The session layer is responsible for managing connections between applications on different endpoints, ensuring data streams stay in sync, and preventing errors. It interfaces with the transport and presentation layers and is important for protocols like RPC, SQL, X Windows System, and NFS.
The session layer is the fifth layer of the Open Systems Interconnection (OSI) model developed by the International Organization for Standardization (IOS). It is responsible for establishing, managing and closing end-to-end connections, called sessions, between applications located at different network endpoints. The dialog control management provided by the session layer includes full-duplex, half-duplex, and simplex communications. Session layer management also helps ensure that multiple data streams stay in sync with each other, as is the case for multimedia applications such as video conferencing, and helps prevent application-related data errors. The session layer is also responsible for ensuring proper data communication during remote procedure calls (RPC).
There are seven distinct functional units, called layers, which the OSI model divides, or modularizes, into the different tasks for which network hardware and software are responsible. The layers below the session layer are primarily concerned with transmitting data over the network. The session layer, also commonly referred to as layer 5 in computer networking, and higher levels focus primarily on interacting with and providing functionality to applications within a computer system. It interfaces with the transport layer, also called layer 4, so that data streams from applications can be segmented and seamlessly transmitted to the network. Layer 5 also passes incoming data to the presentation layer, known as layer 6, for further processing before being given to an application.
When the OSI model was originally developed, there was some discussion as to whether session layer protocols would be useful or not. Several network services, proprietary protocols and OSI protocols have been successfully developed and demonstrate the importance of this layer. These include RPC, Structured Query Language (SQL), X Windows System and X Terminal, Network File Services (NFS), Apple Talk Session Protocol (ASP), and Apple Talk Zone Information Protocol (ZIP). Without the session layer, it would be much more difficult to control information about computer applications that pass across networks.
These services and protocols perform a variety of functions. SQL is a computer language used to create requests that are sent to databases in order to retrieve specific information structured in the desired way. The X Windows System and X Terminal allow networked computers, some with limited capabilities, to connect to server computers so that client computers have graphics capabilities and interface with devices such as monitors, mice, and keyboards. The NFS protocol emulates local access to files located on remote networked file servers. The Apple Talk ASP and ZIP protocols were used with early versions of the Macintosh operating systems, but were replaced by IP-based protocols after the release of the Mac OS X operating system.
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