In some protocols, the transport layer provides a simplex delivery service; this may be sufficient for some applications. However, other applications may need a half-duplex service. The session layer can be used with the transport layer to create a half-duplex service for the application program. The session layer can also use two transport layer connections to provide a full-duplex service. For this reason, the session layer is sometimes called the dialog controller because it can create communication between two end users that like a one-at-a-time dialogue or two-at-a-time dialog.
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