HP-UX IP Address and Client Management Administrator's Guide (October 2009)

empty. If the client wants to discover all DAs within its multicast radius, it does not include
any scope in the service request. DAs are required to reply with their service URLs and the
scopes that they support. The response to the service request message, however, is not a
service reply, but rather a DA advertisement. The service URL of a DA includes the host
name or IP address of the DA as shown in the following examples:
service:directory-agent://test.org service:directory-agent://15.10.20.2
• Passive Discovery In passive DA discovery, DAs multicast periodic unsolicited
advertisements of their services in case any SAs or UAs fail to receive the initial advertisement
sent by the DAs.
• Using DHCP Options for SLP DA A host that uses DHCP may use it to obtain a DA’s IP
address. The DHCP options, SLP Directory Agent, and SLP Service Scope can be used to
discover the DA.
Figure 1-7 illustrates how active and passive DA discovery work.
Figure 1-7 Active and Passive DA Discovery
The UA and SA at the top of the figure have initiated active discovery by multicasting a service
request (SrvRqst) with service type service:directory-agent. The DA at the bottom of
the figure responds by unicasting a DA advertisement (DAAdvert) with its URL and the list of
scopes that it supports. At the bottom of the figure, the DA periodically multicasts a DAAdvert
with its URL and scope list. The UA and SA listen on port 427 for the unsolicited DAAdvert and
add the DA to their list of active DAs.
How SLP Works
Consider the example of a user who needs to use the printer services on a network. The following
series of events occur at the server and the client side:
1. A printer initializes and registers its URL with the SA and the DA server.
2. Service Agents register current attribute information periodically, allowing User Agents to
ascertain their status and other characteristics.
58 Overview