OSI Layer 5: Session Layer

Definition

The Session Layer controls the dialogues (connections) between computers. It sets up, coordinates, maintains, and terminates communication sessions between local and remote application endpoints.

Key Functions

1. Session Establishment, Maintenance, & Termination: Opening, keeping alive, and closing connections cleanly.
2. Dialog Control / Modes:
   • Simplex: One-way communication.
   • Half-Duplex: Two-way communication, but only one party transmits at a time.
   • Full-Duplex: Simultaneous two-way transmission.
3. Synchronization & Checkpointing: Inserting sync checkpoints into long data streams so that in case of failure, retransmission restarts only from the last checkpoint rather than the beginning.

Common Protocols & APIs

• RPC (Remote Procedure Call)
• NetBIOS
• PPTP (Point-to-Point Tunneling Protocol)
• SOCKS Proxy Protocol
• SMB (Session management functions)

Protocol Data Unit (PDU)

• Data

Adjacent Layers

• Sits below OSI Layer 6: Presentation Layer and above OSI Layer 4: Transport Layer.