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From: cliff@violet.berkeley.edu (Cliff Frost)
Newsgroups: comp.dcom.lans
Subject: Re: What services does X.25 provide?
Summary: Why not TP4 over X.25?
Keywords: x.25, services, login, e-mail, file transfer, IPC
Message-ID: <1989Oct10.210812.13144@agate.berkeley.edu>
Date: 10 Oct 89 21:08:12 GMT
References: <796@maxim.erbe.se> <3279@wasatch.utah.edu> <522@wet.UUCP> <6624@pdn.paradyne.com> <23189@cos.com>
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In article <23189@cos.com> howard@cos.com (Howard C. Berkowitz) writes:
>
>One of the uglier problems in current OSI is that Transport (at least
>connection-oriented transport) does have expectations about the
>characteristics of the underlying Network Service.   Transport
>over CNLS cannot directly interoperate with transport over CONS.
>A number of factors are involved, including whether Transport depends
>on the lower layer (i.e., network) to tell it whether a connection
>has broken.
>

Caveat:
I knew next to nothing about OSI before InterOp this year, and I don't
know much (if any) more now.  I'd like to learn, which is the why of
this note.

Why not just run TP4 everywhere?  It could run over X.25 just as well
as over connectionless, couldn't it?  That would at least reduce the
number of complexities in OSI by a large amount.  What do TP0-TP3
give you that TP4 doesn't?  (This question was raised by someone at
InterOp and no one even tried to answer it--does this mean it was
an extremely stupid question or an extremely good one?)

The only argument I've heard is that TP4 is too expensive a 
transport protocol for some cases--but Van Jacobson's work pretty
soundly squashes that reasoning.  What key point(s) am I missing? 
(I mean, technically--not politically.)

>There are a variety of solutions to this problem, including transport
>relays.  The definitive solution is still evolving.  Note that
>X.25 over LANs is used in Europe as a circumvention to this
>difficulty.

Transport relays mean you lose all hope of end-to-end checksumming.  
After several years of maintaining a production network this is too grim
for me to contemplate.

X.25 over LANs?  Somehow this sounds like a "worst of both worlds"
scenario.  I thought X.25 and virtual circuits were supposed to buy you 
things like congestion control and "guaranteed" throughput.  How do
they do that over ethernets?

	Thanks very much,
		Cliff Frost
		Central Computing Services
		University of California, Berkeley
		<cliff@berkeley.edu>