Path: utzoo!attcan!uunet!samsung!usc!rutgers!mcdchg!laidbak!stevea
From: stevea@i88.isc.com (Steve Alexander)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: SCO TCP/IP
Keywords: SCO SLIP runt packet
Message-ID: <1990Jul28.201811.7461@i88.isc.com>
Date: 28 Jul 90 20:18:11 GMT
References: <5@mplex.UUCP>
Sender: usenet@i88.isc.com (Usenet News)
Distribution: na
Organization: INTERACTIVE Systems Corporation, Naperville, IL
Lines: 27

In article <5@mplex.UUCP> fff@mplex.UUCP (Fred Fierling) writes:
>We are running SCO Xenix 386 TCP/IP v1.0.1h (well, trying too!) and have 
>noticed a little 4 byte packet prepended to every full size packet 
>on our SLIP link.

SCO TCP/IP is derived from System V STREAMS TCP, which was originally
delivered to Lachman Associates (now a part of INTERACTIVE) by Convergent
Technologies in early 1987.  The 4-byte "runt" is a SAP identifier packet 
which was added by Convergent to support running non-IP protocols over the 
SLIP frame encapsulation.  It notifies the receiver that the next frame
is destined for the client bound to specified LSAP.  This code has never 
been used by Lachman for anything, but it survives in the SLIP driver to 
this day.  Obviously, it would only work between cooperating implementations.
I don't know if Convergent uses this for anything useful.

The 00 00 08 00 is 800 hex, which conveniently happens to be the Ethernet 
packet type for IP.  The idea was that network drivers would bind to SLIP
using the same SAP as they do for Ethernet.

I would expect that this "feature" will go away at some point, now that
we are committed to PPP.

Hope this clears up the mystery.

--
Steve Alexander, Software Technologies Group    | stevea@i88.isc.com
INTERACTIVE Systems Corporation, Naperville, IL | ...!{sun,ico}!laidbak!stevea