Session layer
Encyclopedia
The session layer is layer 5 of the seven-layer OSI model
of computer network
ing.
The session layer provides the mechanism for opening, closing and managing a session
between end-user application processes, i.e., a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session-layer services are commonly used in application environments that make use of remote procedure call
s (RPCs).
An example of a session-layer protocol is the OSI protocol suite session-layer protocol, also known as X.235 or ISO 8327. In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the session-layer protocol may close it and re-open it. It provides for either full duplex
or half-duplex
operation and provides synchronization points in the stream of exchanged messages.
Other examples of session layer implementations include Zone Information Protocol (ZIP) – the AppleTalk
protocol that coordinates the name binding process, and Session Control Protocol (SCP) – the DECnet
Phase IV session-layer protocol.
Within the service layering semantics of the OSI network architecture, the session layer responds to service requests from the presentation layer
and issues service requests to the transport layer
.
The session layer of the OSI model is responsible for session checkpointing and recovery. It allows information of different streams, perhaps originating from different sources, to be properly combined or synchronized.
An example application is web conferencing
, in which the streams of audio and video must be synchronous to avoid so-called lip synch problems. Floor control
ensures that the person displayed on screen is the current speaker.
Another application is in live TV programs, where streams of audio and video need to be seamlessly merged and transitioned from one to the other to avoid silent airtime or excessive overlap.
protocols, or otherwise considered the realm of the application layer
protocols. TCP/IP's layers are descriptions of operating scopes (application, host-to-host, network, link) and not detailed prescriptions of operating procedures or data semantics.
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
of computer network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
ing.
The session layer provides the mechanism for opening, closing and managing a session
Session (computer science)
In computer science, in particular networking, a session is a semi-permanent interactive information interchange, also known as a dialogue, a conversation or a meeting, between two or more communicating devices, or between a computer and user . A session is set up or established at a certain point...
between end-user application processes, i.e., a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session-layer services are commonly used in application environments that make use of remote procedure call
Remote procedure call
In computer science, a remote procedure call is an inter-process communication that allows a computer program to cause a subroutine or procedure to execute in another address space without the programmer explicitly coding the details for this remote interaction...
s (RPCs).
An example of a session-layer protocol is the OSI protocol suite session-layer protocol, also known as X.235 or ISO 8327. In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the session-layer protocol may close it and re-open it. It provides for either full duplex
Duplex (telecommunications)
A duplex communication system is a system composed of two connected parties or devices that can communicate with one another in both directions. The term multiplexing is used when describing communication between more than two parties or devices....
or half-duplex
Duplex (telecommunications)
A duplex communication system is a system composed of two connected parties or devices that can communicate with one another in both directions. The term multiplexing is used when describing communication between more than two parties or devices....
operation and provides synchronization points in the stream of exchanged messages.
Other examples of session layer implementations include Zone Information Protocol (ZIP) – the AppleTalk
AppleTalk
AppleTalk is a proprietary suite of protocols developed by Apple Inc. for networking computers. It was included in the original Macintosh released in 1984, but is now unsupported as of the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking...
protocol that coordinates the name binding process, and Session Control Protocol (SCP) – the DECnet
DECnet
DECnet is a suite of network protocols created by Digital Equipment Corporation, originally released in 1975 in order to connect two PDP-11 minicomputers. It evolved into one of the first peer-to-peer network architectures, thus transforming DEC into a networking powerhouse in the 1980s...
Phase IV session-layer protocol.
Within the service layering semantics of the OSI network architecture, the session layer responds to service requests from the presentation layer
Presentation layer
The presentation layer is layer 6 of the seven-layer OSI model of computer networking and serves as the data translator for the network. It is sometimes called the syntax layer.-Description:...
and issues service requests to the transport layer
Transport layer
In computer networking, the transport layer or layer 4 provides end-to-end communication services for applications within a layered architecture of network components and protocols...
.
Services
- AuthenticationAuthenticationAuthentication is the act of confirming the truth of an attribute of a datum or entity...
- PermissionPermissionPermission, in philosophy, is the attribute of a person whose performance of a specific action, otherwise ethically wrong or dubious, would thereby involve no ethical fault. The term "permission" is more commonly used to refer to consent...
s - Session restoration (checkpointing and recovery)
The session layer of the OSI model is responsible for session checkpointing and recovery. It allows information of different streams, perhaps originating from different sources, to be properly combined or synchronized.
An example application is web conferencing
Web conferencing
Web conferencing refers to a service that allows conferencing events to be shared with remote locations. Most vendors also provide either a recorded copy of an event, or a means for a subscriber to record an event. The service allows information to be shared simultaneously, across geographically...
, in which the streams of audio and video must be synchronous to avoid so-called lip synch problems. Floor control
Floor control
Floor control allows users of networked multimedia applications to utilize and share resources such as remote devices, distributed data sets, telepointers, or continuous media such as video and audio without access conflicts. Floors are temporary permissions granted dynamically to collaborating...
ensures that the person displayed on screen is the current speaker.
Another application is in live TV programs, where streams of audio and video need to be seamlessly merged and transitioned from one to the other to avoid silent airtime or excessive overlap.
Comparison with TCP/IP model
The TCP/IP reference model does not concern itself with the OSI model's details of application or transport protocol semantics and therefore does not consider a session layer. OSI's session management in connection with the typical transport protocols (TCP, SCTP), is contained in the transport-layerTransport layer
In computer networking, the transport layer or layer 4 provides end-to-end communication services for applications within a layered architecture of network components and protocols...
protocols, or otherwise considered the realm of the application layer
Application layer
The Internet protocol suite and the Open Systems Interconnection model of computer networking each specify a group of protocols and methods identified by the name application layer....
protocols. TCP/IP's layers are descriptions of operating scopes (application, host-to-host, network, link) and not detailed prescriptions of operating procedures or data semantics.