The Session Layer

The Session Layer is level five of the seven level OSI model. It responds to service requests from the Presentation Layer and issues service requests to the Transport Layer.

The Session Layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e. a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session Layers are commonly used in application environments that make use of remote procedure calls (RPCs).

An example of a Session Layer protocol is the OSI protocol suite Session Layer Protocol, also known as X.225 or ISO 8327. In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the Session Layer protocol may close it and re-open it. It provides for either full duplex or half-duplex operation and provides synchronization points in the stream of exchanged messages.[1]

Other examples of session-layer implementations include Zone Information Protocol (ZIP) – the AppleTalk protocol that coordinates the name binding process; and Session Control Protocol (SCP) – the DECnet Phase IV Session Layer protocol.
In brief: the Session Layer establishes, manages and terminates connections (sessions) among cooperating applications. It also adds traffic flow information

Application Layer

The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).