The Session Control Layer

The top Application Layer of the OSI Reference Model is responsible for making a request to access the information resource - a Computer. With the Session Control Layer,  the access to information resource is performed. 

The link of the user Computer ( featured in Application Layer) is established with a Computer ( a Computer that's part of the Session and located at the other end ) that can provide information to the user. 

The Computer that's located at the other end comes in the picture as a result of activities that are directed by the Session Control Layer.

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Note: It must be noted that a Session underlines the existence of at least two objects. In a Session - the  objects can speak  ( transmit ) or listen ( receive ). When one speaks, the other listens and vice vice versa.

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In other words, with the help of the Session Control Layer, connection establishment with a Computer that stores the required information is possible. 

'Log in'/ 'log on' and 'log out'/ 'log off' are the terms that we commonly use to  listen in the world of Computers. 'Log in' activity  is related to the connection establishment with a Computer that can serve the user. 

'Log in' / 'log on' is performed to meet the user Computer requirements. The user Computer is hungry for information. The hunger is served through a Session.

So, the requirement of a controlled Session is fulfilled. The requirement is of a connection establishment. 'Log in' activity is an act that serves the requirement of connection establishment pretty well. 

 'Log out' / 'log off' is related to the release of the connection link that was established. 'Log out' is performed when the user Computer has been served the information it required. So, the requirement of a connection link is over.

When we no longer require the link that is used to connect, we can do away with the connection link. That's what is exactly done when 'log out' is performed. 

When the user Computer ( information seeking Computer) and the information resource Computer seek to part ways they can do so. Usually parting of ways is necessary when being connected with a connection link becomes useless. 

So, 'log in' marks connection establishment and 'log out' means connection release. These two are the prime activities of the Session Control Layer.