Document revision date: 30 March 2001

This chapter describes how the LAT software works and the tasks you must perform to implement and manage the LAT software on your system.

Information Provided in This Chapter

This chapter describes the following tasks:

Task	Section
Starting up the LAT protocol	Section 26.5
Customizing LAT characteristics	Section 26.6
Creating a service	Section 26.6.1
Setting up ports	Section 26.6.2
Setting up printers	Section 26.6.2.1
Setting up special application services	Section 26.6.2.2
Enabling queued incoming requests	Section 26.6.3
Enabling outgoing LAT connections	Section 26.6.4
Managing the LATACP database size	Section 26.7

This chapter explains the following concepts:

Concept	Section
LAT protocol	Section 26.1
LAT network	Section 26.2
LAT configurations	Section 26.3
LAT Control Program utility	Section 26.4

26.1 Understanding the LAT Protocol

The operating system uses the LAT (local area transport) software to communicate with terminal servers and other systems within a local area network (LAN). Terminal servers are communication devices dedicated for connecting terminals, modems, or printers to a LAN. They offer the following features:

• Provide a cost-effective method of connecting many user terminals to a computer
• Save on cable requirements
• Maximize the number of devices that can access a computer

With the LAT software, which implements the LAT protocol, the operating system can offer resources, or services, that the terminal servers can access. A system that offers LAT services is called a service node. In addition, nodes can access LAT services by enabling outgoing connections (using LATCP) and using the DCL command SET HOST/LAT. (In the remainder of this chapter, "servers" refers both to dedicated terminal servers and to nodes that allow outgoing access to other LAT services.)

A LAT service can consist of all the resources of a computer system, or it can be a specific resource on a computer system, such as an application program. You can set up your system as a general timesharing service, meaning that all of its resources are available to users in the LAN, or you can restrict access to a specific service (application program) on the system. This chapter and the OpenVMS I/O User's Reference Manual outline the procedure you use to set up access to a dedicated application program.

26.1.1 How the LAT Protocol Works

The LAT protocol allows the terminal servers and computers to communicate within a LAN, such as the Ethernet or the Fiber Distributed Data Interconnect (FDDI). The LAT protocol matches terminals and other devices to the computing resources (services) of the LAN. Because LAT terminals are not connected directly to the computer (service node) they are accessing, the local server must listen for service requests from its terminals and be able to match the terminals with computers that provide the desired services.

Using the LAT protocol, then, the operating system announces its available services over the LAN. Servers listen to the LAN announcements and build a database of service information so that they can locate an appropriate system when a user terminal requests computing services. For example, a user terminal might request general processing service or a data entry program on the operating system. A server uses the LAT protocol to establish and maintain a connection between the requesting terminal and the operating system.

Sometimes the operating system can request services from a terminal server. The LAT protocol allows systems to ask for connections to printers or other devices attached to a terminal server.

26.1.2 Advantages of the LAT Protocol

Using the LAT protocol on your system has many advantages:

• The LAT protocol lets you make the resources of any computer on a local area network available to any user in that network.
• In addition to general processing resources, you can set up terminals, printers, and modems so they are available from multiple systems in the local area network. This lets you efficiently use these resources and keep them available even if one of the systems in the network must be shut down.
• You can also set up application programs, such as data entry programs or news services, as resources. When a user requests a connection to the resource, the LAT protocol sets up a connection directly to the application program. No login procedure is necessary.
• The LAT protocol provides load balancing features and recovery mechanisms so users get the best, most consistent service possible. In their broadcast messages, systems rate the availability of their services so that servers can establish connections to computing resources on the least busy node. If a node becomes unavailable for any reason, the servers attempt to provide access to alternate services.
• Users can establish multiple computing sessions on their terminals, connecting to several different computers and switching easily from one computing session to another. After switching from one session to another, users can return to the previous session and pick up where they left off. This saves users the time normally required to close out and reopen files or accounts and to return to the same point in a session.
• Finally, the LAT protocol can provide improved system performance. Because the servers bundle messages onto a single LAN interface, a server interface decreases the network traffic and reduces the number of computer interrupts realized in systems where terminals, modems, and printers each have a physical connection to the computer.

A LAT network is any local area network where terminal servers and operating systems use the LAT protocol. A LAT network can coexist on the same LAN with other protocols. The LAT protocol, which operates on both terminal servers and the operating systems, is designed to ensure the safe transmission of data over the LAN.

The LAT network consists of the following components:

Component	For More Information
Service nodes	Section 26.2.1
Terminal server nodes	Section 26.2.2
Nodes allowing outgoing connections	Section 26.2.3
LAN cable	Section 26.2.4

Service nodes supply computing resources for the local network, while terminal server nodes (or nodes allowing outgoing connections) port their terminals, modems, or printers to those resources upon request from a user terminal or an application program.

Note that in a LAT network, nodes that access services are often referred to as master nodes, which distinguishes them from nodes that only provide services.

You can use the LAT Control Program (LATCP) to configure the LAT characteristics for your system. LATCP allows you to set up your system to support:

• Incoming access only
• Outgoing access only
• Both incoming and outgoing access

The systems that support incoming LAT connections are service nodes. (Using LATCP, you can also set up your system so that it supports neither incoming nor outgoing access.)

26.2.1 Service Nodes

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.

26.2.1.1 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.

26.2.1.2 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 26.7 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.)

26.2.1.3 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 14.1.3 and Section 14.2.2.4.

26.2.2 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.

26.2.2.1 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.

26.2.2.2 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.

26.2.2.3 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.

26.2.3 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 26.7 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.

26.2.4 Components of a LAT Network

Figure 26-1 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 26-1, 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 26-1, 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 26-1 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.

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

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