Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site cheviot.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!allegra!bellcore!decvax!genrad!teddy!panda!talcott!harvard!seismo!mcvax!ukc!cheviot!robert
From: robert@cheviot.UUCP (Robert Stroud)
Newsgroups: net.lan
Subject: Help with Xerox vs ISO Ethernet incompatibilities (long)
Message-ID: <242@cheviot.UUCP>
Date: Thu, 24-Jan-85 14:32:26 EST
Article-I.D.: cheviot.242
Posted: Thu Jan 24 14:32:26 1985
Date-Received: Wed, 30-Jan-85 05:11:55 EST
Reply-To: robert@cheviot.UUCP (Robert Stroud)
Organization: U. of Newcastle upon Tyne, U.K.
Lines: 62

Thanks to the Type vs Length field mess, the ISO/IEEE and Xerox 
Ethernet specs are incompatible from the data link level upwards, 
(and perhaps even the physical level).

We have a variety of machines which support one family or the
other and we wish to connect them together on our Ethernet using the
Newcastle Connection. 

For various political and technical reasons, we believe we will
have to support BOTH worlds - we can't change entirely to one or
the other. However, we are prepared to have at least one machine 
which understands both protocol families.

I've been writing a paper about all this and what we're going to
do about it, but would like to clear up a few points.

If anyone can answer any of the following questions I will be
very grateful. I'll summarise any replies to the net.

1) Is "Ethernet 2" the same as IEEE-802.3 or is it an upgraded version
   of the original Xerox/DEC/Intel Ethernet specs?

2) Are the hardware differences between 802.3 and Xerox likely to be a
   problem in practice, and how will the discrepancies be resolved?
   Are transceivers that correspond to the same standard compatible?
   (we certainly hope they are :-)!

3) What is Xerox's position regarding allocation of Type field values
   to ISO protocols? Have they reserved the block 0-1500 for use by ISO,
   (we believe these values correspond to the only valid 802.3 length fields)
   or have some of these values been allocated to other protocols? We think
   that PUP in particular has been allocated the value 512, although the
   4.2 header files are contradictory on this point.

4) Will the following algorithm suffice to distinguish IEEE packets from
   Xerox packets at the data link level, and pick an appropriate protocol
   handler?

	Examine the Type/Length field. If it is less than 1500 then assume
	that it's an IEEE packet so switch on LSAP, otherwise assume it's
	a Xerox packet and switch on the field itself.

	If an IEEE only station receives a Xerox packet (or vice-versa)
	then throw it away. [It should only be a broadcast packet].
	Also throw away any packets for LSAP's (which we use as type
	fields) or Type fields that we don't know about.

5) Just what do the ISO people mean by a SAP (Service Access Point)?
   Is it fair to say that they are using SAP's as sub-addresses AND
   type fields, confusing two notions in one? If not, how do you reconcile
   the allocation of particular SAP values to particular protocols,
   (e.g. IP and ISO internet protocol) with the statement in 802.2 that
   it should be possible to send packets between any pair of SAP's??

Thank you very much.

Robert Stroud,
Computing Laboratory,
University of Newcastle upon Tyne.

ARPA robert%cheviot%newcastle.mailnet@mit-multics.arpa
UUCP ...!ukc!cheviot!robert