Topic Overview

OSI Model (7 Layers)

Learn the OSI 7-layer model for understanding network communication protocols.

The OSI (Open Systems Interconnection) model is a conceptual framework that divides network communication into 7 layers, each with specific functions.

The 7 Layers

Layer 7: Application

Purpose: Interface between user and network Examples: HTTP, HTTPS, FTP, SMTP, DNS Data unit: Message

Layer 6: Presentation

Purpose: Data translation, encryption, compression Examples: SSL/TLS, JPEG, MPEG Data unit: Message

Layer 5: Session

Purpose: Establish, manage, terminate sessions Examples: NetBIOS, RPC Data unit: Message

Layer 4: Transport

Purpose: End-to-end communication, error recovery Examples: TCP, UDP Data unit: Segment (TCP) / Datagram (UDP)

Layer 3: Network

Purpose: Routing, logical addressing Examples: IP, ICMP, OSPF, BGP Data unit: Packet

Purpose: Framing, error detection, MAC addressing Examples: Ethernet, PPP, Switch Data unit: Frame

Layer 1: Physical

Purpose: Physical transmission of bits Examples: Cables, hubs, repeaters Data unit: Bit

Data Flow

Application Data
    ↓
[Layer 7] Adds application header
    ↓
[Layer 6] Adds presentation header
    ↓
[Layer 5] Adds session header
    ↓
[Layer 4] Adds transport header (TCP/UDP)
    ↓
[Layer 3] Adds network header (IP)
    ↓
[Layer 2] Adds data link header (Ethernet)
    ↓
[Layer 1] Transmits bits over physical medium

Encapsulation: Each layer adds its header to the data from the layer above.

Decapsulation: Each layer removes its header and passes data to the layer above.

Examples

HTTP Request Flow

1. Application Layer: Browser creates HTTP request
   GET /index.html HTTP/1.1
   Host: example.com

2. Transport Layer: TCP adds port numbers
   Source Port: 49152
   Dest Port: 80

3. Network Layer: IP adds IP addresses
   Source IP: 192.168.1.100
   Dest IP: 93.184.216.34

4. Data Link Layer: Ethernet adds MAC addresses
   Source MAC: 00:1B:44:11:3A:B7
   Dest MAC: 00:0C:29:AB:CD:EF

5. Physical Layer: Transmits bits over cable

Common Pitfalls

  • Confusing layers: Not understanding which layer handles what. Fix: Remember "All People Seem To Need Data Processing" (Application to Physical)
  • Mixing OSI and TCP/IP: TCP/IP has 4 layers, OSI has 7. Fix: Understand both models
  • Layer boundaries: Some protocols span multiple layers. Fix: Focus on primary function
  • Not understanding encapsulation: How data flows through layers. Fix: Trace a packet through each layer

Interview Questions

Beginner

Q: What is the OSI model and why is it important?

A: The OSI model is a 7-layer conceptual framework that standardizes network communication functions.

Why important:

  • Standardization: Common language for networking
  • Troubleshooting: Identify which layer has issues
  • Design: Understand how protocols work together
  • Education: Foundation for networking concepts

Layers (top to bottom): Application, Presentation, Session, Transport, Network, Data Link, Physical.

Intermediate

Q: Explain how an HTTP request flows through the OSI layers. What happens at each layer?

A:

Flow:

  1. Application (Layer 7): Browser creates HTTP request

    GET /page.html HTTP/1.1
Host: example.com

  2. Presentation (Layer 6): Data encoding/compression (if any)

  3. Session (Layer 5): Establishes session (HTTP is stateless, minimal here)

  4. Transport (Layer 4): TCP adds:

    • Source port (ephemeral, e.g., 49152)
    • Destination port (80 for HTTP)
    • Sequence numbers, ACK numbers
    • Creates TCP segment

  5. Network (Layer 3): IP adds:

    • Source IP (192.168.1.100)
    • Destination IP (93.184.216.34)
    • Creates IP packet

  6. Data Link (Layer 2): Ethernet adds:

    • Source MAC address
    • Destination MAC (router's MAC)
    • Creates Ethernet frame

  7. Physical (Layer 1): Transmits bits over cable/wireless

Reverse on receiving side: Each layer removes its header and passes to layer above.

Senior

Q: Design a network troubleshooting system that identifies issues at different OSI layers. How do you detect and diagnose problems at each layer?

A:

Design:

class OSILayerDiagnostics {
  // Layer 1 (Physical): Check physical connectivity
  async diagnosePhysical(): Promise<Diagnostic> {
    const checks = {
      cableConnected: await this.checkCable(),
      linkUp: await this.checkLinkStatus(),
      signalStrength: await this.getSignalStrength()
    };

    if (!checks.cableConnected) {
      return { layer: 1, issue: 'Cable not connected', fix: 'Check physical connections' };
    }

    if (!checks.linkUp) {
      return { layer: 1, issue: 'Link down', fix: 'Check network interface' };
    }

    return { layer: 1, status: 'healthy' };
  }

  // Layer 2 (Data Link): Check MAC addressing, switching
  async diagnoseDataLink(): Promise<Diagnostic> {
    const checks = {
      macAddress: await this.getMACAddress(),
      arpTable: await this.checkARPTable(),
      switchPort: await this.checkSwitchPort()
    };

    if (!checks.arpTable.has(checks.macAddress)) {
      return { layer: 2, issue: 'ARP failure', fix: 'Check ARP table, network segment' };
    }

    return { layer: 2, status: 'healthy' };
  }

  // Layer 3 (Network): Check IP addressing, routing
  async diagnoseNetwork(): Promise<Diagnostic> {
    const checks = {
      ipAddress: await this.getIPAddress(),
      routingTable: await this.getRoutingTable(),
      ping: await this.pingGateway()
    };

    if (!checks.ipAddress) {
      return { layer: 3, issue: 'No IP address', fix: 'Check DHCP or static config' };
    }

    if (!checks.ping) {
      return { layer: 3, issue: 'Cannot reach gateway', fix: 'Check routing table, gateway config' };
    }

    return { layer: 3, status: 'healthy' };
  }

  // Layer 4 (Transport): Check TCP/UDP, ports
  async diagnoseTransport(): Promise<Diagnostic> {
    const checks = {
      tcpConnection: await this.testTCPConnection(),
      portOpen: await this.checkPort(80),
      firewall: await this.checkFirewallRules()
    };

    if (!checks.tcpConnection) {
      return { layer: 4, issue: 'TCP connection failed', fix: 'Check firewall, port availability' };
    }

    return { layer: 4, status: 'healthy' };
  }

  // Layer 5-7 (Session, Presentation, Application)
  async diagnoseApplication(): Promise<Diagnostic> {
    const checks = {
      dnsResolution: await this.resolveDNS('example.com'),
      httpResponse: await this.testHTTP(),
      sslCertificate: await this.checkSSL()
    };

    if (!checks.dnsResolution) {
      return { layer: 7, issue: 'DNS resolution failed', fix: 'Check DNS server, network connectivity' };
    }

    if (!checks.httpResponse) {
      return { layer: 7, issue: 'HTTP request failed', fix: 'Check application server, SSL/TLS' };
    }

    return { layer: 7, status: 'healthy' };
  }

  // Comprehensive diagnosis
  async diagnoseAll(): Promise<DiagnosticReport> {
    const results = await Promise.all([
      this.diagnosePhysical(),
      this.diagnoseDataLink(),
      this.diagnoseNetwork(),
      this.diagnoseTransport(),
      this.diagnoseApplication()
    ]);

    const issues = results.filter(r => r.issue);
    
    return {
      healthy: issues.length === 0,
      issues,
      recommendations: this.generateRecommendations(issues)
    };
  }
}

Tools for each layer:

  • Layer 1: ethtool, link status LEDs
  • Layer 2: arp, ifconfig, switch logs
  • Layer 3: ping, traceroute, ip route
  • Layer 4: netstat, ss, telnet
  • Layer 5-7: dig, curl, wireshark

Key Takeaways

  • OSI model divides networking into 7 layers for standardization
  • Encapsulation: Each layer adds header, wraps data from layer above
  • Decapsulation: Each layer removes header, passes to layer above
  • Layer functions: Application (user interface), Transport (end-to-end), Network (routing), Data Link (framing), Physical (bits)
  • Troubleshooting: Identify which layer has the problem
  • Protocol mapping: Understand which protocols operate at which layers
  • TCP/IP vs OSI: TCP/IP has 4 layers, OSI has 7 (both useful for different purposes)

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