Document revision date: 19 July 1999

A service node is one type of node in a LAT network. (Nodes that are not running an OpenVMS operating system can also be used along with the OpenVMS nodes in a LAT network.) A service node is an individual computer in a LAN that offers its resources to users and devices. Because the OpenVMS operating systems contain the LAT protocol, any OpenVMS system can be configured as a service node within a LAT network.

24.2.1.1 Types of Services

Each node offers its resources as a service. Often, a node offers a general processing service, but it can offer limited services or special application services as well. Any or all of the services can be specialized applications.

For example, your service node might offer services for the following items:

• General processing
• Data entry
• Stock quotations

The general processing service would allow the use of the general computing environment. The data entry and stock services, on the other hand, would be restricted environments, with connections to the application service but to no other part of the service node.

Each service is distinguished by the name the system manager assigns to it. In an OpenVMS Cluster, Compaq recommends that the service name be the same as the cluster name. In an independent node, Compaq recommends that the service name be the same as the node name. With special service applications, the service holds the name of the application.

24.2.1.2 Service Announcements

A service node announces its services over the LAN at regular intervals so that terminal servers (and OpenVMS systems that allow outgoing connections) know about the availability of these network resources. The service announcement provides the physical node name, the service names, a description of services, and a rating of service availability. Servers listen to the LAN announcements and record information in a database. On nodes allowing outgoing connections, this database is maintained by the LAT Ancillary Control Process (LATACP). (See Section 24.7 for more information about managing the LATACP database.)

Whenever a user terminal or application program requests a service, the server node connects to the appropriate service node.

Note that you can disable a local node from multicasting service announcements by using the /NOANNOUNCEMENTS qualifier with the LATCP command SET NODE. However, because remote nodes must rely on the LAT service responder feature in the LAT protocol Version 5.2 (or higher) to connect to the local node, Compaq recommends that you use this qualifier only in a networking environment where newer model terminal servers and hosts are present (all LAT hosts, terminal servers, and PCs are running at least Version 5.2 of the LAT protocol). Otherwise, systems running versions of the LAT protocol prior to Version 5.2 (for example, DECserver 100, 200, and 500 systems) will be unable to connect to any of the systems that have LAT service announcements disabled.)

24.2.1.3 Print Requests

In some cases, service nodes can request services from terminal servers. The most common situation is when the system wants to use a printer that is connected to a terminal server port. The system submits the print request to the terminal server print queue that is set up and initialized in the OpenVMS startup procedure. Then the LAT symbiont (the process that transfers data to or from mass storage devices) requests the LAT port driver to create and terminate connections to the remote printer.

For information about setting up queues for printers connected to LAT ports, see Section 13.1.3 and Section 13.2.2.4.

24.2.2 Terminal Server Nodes

A terminal server node is the second type of node in a LAT network. A terminal server node is usually located near the terminals and printers it supports. The terminals and printers are physically cabled to the terminal server; the terminal server is physically connected to the LAN cable.

24.2.2.1 Locating Service Nodes

Terminal servers build and maintain a directory of services from announcements advertised over the network. Then, when terminal servers receive requests from terminal users, they can scan their service databases and locate the computer that offers the requested service.

Terminal servers not only look for the node that provides the requested service, but they can also evaluate the service rating of that node. If a requested service is offered by more than one node, then the service rating is used to select the node that is least busy. A server establishes a logical connection between the user terminal and the service node.

24.2.2.2 Setting Up Connections

One logical connection carries all the data directed from one terminal server node to a service node. That is, the server combines data from all terminals communicating with the same node onto one connection. A terminal server establishes a logical connection with a service node only if a logical connection does not already exist.

If a connection fails for any reason, a terminal server attempts to find another node offering the same service and "rolls over" the connection so users can continue their computing sessions.

Even though terminal connections are bundled together, each terminal can be uniquely identified by its name. A terminal name consists of two parts: the first part is the name of the port on the terminal server that the terminal line plugs into; the second part is the name of the terminal server node.

24.2.2.3 Servicing Nodes

Although terminal servers are usually the requesting nodes in a LAT network, sometimes service nodes request service from terminal servers. Most commonly, a service node queues print requests to remote printers connected to terminal servers.

24.2.3 Nodes Allowing Outgoing Connections

Nodes can be set up to allow incoming connections, outgoing connections, or both. Nodes (excluding those that offer incoming connections only) such as terminal server nodes can locate service nodes and set up connections. The database of information about available nodes and services is maintained by the LAT Ancillary Control Process (LATACP). (See Section 24.7 for more information about managing the LATACP database.)

On a node that is set up to allow outgoing LAT connections, a user can connect to another node in the LAT network by entering the SET HOST/LAT command. For more information, refer to the SET HOST/LAT command in the OpenVMS DCL Dictionary.

24.2.4 Components of a LAT Network

Figure 24-1 is an example of a LAT network. The network consists of an Ethernet cable connecting service nodes and terminal server nodes.

The three service nodes in Figure 24-1, named MOE, LARRY, and ALEXIS, each offer services to terminal server nodes on the network.

Two of the service nodes, MOE and LARRY, belong to the OFFICE cluster. (The cluster is distinguished by its computer interconnect [CI] and star coupler.) Because MOE and LARRY are clustered, their service names are the same as their cluster name. Because both service nodes offer an OFFICE service, terminal server nodes can assess the work load on both OFFICE nodes and establish a connection to a node that offers the service that is less busy.

The third service node, ALEXIS, is an independent node in the LAT network so its service name is the same as its node name.

In addition to its primary OFFICE service, node MOE offers an application service called NEWS. With this specialized service, user terminals can connect directly to the online news program, without any login procedure but also without general access to the general computer resources of the node.

The node FINANCE, shown in Figure 24-1, is a terminal server node; it supports a number of interactive terminals, a modem, and printers. The node PROCESSING allows outgoing connections; it supports several interactive terminals. The node FINANCE can accept print requests from any of the three service nodes, provided each service node has set up print queues to support remote printers on the terminal server.

Node PROCESSING is also a service node. It offers the service COMPUTE.

Figure 24-1 Components of a LAT Network

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.

24.3.1.1 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.

24.3.1.2 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.

24.3.2 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 24-2 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 24-2 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 24-3 shows two LANs bridged together. However, this configuration will have the same characteristics as the configuration shown in Figure 24-2.

Figure 24-3 Multiple Address LAT Configuration: Two LANs with Mixed Version LAT Nodes

In Figure 24-4, 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 24-4 Multiple Address LAT Configuration: Two LANs with Version 5.3 LAT Nodes

24.3.2.2 Unsupported Configuration

Figure 24-5 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 24-5 violates this rule. The configuration shown in Figure 24-4 is a valid alternative.

24.3.2.3 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.

24.3.2.4 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).

24.3.2.5 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.)

24.3.3 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 24-6 is an example of the configuration that can cause problems.

Figure 24-6 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

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