Better to know some... than all
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Session Layer RoleThe exact role of the session layer in relation to other layers is worth some consideration. Although the session layer is positioned below and receives its requests from the presentation layer, its primary role is to serve the application layer by allowing applications to converse in a structured manner. The presentation layer is completely transparent to this process. The interface between the session layer and the transport layer is very simple. It provides for basic opening and closing of transport connections (a session connection directly maps to a transport connection) and reliable transfer of data over the connections. This simple interface is enriched by the session layer into a diverse set of services for the ultimate use by the application layer.  Functional UnitsIt is generally rare for an application to require the use of all session services. Often a relatively small subset will suffice. To facilitate this, the session layer services are divided into 13 functional units. Each functional unit is a logical grouping of related session services. Functional units are negotiated and selected during connection establishment using the requirements parameter in the connection request. The functional units selected determine what services will be available for the duration of the session. Kernel represents the most basic set of services that applications could live with. All session layer implementations must provide this minimal subset. | |||||