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KNOCK! KNOCK! "WHOýS THERE?"

Using UPnP to Respond to Inquiries
by Edward F. Steinfeld

Start ý UPnP Overview ý Functions ý Device Addressing, Discovery, and Description ý Control, Event Messaging, and Presentation ý UPnP Forum ý Software Tools ý Hardware ý Easier Deployment ý Sources and PDF

DEVICE ADDRESSING

The addressing layer is where control points and devices get their IP address. The addresses can come from a DHCP server or can use Auto IP to assign an IP address. Auto IP is a draft Internet Engineering Task Force (IETF) standard, Dynamic Configuration of Ipv4 Link-Local Addresses. Vendors can deliver UPnP on a variety of physical networks. The only requirement is that the network support IP.

DEVICE DISCOVERY

Searching for devices on the network or advertising "you are here" is the function of the discovery layer. When a device is added to a network, it advertises its presence by sending a multicast variant of HTTP (HTTPMU). UPnP starts with IP (see Figure 3). UDP is used for discovery because it is multicast, and TCP is used for description, control, and events.

Figure 3ýSome UPnP protocol stacks may be unfamiliar to network device implementers. To have automatic discovery and description, new protocols such as GENA, SSDP, and SOAP are used.

A control point will answer using a unicast variant of HTTP (HTTPU). A device uses HTTPMU to send a broadcast to the network that "Iým here," and a control point uses HTTPMU to send "Whoýs there?" Both use GENA (General Event Notification Architecture, an extension to HTTP) and SSDP (Simple Device Discovery Protocol). GENA and SSDP are defined by the IETF. HTTPMU is not part of the HTTP standard; it was created for use in UPnP.

In Photo 1, a control point (the laptop) was added to the network. To find out what UPnP nodes exist, the control point sends out an HTTPMU and each UPnP-enabled device responds with an HTTPU reply.

DEVICE DESCRIPTION

After a control point discovers a device, it can obtain a description from the device. The information sent by the device is expressed in extensible markup language (XML). XML is used throughout the UPnP implementation.

A description includes a device type, URLs for control and eventing, icons, a URL for presentation, as well as the manufacturerýs name, serial number, product code, and other similar information. Device types are defined by the UPnP Forum. Each device type has one or more templates to define the content and presentation of data.

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