Path: utzoo!utgpu!attcan!uunet!lll-winken!ames!haven!purdue!decwrl!hplabs!hpda!hpcuhb!hpindda!tozz
From: tozz@hpindda.HP.COM (Bob Tausworthe)
Newsgroups: comp.protocols.iso
Subject: Re: Transport Protocol Downward Multiplexing
Message-ID: <5560014@hpindda.HP.COM>
Date: 26 Jan 89 06:16:29 GMT
References: <432@manta.NOSC.MIL>
Organization: HP Information Networks, Cupertino, CA
Lines: 32

Hmm. Interesting problem.

My first question is: what is the internet protocol associated with
the NSAPS? If you are going to use CLNP then you have no choice but to
use TP4 as the transport class protocol. If you are using CONS (i.e.
X.25) then you may choose to support both transport classes (0, 4).

The point to remember is that the transport protocol associated with
your TSAP is TP, not TP0 or TP4. During connection establishment phase
for the transport connection, the peer TP protocol modules negotiate
transport class. A primary class and a secondary class are specified
with the secondary class always being less than or equal to the primary
class. So for instance, when requesting a TP connection over the Net
with a low high error rate you would specify class negotiation of (4,4)
meaning primary class of 4 and secondary class of 4. Likewise when
requesting a TP connection over the Net with a low error rate, you would
specify class negotiation (0,0). When recieving connection requests over
the Net with a high error rate you only accept requests that have the
primary class negotiation set to 4. When recieving connection requests
over the the Net with a low error rate you accept any valid transport
class, with the possibility of negotiating down connection requests 
of (4,0) to 0 so as to achieve lower transport overhead.

So there's no problem with having a TSAP which accepts connection requests
with differing transport class, you simply make TP's negotiation 
mechanism aware of the quality of its underlying networks. This shouldn't
be a violation of the OSI model since TP is already forced to do this 
for CLNP vs CONS (i.e. TP must know whether the connection request came
up through CLNP or CONS so that it knows to reject requests for TP0/CLNP).

				Bob Tausworthe
				tozz@hpindda.hp.com