17 Feb
2019
17 Feb
'19
7:45 p.m.
On 2/17/19 2:23 AM, Dave Wade wrote:
I would say the 3174 is acting more as a Terminal
Server rather than
a gateway. So 3270 CO-AX terminals which are directly attached to the
3174 can connect via TN3270 to another host. Often this is Hercules where
Hercules provides a a TN3270 server which is presented to the host as
a channel attached 3174...
I think we should clarify what "Terminal Server" and "Gateway" mean in
this context.
I was using "Gateway" to mean going between two different types of
networks, SNA / 3270 on one side and TCP/IP / Telnet on the other side.
I'm not sure how to define "terminal server". I would think that a
terminal server does what is needed to connect a terminal and drive a
terminal.
I think there's a significant overlap in those two terms and their
associated definitions.
Please correct me as you see fit.
A 3174 can have either a Token Ring or Ethernet
interface but not both.
(it can also have hdlc/sdlc/bi-sync and bus+tag)
The reading that I did last night agrees with that. But I don't see how
that's germane to this discussion.
The 3174 never acts as a telnet server but.........
A channel attached 3174 can be used to pass TCPIP into the mainframe and
there are products that run on the mainframe that act as Telnet/TN3270
servers. So in effect it acts as a channel attached network interface
for the mainframe.
Okay.
Maybe I'm supplementing a poor understanding of what a 3174 is with what
I /think/ a Cisco router with a Channel Interface Processor is. Namely
that the Cisco terminates the TCP/IP connection and generates a new 3270
based terminal connection into the mainframe. Note how there is no
TCP/IP passing through the Cisco + CIP into the mainframe.
Some 3174s have RS232 ports, so locally attached
serial terminals can
connect to the mainframe. This can be via Channels or via remote network
connectivity, SNA token ring, X.25, SNA over Ethernet.
ACK
It's my understanding that 3174s that connect to channels / ESCON /
FICON (via adaptation) are "Local" 3174s. They can have network
(Token-Ring / Ethernet / RS-232 / other) interfaces that are used to
talk to other "Remote" 3174s. The "Remote" 3174s then connect and
drive
local 3270 terminals which communicate across the network.
It depends on what is at the other end. If you are
going TCPIP/TN3270 then
normally you use a router. However if you are going SNA over LAN into
the mainframe, from what I remember SNA LAN protocols are non-soutable
so you need a bridge/gateway.
I think we just fell off the end of the continental shelf into the
ocean. - But IMHO this is fun. This is how we (I) learn new things. :-D
After having skimmed the PDF that Kevin linked to, I've mostly settled
on the following:
The 3174s speak TCP/IP on the downstream (grey) (Token Ring / Ethernet)
LAN / side. (I'm ignoring the protocols across the other circuits that
the 3174 supports between 3174s.)
There are two frame types that are supported on Token Ring, 802.2
Logical Link Control and 802.2 LLC with SubNetwork Access Protocol.
TCP/IP on Token Ring traditionally uses SNAP.
Conversely, there are four frame types that are supported on Ethernet;
LLC, SNAP, Ethernet v2, and 802.3 "Raw". TCP/IP on Ethernet
traditionally used Ethernet v2.
So, we have a discrepancy between the frame types that traditionally
carry TCP/IP on Token Ring and Ethernet. The easiest way to deal with
this is to use a router that uses the proper frame type for the
underlying network. But that's /routing/, and not bridging. Hence why
my qualm / uncertanty was "routing" vs "bridging".
I think it may be possible to have a piece of software / hardware
actually /bridge/ the IP packet or TCP datagram between SNAP and
Ethernet v2 (and vice versa). But I've /rarely/ heard of that being
done. Especially when you have MTU differences between Token Ring and
Ethernet that are difficult to deal with transparently.
I concede that data is being connected between the two networks much
like a bridge allows cars to connect between islands.
So, back to your statement. "TCPIP/TN3270 then normally you use a
router." Where does the TCP/IP connection that is the TN3270 traffic
terminate? Is it on the router? Or is it on the mainframe?
Please elaborate on what you mean by "SNA over LAN". What protocols
would that be seen as on the LAN client? NetBIOS? Something other than
TN3270 over TCP/IP? Are you referring to DLC? It's my understanding
that DLC is a point-to-point protocol and not usable on a LAN.
I ask because I did not see any reference to anything other than TCP/IP
in conjunction with the 3174 LAN interface.
I agree that NetBIOS is not routable.
If you run Hercules on a PC with a Token Ring card
then you don't need
anything.
How is the 3174 going to connect a 3270 terminal to Hercules running on
a PC via Token Ring if not TN3270?
Or is that the difference in terms that I was commenting about above.
Also, I would expect a 3174 with Ethernet to similarly be able to
connect a 3270 terminal to Hercules running on a PC via Ethernet.
Do a search on "CISCO SNA Token Ring
Bridging". Lots of papers on there.
Yep. As with all things mainframe related, it's complex and there are a
lot of options. The trick is finding the options that will work in the
various constraints.
Last time I fired this lot up I used the CISCO as an
IP router and then
used NAT In the CISCO hide the token ring from main network.
NAT on the 2500 series Cisco is decidedly routing and not bridging.
Now I have a 3174 with An Ethernet card and a P390
with a bigger selction
of Oss I have lots of options...
:-)
... but I am currently distracted by a pile of VAXen
~chuckle~
--
Grant. . . .
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