🚀 go-pugleaf

We have been implementing a Sparc-based SNA-TCP/IP gateway on our 
network, and I would like to share a preview of our experiences.  Once
the installation is complete and in production, I will post an updated
version of this document.  If you want a copy by mail, send me mail
now, asking for the update when it is available, and who knows, I may
even send you one :-).




              A Sun Sparc-based TCP/IP-to-SNA gateway
              ========================================

This document describes the experiences of CSIR in implementing a high-
capacity SNA gateway, using Sun Microsystems Sparc computers.

1) Introduction

CSIR is a fairly large organisation, with some 6,000 employees in 40
buildings.  The buildings are on a single campus, and are linked by a
fibre-optic ethernet backbone.  Each building has a thin ethernet LAN
through the offices, running a mixture of TCP/IP and SPX/IPX.  Each LAN
is linked to the fibre-optic backbone by a PC-Route IP router and a
Novell fileserver acting as an IPX router.  The computing centre on
campus houses a number of general-purpose Unix computers, IP routers to
the Internet, and an IBM mainframe (actually an Hitachi) running MVS/ESA.
A few outlying offices are connected to the backbone with ethernet
bridges and 64kbps leased lines.

Until the implementation of the new gateways, CSIR staff gained access to
the IBM mainframe using Novell's SNA gateway products.  These were linked
to the mainframe via two 3174 clones (Mohawk MC400) and token-ring.
Access was limited to 64 LUs per gateway, and the gateways were extremely
unstable.  A number of applications on the mainframe use Software AG's
Natural Connection product on the user's PC to present a slightly less
daunting face.  Natural Connection uses HLLAPI to communicate with a 3270
session on the user's PC.

A major gateway upgrade was required when CSIR management decided to
centralise all accounting, project management and other critical data on
the mainframe.  This requires all project managers to have access to the
mainframe system, with a projected load of 400 concurrent sessions.  In
addition, decentralised printing is required across the entire campus,
and at outlying offices.


2) Options considered

A number of options were considered for the proposed new gateway system,
as detailed below.

The simplest approach would have been simply to increase the number of
Novell gateways.  However, this product has been discontinued by Novell,
and in addition the gateways had proved themselves unstable.

Sticking with Novell, CSIR could have opted to use the new SNA gateway
product, Netware for SAA.  However, this would have entailed upgrading
50-odd fileservers on campus to Netware 3, at considerable expense.

The mainframe has a local ethernet controller attatched, and some thought
was given to using TN3270 to access the mainframe.  At present, some 50
users access the mainframe in this manner, but the load of the TCP/IP
protocol stack alone uses around 20% of the CPU capacity.

IBM offers various SNA connectivity products for their RS/6000 line, as
do Open Connect Systems.  Sun, too, offers various gateways.  Hewlett-
Packard declined to specify what products they had available.

The options for physical connections to the mainframe were to use the
existing MC400 local controllers and token-ring, use a channel adapter or
use SDLC.  The latter option can be ruled out as being too slow for the
number of sessions required, leaving token-ring or channel-attatch.


3) Deciding factors

Strategically, CSIR is moving all applications from the MVS system to
Unix platforms within the next three years.  For this reason, management
were extremely interested in using a small Unix machine as a gateway.
From a price perspective, this was also the most attractive offer, with
Sun Microsystems' Sparc 10 a factor of four cheaper than an equivalent
IBM offering.  The OCS products were extremely expensive, and only run on
IBM at this stage.

In addition, Brixton Software offer "BrxUnixGateway", which would allow a
3270 user on the SNA network to access any of the Unix machines on
campus.  This, too, was extremely attractive.

The cost difference between token-ring and channel attatchment was
surprisingly high.  As the token-ring can run at 2 megabytes/sec (16 Mbps)
as opposed to the channel's 3 megabytes/sec, we decided to use
token-ring.  In either case, the bottleneck would be the ethernet
connection to the rest of the campus, running at 10 Mbps.


4) Configuration

The final approach was to run an SNA gateway on a Sparc computer, with a
TN3270 server attatched to the ethernet controller.  The user's PC would
run TN3270 to communicate with the gateway.  This offloads the 3270
protocol conversion to the end-user's PC, so that the gateway merely
strips the TCP/IP headers and replaces them with SNA headers before
forwarding packets to the mainframe, and does the reverse in the opposite
direction.

For printing, various printers were defined to the Sun LPR subsystem, and
linked to specific logical units in the mainframe.  Each of these
printers are handled as normal Unix printers, and so can be attatched to
remote machines, using LPD.  They can be logically attatched to Novell
print queues using the Charon gateway.


4) Specification/aquisition

The equipment configuration finally used was:

*  Sparc 10 m30, with 64Mb RAM, 400Mb disk
*  Sun TRI/S token-ring adapter
*  Mohawk MC400 local controller

The software to implement the gateway was a mixture of commercial and
freeware:

*  Sunlink SNA3270 gateway software
*  A TN3270 server from Pekka Nikander at Finnish Telecom
*  An alternative TN3270 server from SunConnect
*  A TN3270 client from Kent Fitch at CSIRO in Australia
*  An LPD server for DOS


5) Implementation

Implementation of the above gateway has so far been comparatively
painless.  However, this is not yet complete, and there might be further
problems lurking.

The most serious problem to date had to do with the token-ring connection
between the IBM and the Sun.  In their SNA/3270 manual, Sun recommend a
MAXDATA value of 265.  The word "recommended" should be replaced by
"mandatory".  However, this only applies to links via 37x5 FEPs, and not
via 3174s.  In the case of a 3174 connection, this must be configured on
the control unit itself.

We have just (1/2 hour ago) received the SunConnect tn3270d, and have been
running on Pekka Nikander's tn3270d until now.  We will compare both servers
early in 1993.


6) Functionality

To date, the following functions have been implemented and tested:

* 3278 emulation from the Sun gateway to the IBM mainframe.

* TN3270 connections from PCs on the ethernet LAN, via the Sun gateway,
  to the IBM mainframe.

* 3287 emulation on the Sun gateway feeding the Sun print queues.

* LPD support on a DOS computer on the ethernet LAN.  This makes
  up to three printers on each server PC addressable from MVS.

* Multiple gateways per TRI/S adapter (we have had up to three PUs per
  adapter, with up to 200 LUs per PU)

* Simple HLLAPI calls into Kent Fitch's tn3270.

The following functions have _not_ yet been tested:

* HLLAPI support in the DOS TN3270, and this HLLAPIs conformance with
  Natural Connection's requirements.

* Funtionality of the SunConnect tn3270d server.


7) Performance

No performance testing has been done yet (this is going to happen Real 
Soon Now :-)).

8) Future enhancements

Automatic tea-maker.
-- 
 Paul Nash                                   (voice) +27-12-8413050
 Network Services, CSIR Infotek                (fax) +27-12-8414109
 PO Box 395, Pretoria, 0001 South Africa    (home) paul@frcs.alt.za