Digital Equipment RapidPrint 500, Reference Manual

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LAT Concepts
2 - 4
2.4 LAT
Digital Equipment Corporation’s LAT (Local Area Transport) networking software is designed to
ease the process of accessing and managing local area networks. LAT software is built around the
concept of services. A service may be provided by a dedicated device, such as a printer, or by a
network host. A device that offers one or more services, such as the RapidPrint 500, is a node.
In general, all services offered by the RapidPrint 500 are associated with a port; exceptions to this
rule will be noted later.
Nodes advertise their services to the network by broadcasting occasional messages about them.
These messages, referred to as multicasts, contain the node’s name and its list of services. By mon-
itoring multicast messages, all hosts on the network know what nodes and services are available
and can provide this information to their interactive users. The Show Services and Show Nodes
commands display this information.
LAT multicast messages contain a rating for each service offered. Ratings range from 0 to 255; 0
means the service is unavailable, while 255 means the service is available and has no current users.
Ratings for a given service may change over time. For example, the rating for a computer accept-
ing logins will generally change as its workload changes. Conversely, ratings for a modem are typ-
ically either 0 (in use) or 255 (not in use). In the example above, the server with eight modems
attached will continue to advertise that the service “modem” is available (a 255 rating) until all
eight modems are in use (a 0 rating).
Service ratings may concern even casual users, since they are used to determine which service a
user will be connected to whenever there is a choice. For example, if a user types
Connect Hub
and five nodes offer service hub, the user will be connected to the least busy node automatically.
In the case mentioned above, where both the local RapidPrint 500 and a remote node offered the
same service, the ratings determine which node will service the connect attempt.
The other major network management feature of LAT is the concept of service groups. The Rapid-
Print 500 parallel port and each service on the network can be thought of as belonging to one or
more groups. When a user or device requests a service connection, the LAT host will check to see if
the groups to which the requester belongs match those of the requested service. If any group num-
ber is common to both the requester and the service, the connection attempt continues. If there are
no common group numbers, the connection attempt fails. Note that there may be additional access
restrictions on the service, such as password protection.
NOTE: See the Set/Define Port Authorized Groups and Set/Define Server Service Groups
commands for more information.
Group numbers also are useful to nodes because each node only needs to pay attention to multi-
casts that involve its users’ groups. As a result, groups can hide services that would otherwise be
visible. The server manager can also hide services from a set of ports. Setting up and managing
services and groups is discussed in more detail in the Server Configuration chapter.
LAT is significantly different from other protocols in two important ways. First, LAT is not
routable. There is no way to divide LAT networks into smaller subnetworks and use routers to
reduce traffic between nodes. Second, LAT is a timer-driven protocol. Packets are expected at cer-
tain intervals, and the protocol cannot adapt to slow network links dynamically. For these reasons,
LAT traffic over wide areas is typically carried inside (or encapsulated in) TCP/IP or IPX/SPX
packets. The latter two protocols are fully routable, and can handle wide-area, slow network links.
NOTE: If LAT is bridged across slow links, session time-outs and errors are very likely.