Subject: Architecture Enhancements for Multiplexed interfaces

From: "jim (j.) cobban" <[email-masked]>
Date: Mon, 4 Jul 1994 14:47:00 -0400

The current APPN architecture does not adequately support the range of
multiplexed serial interfaces which have been introduced in the last 20
years.  Examples of these interfaces are X.25, ISDN, and Frame-Relay.

I have some specific areas where I would like to see improvements:

  1) Permitting both switched and non-switched connections through a
     single Port.
  2) Implementation Agreements on how to express addressing plans other than
     IEEE 802.
  3) Association of link station with CONNECT_IN.

What is a Port?
---------------
In the current architecture there are three kinds of Ports which can be
defined in the NOF:

  - non-switched
  - switched
  - SATF

These are modelled on leased [multi-point] SDLC, switched SDLC, and TRN.  As
a result, for example, a switched port can only have one link station
connected at a time.

I would like to suggest a paradigm shift.  Whether a link connection is
switched or non-switched is a characteristic of that link connection, NOT of
the port through which it came.  However I feel that there is some conceptual
value to users in the existing port types, and in their ability to imply
semantic checks.  Therefore I propose adding a fourth port type: serial
multiplexed.  A serial multiplexed port can have multiple link connections.
Each link connection may be either switched or non-switched.

The type of the link connection must be signalled to the DLC because it will
affect addressing.  The address on a non-switched link connection
will typically be a local identifier (LCN, DLCI, etc.).  The address on a
switched link connection will typically be a global address according to some
specified addressing scheme (X.121, E.164, F.69, etc.)

The specific architectural changes are:

 - The DEFINE_PORT signal to NOF (and from NOF to CS) has an additional
   port type defined: serial multiplexed.  The semantics of the existing port
   types (including the restriction to a single link connection for switched)
   are retained.  The new port type permits any number of switched and
   non-switched link stations to be defined. 

 - The DEFINE_CN signal does not currently specify any semantic checking on
   the port names.  However clearly for a connection network to work all of
   the ports must use the same addressing plan.  Therefore all of the ports
   on a connection network must either be SATF ports (using LAN addressing)
   or serial multiplexed ports (using WAN addressing), but not a mixture of
   the two.

 - The DEFINE_LS signal to NOF (and from NOF to CS) has an additional
   attribute: connection type.  This may be:

    - inherit: this is the default and inherits the switched or non-switched
      or SATF connection type from the port.  This is the only value
      permitted on the existing port types, and is illegal on the new serial
      multiplexed port type.
    - non-switched
    - switched

 - The CONNECT_OUT signal from CS to DLC carries the determined connection
   type (non-switched, switched, or SATF) so that DLC can interpret the
   destination address field.  

In the processing logic for requests all decisions which are currently made
by examining the port type, must be changed to act on the link station
connection type.

Addressing
----------
In the current architecture the type and format of media addresses is not
discussed.  The DLC is expected to perform some implementation dependent
piece of magic to figure out whether the destination address is an SDLC link
station address, an E.163 telephone number, or an IEEE 802 MAC/SAP.  This may
seem straightforward when a port is associated with a particular piece of
hardware which restricts the range of functionality.  With a serial
multiplexed interface the nature of the remote device is often obscured.
This becomes more of an issue because these addresses, as signalled by DLC to
CS, are placed into the topology database and later presented to other nodes
using the TG descriptor vector.

It is reasonable to leave the current default IEEE 802 addressing on SATF
ports, but we must agree on how to express the addressing formats which are
common on serial multiplexed interfaces so that we can implement connection
networks across them.

In order to implement a connection network there must be an agreement on the
format in which DLC addresses appear in the TG Descriptor control vector
(X'46').  I propose the following format for DLC address on serial
multiplexed interfaces such as public switched packet data networks (PSPDN,
X.25), public switched telephone networks (PSTN), integrated services data
networks (ISDN), frame-relay, and native asynchronous transfer mode (ATM) (as
opposed to 802.x LAN simulation) subnetworks:

	   +-----------------------+-----------------------+
   byte 0  |              length of address = n            |
	   +--+--------------------+-----------------------+
   byte 1  | 1|  type of address   |   numbering plan      |
	   +--+--------------------+-----------------------+
   byte 2  |                   address[0]                  |
	   +-----------------------+-----------------------+
	...
	   +-----------------------+-----------------------+
   byte n  |                  address[n-2]                 |
	   +-----------------------+-----------------------+

This is based upon the address format in the Q.931 (ISDN signalling)
specification:

   length of address  - the number of bytes of
			address information which follows.  This may seem
			redundant, since the address is contained inside a
			DLC Signalling subfield, which has a length.  However
			it may prove necessary to include additional routing
			information in the DLC signalling subfield after the
			address itself.  Both X.25 and Q.931 include the
			length field as part of the address.

   type of address    - 000 - unknown
			001 - international, the address begins with the
			      country code
			010 - national, the address excludes the country
			      code
			011 - network specific, for example operator
			      information number
			100 - subscriber number
			110 - abbreviated number
			111 - extension

   numbering plan     - 0000 - unknown
			0001 - ISDN E.164 (E.163)
			0011 - PSPDN X.121
			0100 - Telex F.69
			1000 - national standard
			1001 - private numbering plan
			1111 - extension

   address[i]	      - IA5 (7 bit ASCII) character with 0 parity.

Q.931 encoding, rather than the X.25(1988) encoding which is similar except
that each address "digit" is only 4 bits long, is chosen because it permits
representing any valid X.25 (actually X.121) address, but is also the basis
of the representation of addresses in frame-relay and ATM.  In addition since
E.164, the addressing scheme of ISDN, is a superset of E.163, the addressing
scheme of conventional analog dial networks prior to 1995, this proposal also
includes support of connection networks which use the public switched
telephone network (PSTN).  It is also simpler in that the length field
represents a number of bytes rather than a number of nybbles.
  
Link Station Identification
---------------------------
In the current architecture Link stations which arrive at CS as a CONNECT_IN
are identified by XID (back-level LEN nodes) or CPName,
but only within a particular "Port".  On a switched incoming port CS
does not signal the expected identification (if any) of the link station
on that port, so DLC cannot intelligently route the request.  CS must
therefore be prepared to communicate with a particular link station
regardless of which Port it connects through.

In the VTAM implementation of APPN (and subarea SNA) link stations
are not identified within a single link, but are rather defined in
switched major nodes.  A link station can connect to VTAM through ANY
interface, and still have its particular characteristics associated with
it.  This is particularly critical for link stations containing
dependent LUs, but there are attributes associated with pure APPN nodes
which an administrator would still like to manage.

A particular link station may normally be attached through a non-switched
SDLC link, but if that fails it may attempt to recover the connection through
either a switched SDLC link or a public multiplexed data network.