What is the function of session layer?

What is the function of session layer?

Quick Answer

According to Microsoft Support, the main function of the session layer is to establish connections, otherwise known as sessions, between processes running on different computer terminals or Web-enabled devices, referred to as stations. The session layer is the third layer from the top in the seven layers of the open systems interconnection model that conceptually characterizes and standardizes the internal functions of modern communications systems.

Full Answer

Above it, there are the application and the presentation layers. Below it, there are the transport, the network, the datalink and the physical layers. In addition to its main function, the session layer is also responsible for communicating with the presentation layer above it and interpreting the transport layer below it. An example of the session layer in action would be how each of the individual objects, including graphics, Java applets and text required to load a Web page, are downloaded in the browser from a Web server. To render the Web page, the browser needs to access each of these individuals file separately and then reconstruct the website on the browsing device. This requires the browser to open individual sessions for each of the files, download said files and then terminate the session. During this process, the session layer keeps track of which data packets belong to which file to avoid data corruption.

Definition

Session layer (port layer)

In the Open Systems Interconnection (OSI) communications model, the Session layer (sometimes called the "port layer") manages the setting up and taking down of the association between two communicating end points that is called a connection. A connection is maintained while the two end points are communicating back and forth in a conversation or session of some duration. Some connections and sessions last only long enough to send a message in one direction. However, other sessions may last longer, usually with one or both of the communicating parties able to terminate it.

What Is the Session Layer?

Say you are planning and hosting a large party. Initially, the problems are minor—like small issues with the food or party favors—but as the party activity increases, time and management become critical. To manage the event and keep your party on track, you need additional helpers to interface with the caterer, florist, and baker. With the setup problems solved, the helpers continue providing assistance by giving management support ensuring each activity runs smoothly. At the party's conclusion, the helpers aid in cleaning up, closing party facilities, and terminating connections.

You may be wondering what a party has to do with networking, particularly the session layer? We can make a comparison between setting up, managing, and closing the party with the functions performed by the session layer, which is layer five of the seven layers comprising the Open Systems Interconnection (OSI) model.

Understanding the Session Layer

Networking is defined as a grouping of two or more computers being linked together to exchange data. As layer 5 of the OSI model, the session layer is primarily responsible for setting up and managing communicative sessions, thereby enabling the sending and receiving of data for an undetermined length of time—similar to a telephone call. Additionally, the session layer is responsible for terminating sessions between software processes.

As was the case with our party helpers, the primary responsibilities of session layer protocols (our helpers) are to set up, control, maintain, and close communicative sessions. Common session layer helpers are often referred to as application programming interfaces (APIs).

A common example of an API is the network basic input/output system (NETBIOS), which allows applications on separate computers to communicate over a local area network (LAN). Another example is the use of transfer control protocol/internet protocol (TCP/IP), which standardizes communications. APIs such as these are of primary interest to programmers and developers of application software for completing network communication setup and simplifying management.

As part of the OSI model, the session layer determines what format will be used for data exchange between the engaging nodes on a network. As long as the established format is in place, the communications channel will maintain an open status. Looking back at your party, you had to set up a session (the party) by arranging and managing session formats with items such as the guest list, party favors, food, flowers, and baked goodies. Since you had helpers (APIs), you were able to control the environment, had a great session (party), and closed related activities gracefully.

What Is the Open Systems Interconnect Model?

Before we can begin our session layer discussion in greater detail, we need to look into the networking environment in which the session layer exists: the OSI model. The OSI model has seven protocol layers, with each layer performing a specific and supportive communicative task. For a brief recap, let's go over each of the layers from top to bottom:

• Layer 7: the application layer - provides access to available network resources
• Layer 6: the presentation layer - translates, encrypts, and compresses data
• Layer 5: the session layer - establishes, manages, and terminates communicative sessions
• Layer 4: the transport layer - provides reliable process-to-process message delivery and error recovery
• Layer 3: the network layer, - moves packets from source to destination providing inter networking capabilities
• Layer 2: the data link layer, - organizes bits into frames providing node-to-node delivery
• Layer 1: the physical layer - transmits bits over a medium establishing mechanical and electrical specifications

Here, we see an illustration of data flow to and from the sending and receiving computers on a network. As seen here, the session layer provides communicative support to the OSI model's presentation (6) and transport (4) layers.