OSI Model — The 7 Layers

The Open Systems Interconnection (OSI) model is a conceptual framework that standardises how network communication is structured. It divides networking into seven distinct layers, each with a specific responsibility.

Quick Reference

Layer 7  Application    ← User-facing (HTTP, DNS, FTP)
Layer 6  Presentation   ← Data format & encryption (SSL/TLS, JPEG)
Layer 5  Session        ← Connection management (NetBIOS, RPC)
Layer 4  Transport      ← Reliable delivery (TCP, UDP)
Layer 3  Network        ← Routing & addressing (IP, ICMP)
Layer 2  Data Link      ← Framing & MAC addressing (Ethernet, Wi-Fi)
Layer 1  Physical       ← Bits on the wire (cables, signals)

Layer 1 — Physical

The lowest layer. Deals with the physical transmission of raw bits over a communication channel.

Key concepts:

• Voltage levels, signal timing, data rates
• Connector types (RJ45, SFP, coaxial)
• Transmission modes: simplex, half-duplex, full-duplex
• Encoding schemes (Manchester, NRZ)

Layer 2 — Data Link

Provides node-to-node data transfer and handles error detection at the frame level. Split into two sub-layers: LLC (Logical Link Control) and MAC (Media Access Control).

Key concepts:

• MAC addresses (48-bit hardware addresses, e.g. AA:BB:CC:DD:EE:FF)
• Frame structure: preamble, destination MAC, source MAC, type, payload, FCS
• Error detection via CRC (Cyclic Redundancy Check)
• VLANs (802.1Q) for logical network segmentation
• STP (Spanning Tree Protocol) to prevent switching loops

Layer 3 — Network

Handles routing and logical addressing. Determines the best path for data to travel from source to destination across multiple networks.

Key concepts:

• IP addressing (IPv4: 32-bit, e.g. 192.168.1.1; IPv6: 128-bit)
• Subnetting and CIDR notation (/24, /16, etc.)
• Routing tables and routing protocols (static vs dynamic)
• TTL (Time To Live) to prevent infinite loops
• NAT (Network Address Translation) for private-to-public IP mapping
• Fragmentation and reassembly of packets

Layer 4 — Transport

Ensures reliable (or fast) end-to-end data delivery between applications. Manages flow control, segmentation, and error recovery.

TCP vs UDP

Key concepts:

• Port numbers (0-65535): well-known (0-1023), registered (1024-49151), dynamic (49152-65535)
• TCP three-way handshake: SYN → SYN-ACK → ACK
• Flow control via sliding window
• Congestion control (slow start, congestion avoidance)

Layer 5 — Session

Manages sessions (connections) between applications. Handles setup, maintenance, and teardown of communication sessions.

Key concepts:

• Session management: opening, maintaining, closing connections
• Synchronisation points for long data transfers (recovery after failure)
• Dialog control: full-duplex vs half-duplex communication
• Authentication and authorisation at session level

Layer 6 — Presentation

Translates data between the application layer and the network. Handles data formatting, encryption/decryption, and compression.

Key concepts:

• Data format translation (e.g. EBCDIC ↔ ASCII)
• Encryption/decryption (SSL/TLS handshake happens here conceptually)
• Data compression to reduce bandwidth
• Serialisation formats (JSON, XML, Protocol Buffers)

Layer 7 — Application

The layer closest to the end user. Provides network services directly to user applications.

Key concepts:

• HTTP methods: GET, POST, PUT, DELETE, PATCH
• DNS resolution: domain name → IP address
• DHCP: automatic IP assignment
• SMTP/IMAP/POP3: email protocols
• SSH: secure remote access
• REST APIs and web services

Mnemonic

A common way to remember the layers from bottom to top:

Please Do Not Throw Sausage Pizza Away

OSI vs TCP/IP Model

The TCP/IP model is the practical implementation used on the internet. It combines several OSI layers:

OSI Model              TCP/IP Model
─────────              ────────────
7. Application    ┐
6. Presentation   ├──→  Application
5. Session        ┘
4. Transport      ───→  Transport
3. Network        ───→  Internet
2. Data Link      ┐
1. Physical       ┘──→  Network Access

Troubleshooting by Layer

When diagnosing network issues, work from the bottom up:

1. Physical — Is the cable plugged in? Link light on?
2. Data Link — Can you see MAC addresses? ARP working?
3. Network — Can you ping the gateway? Routes correct?
4. Transport — Is the port open? Firewall blocking?
5. Session — Can a session be established?
6. Presentation — Certificate errors? Encoding issues?
7. Application — Does the app respond? Correct config?

# Layer 1: check interface status
ip link show

# Layer 2: check ARP table
arp -a

# Layer 3: test connectivity
ping 8.8.8.8
traceroute 8.8.8.8

# Layer 4: test port connectivity
nc -zv example.com 443
ss -tulnp

# Layer 7: test application
curl -I https://example.com