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Implementing the Service Location Protocol 395
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Figure 15-1
SLP User Agent and Service Agent Interaction
When a client application queries the User Agent for a network service, the User Agent in search of
service information multicasts a Service Request. The Service Agent receives the Service Request
and consults its local service cache to see if it holds a service matching the criteria of the Service
Request. If so, the Service Agent containing the requested service information unicasts a Service
Reply to the User Agent.
If multiple Service Agents reply, the User Agent combines the results before returning them to the
client application. This same scenario occurs for Service Type and Attribute Requests. When the
network service is terminated, it deregisters its service with the Service Agent, which deletes the
service from its local service cache. The Service Agent remains silent.
15.2.2 SLP with a User Agent, Service Agent, and Directory
Agent
Figure 15-2 on page 396 illustrates how Service Agents and User Agents interact with Directory
Agents to advertise and locate network services. When a network application is started, it registers
its service URL and attributes with the Service Agent. The Service Agent maintains its own copy of
the service information and then unicasts a Service Registration (that has the new service
information) to the Directory Agent. The Directory Agent saves the service information in its local
service cache.