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Session layer

(also layer 5)

Session layer definition

The session layer is the fifth layer in the Open System Interconnection (OSI) model. It is responsible for establishing, managing, and ending communication sessions (requests and responses) between multiple devices.

Real session layer functions

  • Dialog controller. The session layer allows systems to communicate with each other through using half-duplex (data transmission in one direction at a time) or full-duplex (data transmission in both directions at the same time) operations.
  • Token management. The session layer stops more than one user from trying to access the same critical operation at the same time.
  • Communication synchronization. The session layer implements data stream checkpoints to ensure that long communications remain coherent. These checkpoints mark a portion of the data stream as successfully received, meaning that only unvalidated parts after the checkpoint need to be re-transmitted in case of failure.

Examples of session layer protocols

  • ADSP (AppleTalk Data Stream Protocol)
  • ASP (AppleTalk Session Protocol)
  • H.245 (Call Control Protocol for Multimedia Communication)
  • iSNS (Internet Storage Name Service)
  • ISO 8327
  • L2F (Layer 2 Forwarding Protocol)
  • L2TP (Layer 2 Tunneling Protocol)
  • NetBIOS (Network Basic Input/Output System)
  • PAP (Password Authentication Protocol)
  • 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 (Socket Secure)
  • SDP (Sockets Direct Protocol)
  • X.225
  • ZIP (Zone Information Protocol)