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From: martillo@ATHENA.MIT.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Separation of Layers
Message-ID: <8711050031.AA11673@PARIS.MIT.EDU>
Date: Wed, 4-Nov-87 19:31:27 EST
Article-I.D.: PARIS.8711050031.AA11673
Posted: Wed Nov  4 19:31:27 1987
Date-Received: Wed, 11-Nov-87 02:31:36 EST
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Yes, it sounds like my interpretation won't work.  I really was not
trying to get TCP/IP to fit into the ISO model which I consider
malformed at best.  I have some Basic Rate and Primary Rate Interface
cards for various machines, and I want to run TCP/IP over them.  But I
want the option of establishing point to point serial links between
machines or X.31 connections to PSPDNs rather than just packet calls
to my PBX or local switch which seem to have either miserable or
non-existent packet handlers.  In this environment I want to
dynamically assign IP addresses to channels or groups of channels and
I want to tear down links over which there is no traffic because I
want to avoid phone bills which could be large in the case of
point-to-point rather than packet billing.  In such an environment I
could anticipate the physical link for an rlogin session going away
and then suddenly when there was activity over the TCP VC a new
physical link on perhaps a different interface might be established
which might end up with a new IP address. 

Alternatively if possible I would probably like to move calls off the
ISDN medium onto the ethernet medium if a dead network suddenly became
alive.  

But given that IP and TCP are apparently really only one layer it
can't be done.  

In any case the problem probably isn't too important since ISDN is too
slow to be useful for real networking.  Of course for multihomed hosts
the reliability issue might be important, but I guess it just can't be
done.