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From: martillo@bloom-beacon.UUCP
Newsgroups: comp.protocols.iso,comp.dcom.lans
Subject: Re: OSI-model software
Message-ID: <920@bloom-beacon.MIT.EDU>
Date: Sun, 14-Jun-87 19:22:42 EDT
Article-I.D.: bloom-be.920
Posted: Sun Jun 14 19:22:42 1987
Date-Received: Mon, 15-Jun-87 05:49:11 EDT
References: <1204@botter.cs.vu.nl> <1680@munnari.oz> <192@ditmela.OZ>
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Reply-To: martillo@athena.mit.edu (Yakim Martillo)
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Keywords: osi, iso, internetworking
Xref: utgpu junk:5260 comp.dcom.lans:481

Personally, I have always found this confusing because I have
read the ISO documents and have never seen anything which implies
there must be only one protocol at every layer.  Further, I thought
swap in and swap out of protocol layers should be possible.

TCP and IP are tailored to each other but I have seen nothing
which implies that TCP could not run over ISO IP or which implies
TP4 could not run over IP.

In fact there might be reason to tailor a transport protocol for
certain classes of protocols.  A remote virtual disk server for a PC
can certainly get quite hosed when the PC is unexpectedly turned off,
why should the host also have to deal with abnormal termination of the 
transport layer.  Perhaps VCs at level 4 are not always a good idea.
Unfortunately TP0-TP4 are all VC oriented (I'm not sure) maybe a UDP
like level 4 should be defined, in fact maybe remote virtual disk
applications are sufficiently unique that a special level 4 should
be defined.

In any case I don't quite understand why IP and ISO IP can't be
coresident in the network.  The communications subnet protocol will
have some way of indentifying the level 3 protocol to the host level 3
layer software (like the type field in ethernet 2.0 or like ssap/dsap
tuples in IEEE 802.2)

IEEE 802.2 and ISO level 2 puzzle me as well.  These are protocols for
the communications subnet and I don't quite understand why IEEE and
ISO are trying to define communications subnet protocols for all time.
Shouldn't communications subnet protocols be medium-dependent?