In the seven-layer

OSI model The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...

computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections ar ...

ing, the session layer is layer 5. 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 In distributed computing, a remote procedure call (RPC) is when a computer program causes a procedure (subroutine) to execute in a different address space (commonly on another computer on a shared network), which is coded as if it were a normal (lo ...

s (RPCs). An example of a session-layer protocol is the OSI protocol suite session-layer protocol, also known as X.225 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 A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...

half-duplex A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...

operation and provides

synchronization point In computer science, synchronization refers to one of two distinct but related concepts: synchronization of processes, and synchronization of data. ''Process synchronization'' refers to the idea that multiple processes are to join up or handsha ...

s in the stream of exchanged messages. Other examples of session layer implementations include Zone Information Protocol (ZIP) – the

AppleTalk AppleTalk is a discontinued proprietary suite of networking protocols developed by Apple Computer for their Macintosh computers. AppleTalk includes a number of features that allow local area networks to be connected with no prior setup or the ...

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 In the seven-layer OSI model of computer networking, the presentation layer is layer 6 and serves as the data translator for the network. It is sometimes called the syntax layer. Description Within the service layering semantics of the OSI ne ...

and issues service requests to the

transport layer In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet protocol suite and the OSI model. The protocols of this layer provide end-to-end ...

Services

Authentication Authentication (from ''authentikos'', "real, genuine", from αὐθέντης ''authentes'', "author") is the act of proving an assertion, such as the identity of a computer system user. In contrast with identification, the act of indicat ...

Authorization Authorization or authorisation (see spelling differences) is the function of specifying access rights/privileges to resources, which is related to general information security and computer security, and to access control in particular. More f ...

* 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 usage of the session layer is session beans, which are only active as long as the session is active, and are deleted when the session is disconnected. Java developers can use them to store information about the user during a web session. An example application is

web conferencing Web conferencing is used as an umbrella term for various types of online conferencing and collaborative services including webinars (web seminars), webcasts, and web meetings. Sometimes it may be used also in the more narrow sense of the peer-l ...

, in which the streams of audio and video must be synchronous to avoid so-called

lip sync Lip sync or lip synch (pronounced , the same as the word ''sink'', short for lip synchronization) is a technical term for matching a speaking or singing person's lip movements with sung or spoken vocals. Audio for lip syncing is generated th ...

problems. Flow 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

*ADSP, AppleTalk Data Stream Protocol *ASP,

AppleTalk Session Protocol AppleTalk is a discontinued proprietary suite of networking protocols developed by Apple Computer for their Macintosh computers. AppleTalk includes a number of features that allow local area networks to be connected with no prior setup or the n ...

*H.245, Call Control Protocol for Multimedia Communication *ISO-SP, OSI session-layer protocol (X.225, ISO 8327) *iSNS,

Internet Storage Name Service In computing, the proposed Internet Storage Name Service (iSNS) protocol allows automated discovery, management and configuration of iSCSI and Fibre Channel devices (using iFCP gateways) on a TCP/IP network. Features iSNS provides management se ...

*L2F, Layer 2 Forwarding Protocol *L2TP, Layer 2 Tunneling Protocol *NetBIOS, Network Basic Input Output System *PAP,

Password Authentication Protocol Password Authentication Protocol (PAP) is a password-based authentication protocol used by Point-to-Point Protocol (PPP) to validate users. PAP is specified in . Almost all network operating systems support PPP with PAP, as do most network access ...

*PPTP, Point-to-Point Tunneling Protocol *RPC, Remote Procedure Call Protocol *RTCP, Real-time Transport Control Protocol *SMPP, Short Message Peer-to-Peer *SCP, Session Control Protocol *SOCKS, the

SOCKS A sock is a piece of clothing worn on the feet and often covering the ankle or some part of the calf. Some types of shoes or boots are typically worn over socks. In ancient times, socks were made from leather or matted animal hair. In the lat ...

internet protocol *ZIP, Zone Information Protocol *SDP, Sockets Direct Protocol {{div col end

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-layer protocols, or otherwise considered the realm of the

application layer An application layer is an abstraction layer that specifies the shared communications protocols and interface methods used by hosts in a communications network. An ''application layer'' abstraction is specified in both the Internet Protocol ...

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.

References

OSI model

Services

Protocols

Comparison with TCP/IP model

See also

References