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From: eskovgaa@CUE.BC.CA (Erik Skovgaard)
Newsgroups: comp.protocols.iso
Subject: OSI CLNS on X.25??
Message-ID: <196*eskovgaa@cue.bc.ca>
Date: 13 Feb 91 16:58:00 GMT
References: <13375@hacgate.UUCP>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 37

You should probably look at a standard named "The Internal Organization
of the Network Layer" (IONL) ISO 8648.

When you run CLNS over X.25, you can view the Network Layer as being
composed of three "sublayers":  The Subnetwork Independent Convergence
Protocol (SNICP), the Subnetwork Convergence Function (SNDCF) and
the Subnetwork Access protocol (SNAcP).

SNICP in you case is CLNP and SNAcP is X.25 (PLP).  The function of
SNDCF is to map the Network Address to the Subnetwork Access Point
of Attachment (SNPA) address and in this case, to manage X.25 VCs.

You can now open a VC when an IP packet is sent out.  You do not have to
close the VC, but can keep it open for the next IP packet.  A timer
takes care of the closing the VC if no traffic is taking place on the
VC for a while.  Otherwise it remains open. 

It would be too much overhead to open and close a VC for each IP
packet.

SNDCF is a local function and does not generate PDUs on its own.  Other
combinations of SNICP/SNAcP may require the use of a convergence
protocol that generated PDUs.  In that case, the middle sublayer
is referred to as SNDCP (p - for Protocol).

This is a gross simplification of IONL and SNDCF, but it should
give you the general idea.

Using VC's as opposed to PVCs allows you flexibility in routing.

Have fun!                                ....Erik.


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Erik Skovgaard
PSC (Pacific) Inc.
Vancouver, B.C.