IBM Communications Server for AIX, V6 New Features and Implementation Scenarios

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 site’s 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 user’s 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 agent’s 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.