A Guide to the Session Layer | OSI Model Layer 5

The Open System Interconnect (OSI) model in computer networking has seven layers, each of which is assigned a set of functions. Layer 5, or the session layer, is essential in initiating requests and responses of dialogue within systems. The model helps establish, manage, and terminate communication. Learn what the OSI session layer is, its primary functions and protocols, and how it fits into the OSI model.

What is the session layer?

The session layer is the fifth layer in the OSI model.

The session layer is the fifth layer of the OSI (Open Systems Interconnection) Model. It's also called the network dialogue controller. It relies on the layers above and below it - the presentation layer and transport layer - to carry out its function.

Layer 5's main task is to start requests and responses of dialogues within communication systems. The network dialog controller uses Remote Procedure Calls (RPCs) to implement its services on web browsers using various protocols. Dialogues, sessions, and communication channels between peer devices are also created at this level. They facilitate data transmission.

The layer allows for simplex, half-duplex, and full-duplex procedures for remote and local applications. However, it was designed for explicit use with applications that use RPC.

Essentially, the session layer controls the connection between computers, monitoring dialogues, and establishing, controlling, and ending sessions between local and remote applications. It also maintains connections to prevent interruption of data transmission.

How does the session layer work?

The session layer relays information between the presentation layer and the transport layer, so it needs a stable connection. The session layer must do the following things to establish and maintain a connection between entities:

  • Map the session address to the shipping address
  • Select Quality of Service (QoS) parameters
  • Handle negotiations between session parameters
  • Transmit limited transparent user data
  • Monitor the data transfer phase

Components of the fifth layer

There are several sessions that all are a part of the OSI session layer.

The Network File System (NFS) allows a client computer to access files stored on another machine or server over a network, just as they would access a local file. The server sends a mount prompt to the client, and if it's successful, it then grants access. The server also acknowledges and validates the client and decides what files to avail when managing a session.

The Remote Procedure Calls (RPC) simplifies the complexity of the OSI model by providing network applications for accessing files in a remote computer. RPC is an independent client-server model that uses various methods to authenticate a client's requests.

The AppleTalk Session Protocol assigns a session reference number to track communication easily. The protocol allows multiple applications to establish one session simultaneously.

The Structured Query Language (SQL) supports communication with TCP/IP. It supports tunneling and provides encryption, hiding the client's details. SQL network protocols are installed by default when installing the SQL server.

The X Window System is a computing protocol used for transmitting network-transparent data. An X Windows session is a client-server windowing system developed from DEC Session Control Protocol for graphics workstations.

Finally, the DEC Session Control Protocol is a simple model that enables clients and servers to have multiple conversations over a single connection.

Functions of the session layer

Each layer in the OSI model performs a unique function within a certain network. The session layer, therefore, is an indispensable layer with a number of useful protocols that enable the functions of the system.

The session layer performs dialogue control, offering both half-duplex and full-duplex modes of communication. It also recovers dialogues in case of interruptions; the dialogue controller has checkpoints that allow users to resume the session at the point where the failure occurred. It manages communication, deterring two users from performing similar options simultaneously, and ensures that only one user can perform a critical operation on a system at once.

The fifth layer also performs synchronization, meaning it terminates, restarts, checkpoints, and adjourns procedures that allow information synchronization to avoid overlapping.

Finally, the session layer is responsible for facilitation - both of requests and of connections. The fifth layer provides a network for managing processes and allows the opening and closing of end-user processes. It also links the transport layer, the fourth layer, and the presentation layer, the sixth layer.

Essentially, the functions of the session layer can be summed up as session restoration, authentication, and authorization, which are provided for by its protocols.

Session layer protocols

OSI session layer protocols provide security, accuracy and safety in communication between two applications.

These protocols include AppleTalk Data Stream Protocol, Point-to-Point Tunneling Protocol, Secure Copy Protocol, and more. Learn about each of the OSI session layer protocols below.

AppleTalk Data Stream Protocol (ADSP)

AppleTalk Data Stream Protocol (ADSP) is a development by Apple Inc. with multiple features that allow a network to configure itself. Additionally, two other protocols (AARP and NBP) allow connection without other setup. Similarly, AppleTalk Session Protocol (ASP) lets multiple applications be a part of one session at the same time. It assigns a session reference number to track communication.

Password Authentication Protocol (PAP)

Password Authentication Protocol (PAP) ensures security by limiting access to users. It asks for a password request. Then, it gives access upon verifying the password. It's done at the link establishment stage.

Remote Procedure Call Protocol (RPCP)

Remote Procedure Call Protocol (RPCP) serves the request-response function of the session layer, creating a client-server interface. The programmer can perform a procedure without coding the details.

Real-time Transport Control Protocol (RTCP)

Real-time Transport Control Protocol (RTCP) is the protocol responsible for making sure the quality of service stays high. It gives statistical information, such as packet counts, octet and packet loss, to users as feedback for the quality of media transmitted through a network.

Call control for multimedia communication (H.245)

H.245 acts as a control channel for conveying information for multimedia communication. It facilitates encryption, preference requests, jitter management and flow control within a communication session.

See Internet Socket (SOCKS)

SOCKS s5 is an Internet protocol that requires authentication to authorize access. Communication happens through a proxy server.

Additional session layer protocols

  • Point-to-Point Tunneling Protocol (PPTP): PPTP protocol is similar to a VPN, as it gives security through remote access.
  • Sockets Direct Protocol (SDP): An advanced version of TCP, this shares limited data and also supports socket streams over RDMA (Remote Direct Memory Access).
  • Short Message Peer-to-Peer (SMPP): SMPP works for telecommunications. It gives a flexible data transmission interface for short message communication.
  • Secure Copy Protocol (SCP): SCP enables the transfer of computer files from a local host to a remote host between two remote applications.
  • Zone Information Protocol (ZIP): ZIP gives access to processes and applications that relate to a certain zone. Therefore, no application with a different zone name can go to another zone.
  • Internet Storage Name Service (ISNS): ISNS automates the management and discovery of devices on a storage area network. It reveals the devices on the network. It also reveals their zone names, IP addresses, and portal groups.
  • Layer 2 Forwarding Protocol (L2F): Layer 2 Forwarding is a development of the Cisco systems that provides VPN connections for tunneling protocols.
  • Network Basic Input Output System (NetBIOS):NetBIOS is a network protocol that gives a platform for applications on different devices to share information across a local area network.

Frequently asked questions

What are the other layers in OSI models?

Though the transport layer is an important layer in the OSI model, there are others. The other six layers in the OSI model include the application layer, the presentation layer, the transport layer, the network layer, the data link layer, and the physical layer.

What does the session layer do?

The session layer facilitates dialogues and communication between systems. The layer is also responsible for establishing and ending sessions between applications.

What is an example of session layer protocol?

Though there are many examples of session layer protocols, a few common examples are AppleTalk Session Protocol (ASP) and Password Authentication Protocol (PAP).