21 Feb
2019
21 Feb
'19
3:02 a.m.
I'll give you that a "gateway", as in a network layer gateway or router,
does
have network protocols that it routes / bridges between. (They may be on a
single interface, a la one-armed-router.)
I think we have a layer disconnect here. There is a PHYSICAL connection, but logically the
SNA traffic just passes through the box.
You define the MAC addresses of the end points in the 3174 but it knows nothing of the
traffic passing through.
As far as the end points are concerned its just a router like the one in my wardrobe that
links to the VDSL that?s passing my traffic TCPIP traffic through
Except that my router has to do NAT. These are all in the same SNA Domain and have unique
SNA addresses.
When doing SNA the terminal on the remote 3174 connects the VTAM...
The 3174 NEVER accepts any sort of incoming
connections. Just physical
terminals.
Um ? what do you consider the connections from remote 3174s (physically /
logically) connecting to a local 3174 via Token Ring / Ethernet / SDLC to be if
not connections to the local 3174?
I'm using IBM's definition for
"local" and "remote" in this context.
I can see some wiggle room for the connections from the remote 3174 being
to the mainframe via the local 3174 and not actually to the local 3174.
That being said I still think that the 3270 connection from the RS/6000 are
addressed /to/ the local 3174's Token Ring (MAC) address. Or is this the
above wiggle room too?
That?s just like IP routing. If we are doing SNA then each terminal has an SNA address
that?s defined on the host. Its kind of like IP routing.
My workstation knows that to route IP traffic to a specified non-local IP address it has
to send it to my router.
Same with SNA. The SNA connection is between the terminal and the mainframe. The 3174
simply routes traffic....
When used to
connect network traffic to a mainframe the 3174 does not
terminate the TCPIP connection., it passes the frames across to the
channel. I may be wrong its been a long time since I did this and I
don't want to go delving into the VTAM documentation.
The reading that I've done since the start of this thread makes me think that
the connections from the RS/6000 would be SNA over Token Ring. As I
understand it, this means that they are 802.2 LLC SNAP frames carrying
something other than TCP/IP.
Perhaps the 3174 is receiving those frames and passing them on to the
mainframe via some form of routing or bridging.
Or perhaps the 3174 is extracting the SNA data off of
the Token Ring frames
and passing just the SNA application layer data to the mainframe.
Theoretically VTAM has application layers. In practice it?s a mess...
I suspect that VTAM documentation is in my future if I
truly want to
understand this. Or maybe I'll get lucky and someone can answer my
pointed questions.
Yes, the VTAM documentation is a good place to start (well perhaps not).
But basically VTAM (used to, I haven't looked for a while) have huge tables
that list every device on the network.
So if you want to use 10 SNA 3270 terminals concurrently on an RS6000
There must be a VTAM/SNA node defined for the RS6000, and
10 VTAM/SNA nodes for the terminals.
This lead to PC type gate products like Microsoft SNA server where you had
TN3270 sessions to SNA server then connected via SDLC/Token Ring/X.25 to
SNA on the host.
If you wanted 20 concurrent session, you defined 20 nodes in VTAM and SNA server.
SNA Server would assign in bound TN3270 sessions SNA addresses from its pool and associate
them
with the terminal...
Now I have remembered something nasty. There are SNA management sessions, so if he is
feeling nasty,
the VTAM operator can disable any terminal on any 3174, or any 3174 .....
... so yes there may be SNA sessions between 3174s for management purposes.
I also haven't considered LU6.2 but LU6.2 is SNA app to SNA app so I can't see and
SNA LU6.2 session terminating within a 3174.
Its kind of
odd. RS232 (so X.25/SDLC/HDLC/Bi-Sync) connections can
only be used to connect to a Mainframe, not another 3174.
That's contrary to what I have been reading this week.
Based on the reading that I've done (I can dig for sources if you want me to),
a remote 3174 can connect to a local 3174 via Token Ring / Ethernet / SDLC.
This implies that the remote 3174 is connecting to another 3174. (See
additional comments below.)
The Token Ring
or Ethernet interface can be used to connect traffic to
the mainframe But from what I remember the 3174 isn't too involved at
this level it is acting as a network router/bridge.
"too involved" is critical.
Just to confuse things this is an IBM manual
where IBM does use it as
a "gateway"...
~chuckle~ Very little about IBM is simple.
http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/lan/GG24-3366-0_3
174_Remote_Token-Ring_Gateway_Feb89.pdf
I have seen virtually identical diagrams to the one on page 15 where the NCP
was a local 3174 instead of the 3720 / 3725 / 3745.
Notice how the listed 3174 sub models are all the remote variety.
Take a look at page 54 of the following pdf.
http://www.bitsavers.org/pdf/ibm/3174/GG24-4172-
0_Using_3174_in_TCP_IP_Networks_Jun94.pdf
The downstream 3174-13R can talk to either the upstream 3174-11L or the
3172.
Figure 244 on page 259 shows the same.
TCP/IP support improved over the years, yes...
so using the Token Ring interface on a remote
3174 to connect SNA
traffic to the host via SDLC.... ... again no TCPIP, working at the
frame level, and the host end cannot be a 3174...
Figure 244 on page 259 tends to refute that.
This document seems to be from 94 verses the document you linked to
seeming to be from 89. Maybe things changed in the intervening years.
That really
muddies the waters because it uses the term "3270"
connection in two senses. It uses it to refer to the co-ax type
connection from a work station (CUT or DFT) with with 3270 over
Channel/SNA as defined in the 3270 data streams manual and these really
are
different protocols.
I agree to your prior comment that this traffic between the terminals and the
3174 terminal controller is not 3270.
That?s where you are going wrong. The protocols
that the 3174 supports
between other 3174s are IBM SNA protocols. The "other 3174s" do not
need to be 3174s and can be any SNA device.
Some of the documents that I've looked at this week have explicitly shown a
routed TCP/IP network between the upstream / host 3174 and the
downstream 3174.
Where does it say that?. In particular on page 39
it says..
I've lost track of what document you're referencing. I don't see anything
like
the following on (file) page 39 and page (number) 39 is a source code listing.
IEEE 802.2
? PU 2/LU 2
? PU 2.1/LU 6.2 (in migration mode)
In the sort of use Kevin is talking about for connecting to Mainframe
channels there is generally no TCPIP on the 3174. In effect it looks
like a Mainframe NIC...
But the 3174 generally doesn't use TCPIP on the ring...
I'll agree that the RS/6000 may be using SNA on 802.2 LLC SNAP frames
directly and not using TCP/IP.
The TN3270 traffic originates from the 3174 and
terminates on the
Mainframe.
Hum....
TN3270 (and normal Telnet) traffic NEVER
terminates on the 3174...
Normal telnet traffic most certainly does terminate on the 3174 when it's
being used as a gateway for 3270 / CUT terminals to Telnet sessions on other
systems, like the RS/6000.
But that's not what we've been discussing.
If its application aware LU6.2 would show up. That?s like saying a network sniffer
can't see TCP/IP...
IBM describes it as LU6.2.....
LU6.2 doesn't translate to what a network sniffer would show things as,
which is what I'm trying to determine.
I've gathered that it's 802.2 LLC SNAP frames.
Which work perfectly fine on
both Ethernet and Token Ring. As such, they can also be bridged between
the two.
I would say 99% of 3174's had 3270 screens and connected them to the Mainframe.
That?s just bread and butter operation so any mainframe programmer can set on up in his
sleep...
... so there isn't much info on it, so it looks like its not the way things are
done....
I guess that a good proportion also had Token Ring or LAN interfaces that allowed the
mainframe to talk LAN protocols.
That was cheaper than 37xx box....
See above....
That?s how it connects, but this is not the normal operating mode of a
3174.
I get the impression that "normal" is highly subjective. Especially with
later
firmware on 3174s.
Yes, but a
3270 terminal does not talk 3270 protocol to the 3174....
ACK
Yes and the waters get muddied because the 3174
has had extra features
added along the way that allow it to be used in odd ways....
Agreed. I think I'm talking about things on the odder end of the spectrum.
--
Grant. . . .
unix || die
Dave