18.104.22.168 Opening a DVC
The following diagram illustrates the sequence of operations involved in the creation of a DVC.
Figure 4: DVC open sequence
A DVC consists of two endpoints logically connected over a network. One endpoint is an application running on a TS server, and the other endpoint is an application running on a TS client. The applications running on the TS client are referred to as DVC listeners. These listeners are service providers to the applications running on the TS server.
Channels are established by the DVC managers exchanging Create Request and Create Response PDUs. Channels are created by a DVC server manager in response to a channel-create request by an application. When an application makes a request to a DVC server manager to create a channel, the server generates a channel identifier (that is, a unique number for the requested session), and sends this identifier (and the listener name the application is requesting a connection to) in a Create Request PDU to the DVC client manager. The DVC client manager locates the requested listener, and the listener creates a DVC using the ChannelId. The DVC client manager binds the endpoint to the ChannelId. The client then sends a Create Response message to the server indicating the endpoint creation status. If the creation is successful, the DVC server manager indicates to the application that the session is established and is ready for sending and receiving data. The client and server maintain the endpoints for the life of the channel.
When a multitransport connection ([MS-RDPEMT] section 1.3) is negotiated for a given RDP connection, the DVC server manager can establish a particular DVC from one of the connected transports (TCP, UDP-R, or UDP-L) based on preferences specified by the application endpoint. For example, an application can request that the DVC be encapsulated in a TCP transport for a feature that requires full-reliability but can be latency-tolerant, such as printer redirection. An application can also request that the DVC be encapsulated in a UDP-L transport for a feature that is loss-tolerant but latency-sensitive, such as audio output redirection.
The server DVC manager sends the Create Request PDU over the selected transport, and the client responds by sending the Create Response PDU back to the server over the same transport.