16-bit words 
and you've got an undetected error.  The right two-bit error combo will
be undetected.  It's a whole lot better than nothing, but not as good as 
the Fletcher checksum and that's not as good as the more computationally 
intensive CRC (which is usually done in hardware).

SLIP, with no checking whatsoever, is a bad joke.  PPP uses CRC-CCITT,
which is pretty good (for reasonable MTUs).  Ethernet is close to 
bulletproof, with regard to line noise induced error.  (Host error is, 
as the man said, another story.  TCP's checksum is mainly there to 
detect it.)
---
Fred R. Goldstein   goldstein@carafe.enet.dec.com 
                 or goldstein@delni.enet.dec.com
                    voice:  +1 508 486 7388 
opinions are mine along; sharing requires permission