Document revision date: 15 July 2002

When you set up a LAT system, you need to understand the relationship between the LAT software and the network so you can configure your system to operate efficiently. The following sections contain information that will help you understand the following concepts:

• The relationship between LAT software and OpenVMS Cluster software
• The relationship between LAT software and DECnet software
• How the LAT software works in a networking environment that uses multiple LAN adapters
• How to use the LAT software in an Ethernet/FDDI configuration that cannot use large buffers

Although the LAT protocol works independently of OpenVMS Cluster software, Compaq recommends that you configure a service node to complement the OpenVMS Cluster concept. You achieve this by creating a service on each node in an OpenVMS Cluster and assigning the cluster name to this service. A terminal server assesses the availability of cluster services and establishes a connection to the node that is least busy. Thus, the LAT protocol helps balance the cluster load. If one node in the cluster fails, the terminal server can transfer the failed connections to another service node within the cluster.

The LAT software does not use DECnet as a message transport facility, but instead uses its own virtual circuit layer to implement a transport mechanism. The LAT and DECnet software work independently in a common LAN environment. For compatibility, if a service node is also a DECnet node, the service node name should be the same as the DECnet node name.

26.3.1.1 LAT and DECnet Running on the Same Controller

If Ethernet ports will be running both DECnet and LAT, you must start the DECnet software before the LAT software. If you do not start DECnet software first, all existing LAT connections may terminate, and reconnecting to the system via LAT may not be possible.

26.3.1.2 LAT and DECnet Running on Different Controllers

If DECnet is configured on the system (or if the system is part of a cluster), the SCSSYSTEMID system paramater may contain a nonzero value. Normally, this is not a problem unless the system has two or more LAN controllers connected to the same logical LAN.

For example, if your system has an FDDI controller and an Ethernet controller, your site may be configured so that the FDDI ring attached to the FDDI controller and the Ethernet segment attached to the Ethernet controller are bridged by a 10/100 LAN bridge (FDDI-to-Ethernet). In this configuration, it is impossible to run LAT over both controllers.

In such a configuration, you must run LAT and DECnet over the same controller if SCSSYSTEMID is not 0. If they do not run on the same controller, DECnet starts first, which in turn causes the LAT startup on the other controller to fail. This failure occurs because LAT startup tries to use the AA-00-04-00-xx-xx address (the DECnet LAN address); however, because DECnet is already using this address on another controller, the data link layer prevents the LAT startup from using that address. (In a single logical LAN, all data link addresses must be unique. Because both controllers try to use the same address, it is no longer unique.)

Using the following command to create the LAT link also fails because the LAN driver tries to use the address based on SCSSYSTEMID:

LATCP> CREATE LINK LAT$LINK_2 /NODECNET

If SCSSYSTEMID is set to 0, configuring LAT and DECnet on different controllers is possible. However, in a cluster environment, SCSSYSTEMID cannot be set to 0.

26.3.2 Using Multiple LAN Adapters

When you use multiple LAN addresses for one LAT node, you can configure a system with multiple LAN adapters connected to the same logical LAN. The LAT software can run over each adapter simultaneously and can better maintain connections. For example, when a virtual circuit chooses a primary path and uses it for all LAT message transmissions, the LAT software can continue communications through another adapter or logical path if that original path becomes blocked.

Although it is possible to run LAT over multiple LAN adapters, it is still not possible to route LAT from one logical LAN to another. The following examples show supported LAT configurations for nodes running Version 5.3 of the LAT protocol (including nodes running Version 5.2 and 5.1 as well).

The widely used configuration shown in Figure 26-2 has an OpenVMS system running LAT Version 5.3 software over two Ethernet adapters (labeled A and B in the diagram) connected to the same physical LAN as a DECserver 200.

Figure 26-2 Multiple Address LAT Configuration: One LAN with Mixed-Version LAT Nodes

When a LAT connection is started between the DECserver 200 and the OpenVMS system, the LAT software determines that it is possible to use both adapters A and B for the LAT virtual circuit. One of the adapters will be chosen as the primary communications path while the other will be present in the unlikely event that the primary path fails.

For example, if a user connects to the OpenVMS system from the DECserver 200, the OpenVMS system determines that there are two paths but chooses adapter B as the primary communications path. If the user runs a program that generates a large amount of output from the OpenVMS system and adapter B fails in some manner during the output, the LAT software will attempt to continue communications from the OpenVMS system to the DECserver through adapter A.

Figure 26-3 shows two LANs bridged together. However, this configuration will have the same characteristics as the configuration shown in Figure 26-2.

Figure 26-3 Multiple Address LAT Configuration: Two LANs with Mixed Version LAT Nodes

In Figure 26-4, any virtual circuit created between the two OpenVMS systems will have two paths: through controllers B and C or A and D. If one path fails, the virtual circuit will continue over the other path. If both paths fail, the virtual circuit will eventually time out.

Figure 26-4 Multiple Address LAT Configuration: Two LANs with Version 5.3 LAT Nodes

26.3.2.2 Unsupported Configuration

Figure 26-5 Unsupported Multiple Address LAT Configuration

Any configuration similar to this diagram will result in unpredictable results and may not function. In a network environment, LAT Version 5.1 and 5.2 nodes can have only a single logical LAN address. The configuration in Figure 26-5 violates this rule. The configuration shown in Figure 26-4 is a valid alternative.

26.3.2.3 Creating Logical LAT Links

The LAT software regards all paths as equal, point-to-point communications. The LAT software can support a maximum of eight LAN adapters simultaneously (and it is possible to connect all controllers to the same logical LAN). To get the maximum coverage over possible path failures, each logical link should be created prior to setting the LAT node state to ON in SYS$MANAGER:LAT$SYSTARTUP.COM.

For example, if a system has one Ethernet adapter (device ESA0) with two FDDI adapters (FCA0 and FCB0) and the system manager chooses to run LAT over all adapters, the LAT$SYSTARTUP.COM file would contain the following commands:

$! $! Create each logical LAT link with a unique name and $! unique LAN address (forced with /NODECNET). $! $ LCP CREATE LINK ETHERNET /DEVICE=ESA0 /NODECNET $ LCP CREATE LINK FDDI_1 /DEVICE=FCA0 /NODECNET $ LCP CREATE LINK FDDI_2 /DEVICE=FCB0 /NODECNET $! $! Turn on the LAT protocol. $! $ LCP SET NODE /STATE=ON

It is possible to create logical LAT datalinks after the LAT protocol has been started. Any existing virtual circuit will attempt to find any new paths through the newly created logical datalink when it is ready for use. However, Compaq does not recommend that you create links at this point because during the time it takes existing virtual circuits to discover new paths through this newly created datalink, the virtual circuit may fail before the new paths are discovered.

26.3.2.4 Path Discovery

The OpenVMS LAT software uses a combination of the directory service and solicitation to obtain paths for each virtual circuit. To expedite path discovery at virtual circuit startup, Compaq recommends that you configure a system with multiple LAN adapters to maintain a LAT service and node database, by performing the following actions:

• Enabling outgoing LAT connections
• Using the same group code mask for User Groups and Service Groups

An OpenVMS system running with outgoing connections disabled and no service and node database is still capable of running with multiple paths for each virtual circuit. These paths must be discovered through the LAT solicitation process and will take longer (leaving the possibility for virtual circuit failure to occur before all paths have been discovered).

26.3.2.5 Modifying LAT Parameters

In the unlikely event of a path failure, it will take the OpenVMS LAT software time (which will vary depending on the number of adapters to which the remote node has access) to locate another working path. Therefore, Compaq strongly recommends that you modify the following LAT parameters on potential LAT master nodes:

• Retransmit limit - default value is 8. Set to the maximum number of LAN adapters times 8. For example, if an OpenVMS system on the LAN has 3 adapters, each LAT master node should have their retransmit limit set to 24 (3 * 8).
• Keepalive timer - default value is 20 seconds. While the default value may be sufficient in most circumstances, it may be necessary to increase this to 30 or 40 seconds.

Although it is possible to keep virtual circuits running through multiple adapters to LAT Version 5.1 or LAT Version 5.2 master nodes, there is still a possibility that the connections to these nodes may fail.

LAT Version 5.2 and LAT Version 5.1 master nodes do not have the ability to recognize multiple paths to LAT nodes that provide services. They can only communicate with such nodes through one remote address at a time. Therefore, if a LAN path failure occurs when a LAT master node running LAT Version 5.1 or Version 5.2 attempts to connect to a remote LAT Version 5.3 node providing services, the LAT Version 5.3 node might not discover this failure in time and the LAT master node may time out the connection. You can partially solve this problem by increasing the retransmit limit to as high a setting as possible.

In addition, if a LAT Version 5.3 node providing services views the virtual circuit as completely idle during the primary path failure, no attempt will be made to use any of the alternate paths (because of the previously described LAT Version 5.2 and 5.1 limitation). Therefore, although multiple LAN adapters will work with older LAT implementations, you might still need to upgrade to Version 7.0 or higher of the OpenVMS operating system to obtain the LAT Version 5.3 protocol, which will correct this type of problem. (Note that this type of problem affects only those connections that are idle. An example of where this situation could arise is in an office environment if all users were to leave their systems at the same time, either at lunchtime or at the end of the workday.)

26.3.3 Large Buffers in Ethernet/FDDI Configurations

The OpenVMS LAT software will attempt to use large buffers over any virtual circuit that comes in over an FDDI controller. This feature can cause problems if an alternate virtual circuit path must go through an Ethernet. Figure 26-6 is an example of the configuration that can cause problems.

Figure 26-6 LAT FDDI Ring and Large Buffers

In this diagram, it is possible for the two OpenVMS systems to communicate using large packets through the path described by controllers B and C. Large packets may exceed 1500 bytes of data (the maximum Ethernet message can contain 1500 bytes of data). If the path described by controllers B and C were to fail, it will not be possible for communication to continue through the path described by A and D.

The path described by controllers A and D pass through an Ethernet LAN segment. The messages that are routed through the 10/100 bridges cannot be larger than the maximum Ethernet message. Problems can occur because the OpenVMS LAT software cannot always detect this kind of configuration.

There are two ways to prevent problems with the previously described configuration. The first and easiest option is to create a logical LAT link using an Ethernet adapter (if either system has an Ethernet LAN adapter). This will force the message size negotiation to be no larger than the maximum sized Ethernet message.

If neither system has an Ethernet controller (thus making the first option not possible), the second option is to override the use of large buffer support (which is enabled by default) by using the new LATCP command qualifier, /[NO]LARGE_BUFFER. For example:

$ MCR LATCP SET NODE/NOLARGE_BUFFER

Compaq recommends that you use the SET NODE/NOLARGE_BUFFER command after all logical LAT links have been created and before the LAT node has been turned on. For example, note the order of the commands in LAT$SYSTARTUP.COM:

$! $! Create each logical LAT link with a unique name and $! unique LAN address (forced with /NODECNET). $! $ LCP CREATE LINK FDDI_1 /DEVICE=FCA0 /NODECNET $ LCP CREATE LINK FDDI_2 /DEVICE=FCB0 /NODECNET $! $! Don't use large buffer support (force packet $! sizes to be no larger than what Ethernet can $! support). $! $ LCP SET NODE /NOLARGE_BUFFER $! $! Turn on the LAT protocol. $! $ LCP SET NODE /STATE=ON

26.4 Understanding the LATCP Utility

The LAT Control Program (LATCP) utility configures and controls the LAT software on OpenVMS systems. LATCP commands let you stop and start the LAT driver (which implements the LAT protocol) and modify or display LAT characteristics of the OpenVMS node.

With LATCP, you can set up your system as a service node, which offers one or more resources (services) for access by users on other systems in the local area network (LAN).

You can also use LATCP to set up the system to allow its users to access services on other systems in the LAN. In this case, the system can act like a terminal server: it can manage multiple user sessions simultaneously for connections to services on other nodes.

You can use LATCP to set up your system to support incoming access only, outgoing access only, or both incoming and outgoing access. You can also set up your system so that it supports neither incoming nor outgoing access.

When you set up your system to support outgoing access, the LAT software manages a database of LAT services and nodes. The software builds the database when you enable outgoing access on your node. The software begins to collect LAT service announcements---multicast messages sent by LAT service nodes---and builds the database based on these service announcements. You can use LATCP to display the services and nodes in this database and to control the size of the database. Allow outgoing access on systems that can tolerate the additional overhead, such as standalone systems.

Use LATCP to perform the following actions:

• Specify operational characteristics for your node and its services
• Turn the state of the LAT port driver (LTDRIVER) on and off
• Display the status of LAT services and service nodes in the network
• Display the status of links created on your LAT node
• Display the status of your LAT node
• Show and zero LAT counters
• Create, delete, and manage LAT ports
• Recall previously entered LATCP commands so that you can execute them again without having to reenter them
• Create subprocesses so that you can execute DCL commands without exiting from LATCP

With the LAT protocol, you can set up LAT application ports on the local node so that users can access printers and other asynchronous devices that are connected to LAT terminal servers or service nodes on the LAN. The remote devices must be configured appropriately.

26.4.1 Invoking and Exiting LATCP

Enter the following command to invoke LATCP:

$ RUN SYS$SYSTEM:LATCP LATCP>

At the LATCP> prompt, you can enter LATCP commands. To exit LATCP, enter EXIT or press Ctrl/Z at the LATCP> prompt.

You can also execute a single LATCP command by using a DCL string assignment statement, as shown in the following example:

$ LCP :== $LATCP $ LCP SET NODE/STATE=ON

LATCP executes the SET NODE command and returns control to DCL.

26.4.2 LATCP Commands

Table 26-1 summarizes the LATCP commands.

Table 26-1 LATCP Commands

Command	Function
ATTACH	Transfers control from your current process to the specified process.
CREATE LINK	Creates LAT data links.
CREATE PORT	Creates a logical port on the local node.
CREATE SERVICE	Creates a service on a service node.
DEFINE/KEY	Assigns a command string to a function key on your keypad.
DELETE LINK	Deletes a LAT data link from a node.
DELETE PORT	Deletes an application port or dedicated port.
DELETE QUEUE_ENTRY	Deletes an incoming queued request from the local node.
DELETE SERVICE	Deletes a service on a service node.
EXIT	Returns the user to DCL command level.
HELP	Displays help text for LATCP commands.
RECALL	Recalls LATCP commands that you entered previously so that you can execute them again.
REFRESH	Refreshes your display screen, for example, after your display has been overwritten by output from some other source.
SCROLL	Allows you to retrieve information that has scrolled off the screen.
SET LINK	Modifies characteristics of LAT data links.
SET NODE	Specifies LAT characteristics for a node.
SET PORT	Maps a logical port on a node to either a remote device on a terminal server or a special application service on a remote LAT service node.
SET SERVICE	Changes service characteristics.
SHOW LINK	Displays the characteristics of links on your node.
SHOW NODE	Displays the characteristics of nodes.
SHOW PORT	Displays port characteristics.
SHOW QUEUE_ENTRY	Displays information about requests, or entries, queued on the local node.
SHOW SERVICE	Displays characteristics of LAT services known to your node.
SPAWN	Creates a subprocess.
ZERO COUNTERS	Resets the node counters, service counters, and link counters maintained by your node.

For detailed information about LATCP commands and qualifiers, refer to the OpenVMS System Management Utilities Reference Manual.

	privacy and legal statement
6017PRO_103.HTML