🚀 go-pugleaf

In article <Bwn2LB.DxA@immd4.informatik.uni-erlangen.de>, mskuhn@immd4.informatik.uni-erlangen.de (Markus Kuhn) writes:

   [ ... snip, snip]

|> >3. What are intermediate steps between that and X.25? (Layers 2&3)
|> 
|> Layer 4: TP0, a quite simple transport protocol
|> layer 5: the session layer protocol (which I still haven't understood in
|>          detail :-)
|> layer 6: the presentation layer protocol (perhaps including the optional
|>          old 1984 version)
|> layer 7: The common application service elements ACSE and RTSE for
|>          association control (handshaking) and reliable transfer of huge
|>          messages and the X.400 specific MTSE message transfer service
|>          element which is already a part of X.400.

X.25 is really only layers 2 & and the lowest sub-layer of layer 3.
There are two possibilities for the choices of protocols in the remainder
of layer 3 and for layer 4. These can
be either :-

     X.25 SNDCP - CNLP - TP4

OR

     CONS - TP0 (whatever became of 1,2 & 3)

Where the X.25 SNDCP (Subnetwork Dependent Convergence Protocol for X.25 or
something like that) basically converts the connection oriented service offered
by X.25 to a connection-less service suitable for use by the next highest layer
3 sub-layer, CNLP. CNLP is the Connectionless Network Layer Protocol which is
basically the ISO equivalent of the DoD IP. TP4 is Transport Protocol Class 4
which is (I think) the only ISO Transport Protocol capable of operating over
the kind of unreliable connectionless service offered by CNLP.

CONS is the Connection Oriented Network Service. I don't know if there is also
the usual accompanying protocol since it doesn't really do very much. It really
only acts as a bit of glue between X.25 and the Transport layer, I don't think
any PDUs are exchanged. Some CONS implementations can perform through routing
based on the (source, destination) NSAPs which CONS sends as User Data in the
X.25 Connect Request.

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Gary Watson