The Callbase class handles Session Description Protocol (SDP)-based offers and answers. When a call (created locally, and represented by a Callsubclass instance) is established, an SDP offer is sent that describes the nature of the call. The SDP offer contains a global section and one or more media sections. Each media section indicates the media type described in that section. For example, an SDP offer might include two media types, audio and video, and would therefore have two media sections, as describe in RFC 3264, An Offer/Answer Model with the Session Description Protocol (SDP). When the remote endpoint accepts the offer, it sends an SDP answer that contains the same media sections, but adds a description of the remote endpoint’s capabilities or preferences. For example, a media section might describe the codec formats that are supported. The set of media types supported by the call also should be supported by the media provider that is bound to the call. The Conversationassociated with the call assists the call in selecting the appropriate media provider instance, which is obtained from the platform.

When an incoming INVITE message is processed, the SDP in the INVITE is also processed to determine the set of media types. These media types are used to create the call. When a call is accepted or established but the call does not have a provider, a media provider that is able to handle the media types in the call is created using the appropriate media provider factory. This provider is cached in the call. The conversation helps to bind the call to the media provider using the media provider factory.

A class derived from the Callclass must provide definitions for the two abstract properties on the Callclass. These nonabstract properties on the subclass override their abstract counterparts on the Callclass.

The DefaultMediaTypecan be used to get or set the default media type that will be used to establish an outbound call.

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public abstract string DefaultMediaType { get; set; }

The SupportedMediaTypesproperty holds a list of the types of media intended to be supported in new Callsubclass. The media types appear in the media description line ( m=< media> < port> < proto> < fmt> ...",) in the SDP session description.

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public abstract ReadOnlyCollection<string> SupportedMediaTypes { get; }

A class derived from the Callclass must also provide definitions for four abstract methods on the Callclass.

HandleFlowConfigurationRequested

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protected abstract bool HandleFlowConfigurationRequested(MediaFlow mediaFlow);}

HandleForwardReceived

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protected abstract void HandleForwardReceived(CallForwardReceivedEventArgs e);

HandleTransferNotificationReceived

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protected abstract void HandleTransferNotificationReceived(TransferStateChangedEventArgs e);

HandleTransferReceived

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protected abstract void HandleTransferReceived(CallTransferReceivedEventArgs e);

If the custom call control is intended to provide call forwarding or call transfer functionality, the derived class can raise a strongly-typed event to the application. Alternatively, if the custom call control does not support these capabilities, the event can be ignored (forwarding) or declined (transfer).

If the custom Callsubclass implementation is intended to support call transfer, it should implement the HandleTransferReceivedand HandleTransferNotificationReceivedprotected methods. The expected behavior in these methods is that they expose a first-class event handler so that an application can register for the events that notify it of incoming REFER requests and REFER status.