Many things on ethernet together???

Michael A. Patton (map@GAAK.LCS.MIT.EDU)
Mon, 2 May 88 13:57:36 ast

   Date: 29 Apr 88 19:38:16 GMT
   From: rayssd!turbo! (Robin Alston)
   Organization: Raytheon Company, Marlborough MA

   My question is can this really work?
   Can XNS and TCP-IP share the same coax cable with no possible problems?
   Can we have our own domain (we really have no interest at this time in
   talking to our vaxes), while decnet has its own on the same cable?

There is no problem at all with this, Ethernet was designed with
exactly this type of thing in mind (of course ISO is different, but
can be made to coexist with a simple kludge). We have one Ethernet
here that carries TCP/IP, XNS and ChaosNet (that I know of, I'm sure
there are others that nobody bothers to tell me about) at the same
time. In fact the MicroVAX that I use as a workstation runs both
TCP/IP and ChaosNet on the same interface without any problem. The
designers of Ethernet assumed there would be multiple higher level
protocols and provided a field in the Ethernet packet to say which one
it carried. Any reasonable Ethernet driver should just calmly ignore
(maybe count) any packets with a type field it doesn't recognize.

        Michael A. Patton
        Network Manager
        Lab. for Comp. Sci.
        Mass. Inst. of Tech.

P.S. For those who don't know the kludge for coexistance mentioned in
the body, ISO replaces the Ethernet type field with a length field
which occupies the same bits. Coincidentally Ethernet types are
illegal ISO lengths and vice-versa, this allows the software to tell
them apart, you could look at it as assigning ISO a large number of
Ethernet type codes, which they then select between to encode the
length of the packet.

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