The Service Location Protocol (SLP) provides mechanisms whereby service agent clients can advertise and user agent clients can query for services. The design is very much demand-driven, so that user agents only obtain service information when they specifically ask for it. There exists another class of user agent applications, however, that requires notification when a new service appears or disappears. In the RFC 2608 design, these applications are forced to poll the network to catch changes.

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... this document, we describe a protocol for allowing such clients to be notified when a change occurs, removing the need for polling.