Chapter 4. Service Location Protocol (SLP) 57
For example, a client that specifies a scope of TSO* can select between
servers that advertise scopes of TSOTEST and TSOPROD.
4.2 SLP review
The basic operation in Service Location Protocol is that a client attempts to
discover the location of a service. In smaller installations, each service will be
configured to respond individually to each client. In larger installations,
services will register their services with one or more directory agents, and
clients will contact the directory agent to fulfill requests for Service Location
information.
A large network can be divided and categorized by the use of scopes, so that
information about a TN server is advertised only to TN3270 clients and
directory agents that have the same scope as the TN server. This allows you
to control the range of service searches.
The following describes the operations a user agent would employ to find
services on the sites network. The user agent needs no configuration to
begin network interaction. The user agent can acquire information to
construct predicates that describe the services that match the users needs.
The user agent may build on the information received in earlier network
requests to find the service agents advertising service information.
A user agent will operate two ways:
1. If the user agent has already obtained the location of a directory agent, the
user agent will unicast a request to it in order to resolve a particular
request. The directory agent will unicast a reply to the user agent. The
user agent will retry a request to a directory agent until it gets a reply, so if
the directory agent cannot service the request it must return an response
with zero values, possibly with an error code set.
2. If the user agent does not have knowledge of a directory agent or if there
are no directory agents available on the site network, a second mode of
discovery may be used. The user agent multicasts a request to the
service-specific multicast address, to which the service it wishes to locate
will respond. All the service agents that are listening to this multicast
address will respond, provided they can satisfy the user agents request.
A directory agent acts on behalf of many service agents. It acquires
information from them and acts as a single point of contact to supply that
information to user agents.

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