Operating System - OpenVMS
cancel
Showing results for 
Search instead for 
Did you mean: 

LAT communication over WAN

 
SOLVED
Go to solution
Geoff Hess
Advisor

LAT communication over WAN

I have a VAX/VMS system running VMS V6.2 and DECnet Phase IV. The VAX is performing a disaster recovery role for a production VMS system. There is currently DECnet and TCP/IP connectivity between the two systems over a WAN. I'm trying to establishing LAT communications over the WAN between the DR VAX and LAT terminals servers located at the site of the production VMS system. TSM output on the DR VAX shows a status of Unavailable for the terminal servers. (The terminal servers have been configured properly via DSV$CONFIGURE.COM.)

The attachment shows TSM output on the DR VAX followed by TSM output on the production VAX.

Is there anything that I might be missing or anything that I can check to help diagnose the problem? My network engineering contact tells my that the Cisco router on the DR VAX end should be tunneling the LAT packets between the DR VAX and the remote LAN, but I'm not 100% sure that this is accurate. Any help would be greatly appreciated!
18 REPLIES 18
Highlighted
Jess Goodman
Esteemed Contributor

Re: LAT communication over WAN

Sorry, but the LAT protocol is not routeable so it can not be used over a WAN unless you set up a VPN for it.
I have one, but it's personal.
Robert Gezelter
Honored Contributor

Re: LAT communication over WAN

Geoff,

My solution would be to use an analyzer (e.g., WireShark) to check the actual transmissions between the systems.

That said, LAT over Wide Area Network connections is somewhat problematical because of issues related to propagation delay and the LAT protocol.

I have seen many situations with various protocols where misconfigured networks and routers have failed to forward packets in one direction or the other.

- Bob Gezelter, http://www.rlgsc.com
EdgarZamora_1
Respected Contributor

Re: LAT communication over WAN

Whenever I have this problem I usually ask the network guy if the IP helper is configured properly on the router (between the two addresses).
Geoff Hess
Advisor

Re: LAT communication over WAN

Guys, need to understand this better...
Several years back at the production site our VMS production system was communicating with DECserver 200s located across town via a T1 line. So first question: We must have been bridging LAT through our Cisco routers right?

Second question: Please explain the propagation delay issue. This is of concern because the DR VAX would be communicating with a PLC in which the normal production request/response time is < 320 milli-seconds. The production VAX has no problem maintaining reliable communcations, but I do need to determine how fast the remotely located DR VAX can communicate with this same device.


Robert Gezelter
Honored Contributor

Re: LAT communication over WAN

Geoff,

The LAT protocol has some timeouts that are reasonable in a LAN environment, but not necessarily workable in a WAN environment, particularly when there are any line errors. I do not have the citation handy, but it was a fairly well known fact when LAT was more heavily used.

Today, I would be tempted to use a telnet connection to the host in place of a LAT connection where there is potential for wide area access,

These problems would show up as dropped connections, not failure to see the services. Those are most likely caused by packets not reaching the receiver node.

- Bob Gezelter, http://www.rlgsc.com
Steven Schweda
Honored Contributor

Re: LAT communication over WAN

> We must have been bridging LAT through our
> Cisco routers right?

Right. For LAT, bridging works, routing
doesn't.

> Second question: Please explain the
> propagation delay issue.

I don't remember the fine print (probably
because I never knew it), but LAT has some
sensitivity to latency. That said, once,
upon a time long ago, I was at one end of a
9600b/s leased line between CA and MN with a
(forgotten brand, model) bridge+router at
each end. We never relied on LAT working
over that link, but it always worked when I
tried it (just fooling around, no serious
testing).
Bill Hall
Honored Contributor

Re: LAT communication over WAN

Geoff,

TSM uses MOP, not LAT, to communicate with your terminal servers. MOP (Maintenence Operations Protocol) is a non-routable protocol and has to be bridged or tunneled across a WAN link.

MOP uses three packet types, one for loopback testing, one for dump and load functions and one for remote console. The packet tyes to be bridged are 90-01, 60-01 and 60-02 respectively.

If you are using older DECservers that do not contain flash memory to load their software, you had better bridge MOP or get additional load hosts installed at your primary site. Otherwise if the VAXes are down at the primary site and the terminal servers are rebooted or loose power for what ever reason, they'll never get a down-line load. Your DR plan has a hole in it.

Bill
Bill Hall
Geoff Hess
Advisor

Re: LAT communication over WAN

>>Right. For LAT, bridging works, routing
>>doesn't.

>>TSM uses MOP, not LAT, to communicate with your terminal servers.
>>MOP (Maintenence >>Operations Protocol) is a non-routable >>protocol and
>>has to be bridged or tunneled across a WAN link.

So, IF LAT bridging (or possibly tunneling) is set up correctly, how do I verify that it's working, if not through TSM. It sounds like MOP also needs to be bridged or tunneled.

>>If you are using older DECservers that do not contain flash memory to
>>load their software, you had better bridge MOP or get additional
>>load hosts installed at your primary site. Otherwise if the VAXes are
>>down at the primary site and the terminal servers are rebooted or loose
>>power for what ever reason, they'll never get a down-line load.
>>Your DR plan has a hole in it.

Good catch. The original plan has been to relocate the DR VAX to the LAN where the "event" has taken place. (If a LAN still exists of course...) The DR VAX has the terminal server software on it and can function as a load host. Only recently was I asked to test LAT across the WAN.
Bill Hall
Honored Contributor
Solution

Re: LAT communication over WAN

Geoff,

$MCR LATCP SHOW SERVICE will show you the LAT service messages that the DR system can "hear" on the network. If you see your production server's LAT service you should be able to $SET HOST/LAT if outgoing connection are enabled.

"Outgoing connections" have to be enabled in LATCP. Outgoing connections can be enabled by $MCR LATCP SET NODE /CONNECTIONS=BOTH (both means incoming connections, the default and outgoing connections). This has to be enabled at every startup.

Bill
Bill Hall