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From: jbvb@VAX.FTP.COM (James B. Van Bokkelen)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: SLIP reliability
Message-ID: <9006130114.AA05996@vax.ftp.com>
Date: 13 Jun 90 01:14:04 GMT
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Reply-To: jbvb@vax.ftp.com
Organization: The Internet
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	Does SLIP have the same level of error detection and correction that
	ethernet-based IP has?

IP has header checksums, and they're the same wherever IP is run.
Likewise, the TCP checksum is mandatory.  UDP can optionally use the
same algorithm, but some vendors believe this costs too much speed,
so they omit them, regardless of the network media.  IP and UDP have no
"correction" in and of themselves, but TCP will do retransmits if the
checksum catches damage, or packets are lost altogether.

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.  This sort of
undetected lower-layer damage happens much more often than the
optimistic crowd would like to believe, and is at the root of the
widely held belief that "end-to-end checksums are a Good Thing".

James B. VanBokkelen				FTP Software Inc.
jbvb@ftp.com					617-246-0900