ons © 2009 by Taylor & Francis Group, LLC Home Networking System with SIP Root device Service Service Service Control point Embedded Embedded device device Service Service Service Service State variable Action State variable Action FIGURE 7.1 Structure element in UPnP. (From Nakamoto, Y. and N. Kuri, Proc of 6th IEEE Int Conf Comp Info Tech. 2006, p. 99. With permission.) Root device 1 Service (2) description Service (1b)discovery:search 0)addressing network (1a)discovery:advertisement Root device 2 Control point (2)description FIGURE 7.2 Addressing, discovery and description in UPnP. (From Nakamoto, Y. and N. Kuri, Proc of 6th IEEE Int Conf Comp Info Tech, 2006, p. 99. With permission.) that may result in a change of state variable values. UPnP defines protocols and conventions for the following operations by which a control point accesses and controls devices (see Figure 7.2). (0) addressing: When a device joins the home network, it obtains a unique network address. (1) discovery: Discovery defines how a device announces its presence on the network and how control points discover it. When a device joins the network, it advertises its participation (advertisement). The device multicasts specific device unique identifier (uid), types of the device and services to devices in the home network. A device can be discovered by control points and learn about the device's capabilities by retrieving a device description. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security A control point, once added to the network, may search for particular devices or services (search). A search may be specified in terms of a specific device uid, a particular service type, or a particular device type. Discovery messages are sent to devices through an IP (Internet Pro- tocol) multicast server in the home network. Hypertext Transfer Pro- tocol/Multicast UDP (HTTP/MU), Simple Service Discovery Protocol (SSDP), and General Event Notification Architecture (GENA) are used in the discovery phase. The message 