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  Layer 5 Session Layer


Build and control sessions

The layer 5 (control of logical connections; also session layer) provides inter-process communication between two systems. Here you can find among others the protocol RPC (Remote Procedure Call). To resolve failures of meeting and similar problems, the session layer services for an organized and synchronized data exchange. To this end, recovery points, so-called fixed points (check points) introduced, where the session can be synchronized after a failure of a transport connection again without the transfer must start from the beginning again.

OSI Layer 5 - Session Layer

In the seven-layer OSI model of computer networking, 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 calls (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 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.



  • Authentication
  • Authorization
  • Session restoration

Popular Session Layer Protocols

ADSPAppleTalk Data Stream Protocol
ASPAppleTalk Session Protocol
H.245Call Control Protocol for Multimedia Communication
ISO-SPOSI session-layer protocol (X.225, ISO 8327)
iSNSInternet Storage Name Service
L2FLayer 2 Forwarding Protocol
L2TPLayer 2 Tunneling Protocol
NetBIOSNetwork Basic Input Output System
PAPPassword Authentication Protocol
PPTPPoint-to-Point Tunneling Protocol
RPCRemote Procedure Call Protocol
RTCPReal-time Transport Control Protocol
SMPPShort Message Peer-to-Peer
SCPSession Control Protocol
SOCKSthe SOCKS internet protocol, see Internet socket
ZIPZone Information Protocol
SDPSockets Direct Protocol