Multicast

DPs also provide multicasting, which enables transporting a single stream of data over a network. Clients can then subscribe to this stream of data. The main advantage of multicasting over the traditional unicast model is using a single stream of data for multiple destination systems. Unicast communication requires a separate stream of data for every client system.

A downside for many multicast implementations for image deployment is you must manually coordinate the start of the data stream. All client systems you want to receive the stream must be waiting for the stream prior to its being sent. With WDS multicasting in Windows Server 2008, Microsoft implemented a catch-up feature. This enables clients that joined a stream midway through to continue to receive the entire stream.

WDS tracks when clients join the image stream and replays the stream until all clients subscribed to the stream have received the entire image.

Multicasting is used only for image deployment in OSD; it is not used for any type of package delivery including driver, application, or OS install. If you plan to use multicasting heavily, this may affect the decisions you make about what to put into the actual image. It may make more sense to make the image fatter to improve distribution times when using multicasting, which is often the case when organizations have rigid, timebased service-level agreements in place for deploying Windows to users.

To enable multicasting, follow these steps:

1. Navigate to Administration → Overview → Distribution Points.
2. Select the DP you want to enable PXE on from the list shown in the right pane, and choose Properties from the ribbon bar or the right-click context menu.
3. Select the Multicast tab on the DP's Properties dialog box.
4. Check Enable multicast to simultaneously send data to multiple clients . Note the text under this check box: Windows Deployment Services will be installed if required.
5. Here are the options also available on this tab once multicast is enabled:
   • Multicast Connection Account: This account is used for communication back to the site database. By default, it is set to Use the computer account of the distribution point to connect to the primary database but you can choose another account.
   • Multicast address settings: This allows you to specify a specific multicast address according to Request For Comment (RFC) 3171 or use the default of any available multicast address.
   • UDP port range for multicast: Specify which User Datagram Protocol (UDP) ports to use for multicasting.
   • Client transfer rate: This setting optimizes the performance of the multicast data stream for the selected network type.
   • Maximum clients: This caps the number of clients that this distribution point serves using multicast. This number is cumulative across all multicast sessions.
   • Enable scheduled multicast: Scheduled multicast configures a multicast session to wait for a specific number of clients to join a session or a number of minutes to wait before starting a session. This allows you to coordinate the client systems and ensure they are all online and available before the session starts. The use of the catch-up feature described in the "Utilizing Multicasting" section reduces the importance of this functionality, but it is still available.