ISO/OSI model overview

Stack overview

Here’s a concise OSI-model overview in Markdown, starting with your incipit and then expanding on each layer:

7   Application      HTTP, FTP, SMTP, …  
6   Presentation     SSL, TLS, JPEG, MPEG  
5   Session          Sockets, NetBIOS, RPC  
4   Transport        TCP, UDP, QUIC  
3   Network          IP, ICMP, OSPF, BGP  
2   Data Link        Ethernet, PPP, Wi-Fi, VLAN  
1   Physical         Cables, Hubs, Repeaters, Bits  

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Layer 7: Application

The topmost layer, closest to the end-user.

• Role: Provides network services directly to applications.
• Key functions: File transfers, e-mail, web browsing, remote login.
• Protocols/Examples:
  • HTTP/HTTPS – web pages
  • FTP/SFTP – file transfer
  • SMTP/IMAP/POP3 – e-mail delivery and retrieval
  • DNS – hostname resolution

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Layer 6: Presentation

Ensures that data is in a usable format and is syntax-and-semantics compliant.

• Role: Data translation, encryption/decryption, compression/decompression.
• Key functions:
  • Character encoding (ASCII, EBCDIC)
  • Data serialization (XML, JSON)
  • Encryption (SSL/TLS)
  • Compression (JPEG, MPEG)

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Layer 5: Session

Manages sessions (connections) between applications.

• Role: Establishes, maintains, and terminates dialogues (sessions).
• Key functions:
  • Session establishment, maintenance, teardown
  • Full-duplex, half-duplex, and simplex operation
  • Checkpointing and recovery
• Examples:
  • Sockets API
  • NetBIOS
  • RPC (Remote Procedure Call)

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Layer 4: Transport

Provides end-to-end communication services for applications.

• Role: Segmentation, reassembly, flow control, error correction.
• Key functions:
  • Reliable delivery (ACKs, retransmissions)
  • Multiplexing/demultiplexing ports
  • Congestion control
• Protocols:
  • TCP – reliable, connection-oriented
  • UDP – fast, connectionless
  • QUIC – UDP-based, low-latency

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Layer 3: Network

Determines how data is sent to the receiver over multiple links.

• Role: Logical addressing, routing, packet forwarding.
• Key functions:
  • IP addressing (IPv4/IPv6)
  • Routing decisions (shortest path, policy)
  • Fragmentation and reassembly
• Protocols:
  • IP – logical addressing
  • ICMP – diagnostics (ping, traceroute)
  • OSPF, BGP – routing protocols

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Layer 2: Data Link

Handles node-to-node data transfer and error detection/correction.

• Role: Framing, MAC addressing, error notification.
• Sub-layers:
  • LLC (Logical Link Control)
  • MAC (Media Access Control)
• Key functions:
  • Frame delimiting and recognition
  • MAC addressing (Ethernet, Wi-Fi)
  • Flow control, error detection (CRC)
• Technologies:
  • Ethernet (IEEE 802.3)
  • Wi-Fi (IEEE 802.11)
  • PPP – point-to-point connections
  • VLANs – virtual LAN segmentation

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Layer 1: Physical

Concerned with the transmission of raw bitstreams over a physical medium.

• Role: Electrical/optical/physical interface to the transmission medium.
• Key functions:
  • Bit encoding (NRZ, Manchester)
  • Signaling (voltage levels, light pulses)
  • Medium specifications (cable types, connectors)
• Media & Devices:
  • Copper cables (Cat5e/6), fiber optics
  • Hubs, repeaters, transceivers
  • Wireless radio frequencies

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Tip for Developers:
When troubleshooting, map your tools to OSI layers—e.g., Wireshark at Layers 2–7, ping/traceroute at Layers 3–4, and iperf at Layer 4. Understanding where each protocol lives helps isolate problems quickly.