The session layer, or layer 5, is responsible for setting up, maintaining, and tearing down a session. Before any data is exchanged after initiating contact with the network, the OS must establish a session. The OS creates the appropriate socket, which is an IP address, and a port, so the two endpoints can communicate with one another.

When communicating on the network, you will have multiple concurrent sessions and connections established. You can see your active connections by going to the command line and running netstat, as shown in the following screenshot:

In this screenshot, we only see Transmission Control Protocol (TCP) connections. TCP is a connection-oriented protocol and both endpoints need to communicate the status of the data transaction with one another. As a result, you will see a local and foreign address, along with the state of the transaction.

In addition to setting up a session, this provides other services that include the following:

• Authentication: This validates and identifies an entity by requiring a password or another form of authentication.
• Authorization: This allows access to resources if the entity has the appropriate permissions.
• Checkpointing: This monitors the session for errors and ensures that all data has been received. If there are errors in transmission, the session layer may re-request any missing data.

After the session has ended, the session layer safely closes the session. Next, let's look at some key protocols in this layer.