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Duplex communication allows either party to initiate a message exchange. But Silverlight version 3 clients are not addressable because the browser environment does not allow the creation of a listener. So a traditional duplex solution implemented by Windows Communication Foundation (WCF) for TCP, for example, which requires listening at an address, is not available. Duplex communication with a Silverlight 3 client is achieved by having the client poll a destination server for messages within an established session over HTTP. This is useful in scenarios where data must be pushed to a Silverlight client when it is updated on the server.

In addition to HTTP, two protocols are used by the Silverlight 3 polling duplex feature. The Net Duplex Protocol is implemented to establish the client session with a server and WS-Make Connection (WS-MC) is used to establish a back channel using polling so that server-initiated messages can be delivered to the client.

This section contains procedures on how to build a Silverlight-enabled duplex service and on how to access this duplex service from a Silverlight client application.

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