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building blocks of UPnP home media network

The basic building blocks of a UPnP network are devices, services and control points.

Devices: A UPnP device is a container of services and nested devices. For instance, a VCR device may consist of a tape transport service, a tuner service, and a clock service. A TV/VCR combo device would consist not just of services, but a nested device as well. Services: The smallest unit of control in a UPnP network is a service. A service exposes actions and models its state with state variables. Control points: A control point in a UPnP network is a controller capable of discovering and controlling other devices. After discovery, a control point could:

Retrieve the device description and get a list of associated services
Retrieve service descriptions for interesting services
Invoke actions to control the service
Subscribe to the service's event source. Anytime the state of the service changes, the event server will send an event to the control point

It is expected that devices will incorporate control point functionality (and vice-versa) to enable true peer-to-peer networking.