After a network media device is discovered by Windows 7 and configured by the user, the device must be able to communicate with other devices on the network. To communicate effectively, a network device must comply with all UPnP protocols as specified in the DLNA 1.5 specification. The device description document must include the required device and service identifiers specified by DLNA. Specific network media device requirements can be found at the UPnP Web site.
Additional Metadata Requirements
To ensure that a network of several media devices provides the user with a great experience, Windows 7 requires some information in the device description document in addition to what DLNA requires. The additional elements enable the user to change the friendly name of the device and an indication of the device support for Wake on LAN (WoL).
The device description document for a network media device exposes a friendly name for the device. For the friendly name to be useful, the user must be able to change it. For example, the user might change the friendly name of a device to Living Room Player.
Under Windows 7, a user must be able to change the friendly name of a device by using a Web page that the device manufacturer provides. The URL of that Web page is specified in the presentationURL element of the device description document. The use of a presentationURL element to expose an HTML presentation page is described in the UPnP Device Architecture specification. The presentation page must be written in such a way that it can be parsed and displayed correctly by Windows Internet Explorer 7 or later versions.
Windows 7 requires that a network media device declare its support for WoL in the device description document. Windows uses magic packets to wake devices that are sleeping on the network. In addition, Windows uses this information to inform the user whether a device can be awakened.
Figure 21 shows how a user can determine whether a DMP or a DMR in the network supports waking the PC. If a network media device does not specify the magicPacketSendSupported capability in its device description document, the following informational text appears under the device listing: “This device cannot access your shared media if your computer is sleeping.”
Figure 21: The Share with Media Devices application in Control Panel indicating whether the device can access a sleeping PC
By knowing which network media devices cannot wake the Windows 7 PC when it idles to sleep, a user can determine which option to select in the Share with Media Devices application in Control Panel to Prevent my PC from sleeping so that all devices can access my shared media.
A device that supports being awakened by magic packets must include the following element in its device description document:
A device that does not support being awakened by magic packets must include the following element in its device description document:
A device that supports sending magic packets to wake a sleeping Windows PC must include the following element in its device description document:
A device that does not support sending magic packets to wake a sleeping Windows PC must include the following element in its device description document:
UPnP devices announce their presence and find other devices by using the Simple Service Discovery Protocol (SSDP).
The Windows 7 NSS announces itself as a UPnP MediaServer device by sending out two SSDP NOTIFY messages every 15 minutes. The NSS responds to any correctly formatted M-SEARCH action by searching for a UPnP MediaServer. Sample SSDP NOTIFY messages are shown in Appendix 1.
The NSS provides a manufacturer name, model name, and model number in its device description document. The model number for Windows Media Player differs, depending on the operating system. In Windows 7, the model number for Windows Media Player is 12.0, the manufacturer name is Microsoft Corporation, and the model name is Windows Media Player Sharing.
The NSS tries to discover compatible devices on the network by examining UPnP SSDP NOTIFY announcements or responses to M-SEARCH actions. For a device to be discovered, it must implement the UPnP MediaRenderer:1 device class. Compatible devices include the following:
UPnP Control Points
UPnP MediaRenderers are recognized by a combination of their media access control (MAC) address and a unique device name (UDN). When the NSS discovers a UPnP MediaRenderer device, the MAC/UDN pair of the device is compared to a stored list. If the MAC/UDN pair is not in this list, the NSS stores the MAC/UDN pair in the list and presents the device as a new device.
UPnP control points are recognized only by the MAC address. When the NSS discovers a control point device, the MAC address of the device is compared to a stored list. If the MAC address is not in this list, the NSS stores the MAC address for this device in the list but does not announce the device as a new device.
A network media device must a supply a unique device name (Udn0). A valid UDN value starts with the identifier uuid: followed by a 128-bit value in hexadecimal form. The ABNF syntax for the unique device name is defined as follows:
After a network media device is authorized, the device is automatically unauthorized if the MAC address for that device changes.
The Windows 7 NSS communicates only with devices on the same subnet as the computer on which NSS is running. If the IP subnet of the Windows 7 PC and the device are not the same, the device cannot browse, search, or play content.
Devices can check the source IP address of incoming SSDP announcements from the NSS to confirm that the device and the computer are on the same subnet and to warn the user if they are not.