fff@mplex.UUCP (Fred Fierling) (07/28/90)
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. A datascope connected to the SLIP port shows this being transmitted (hex): c0 00 00 08 00 c0 45 00 00 2D ... The second packet (starts with "45") fully conforms to RFC 791, but what is that little runt doing there? -- Fred Fierling fff@eng.mplex.bc.ca Tel: 604 875-1461 Fax: 604 875-9029 Microplex Systems Ltd 265 East 1st Avenue Vancouver, BC V5T 1A7, Canada
stevea@i88.isc.com (Steve Alexander) (07/29/90)
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