Section 1: Introduction & OSI Model
1.1 What is the OSI Model?
The Open Systems Interconnection (OSI) Model is a conceptual framework that standardizes how different network protocols communicate. Developed by the International Organization for Standardization (ISO) in 1984, it divides network communication into seven distinct layers.
Why OSI Matters
The OSI Model helps us:
- Understand how data flows through a network
- Troubleshoot network problems layer by layer
- Design and implement network protocols
- Communicate using standard terminology
- Analyze packet captures (like in Wireshark)
1.2 The Seven Layers - Visual Breakdown
OSI MODEL - 7 LAYERS
Layer Name Function Protocols/Examples
7 APPLICATION User interface & services HTTP, DNS, FTP,
Network applications SMTP, SSH, SIP
6 PRESENTATION Data formatting & encryption SSL/TLS, JPEG,
Compression, encoding MPEG, ASCII
5 SESSION Session management NetBIOS, RPC,
Dialog control PPTP, SIP
4 TRANSPORT End-to-end delivery TCP, UDP,
Segmentation, flow control SCTP, RTP
3 NETWORK Logical addressing & routing IP, ICMP, IGMP,
Path determination IPsec, ARP*
2 DATA LINK Physical addressing Ethernet, Wi-Fi,
Frame transmission PPP, ARP*
1 PHYSICAL Bit transmission Cables, Hubs,
Electrical/optical signals NICs, Repeaters
* ARP operates between Layer 2 and Layer 3
1.3 Memory Trick - Remember the Layers
Top-Down (Layer 7 to 1)
"All People Seem To Need Data Processing"
| Letter | Layer | Name |
|---|---|---|
| A | 7 | Application |
| P | 6 | Presentation |
| S | 5 | Session |
| T | 4 | Transport |
| N | 3 | Network |
| D | 2 | Data Link |
| P | 1 | Physical |
Bottom-Up (Layer 1 to 7)
"Please Do Not Throw Sausage Pizza Away"
| Letter | Layer | Name |
|---|---|---|
| P | 1 | Physical |
| D | 2 | Data Link |
| N | 3 | Network |
| T | 4 | Transport |
| S | 5 | Session |
| P | 6 | Presentation |
| A | 7 | Application |
1.4 OSI vs TCP/IP Model Comparison
OSI MODEL (7 Layers) TCP/IP MODEL (4 Layers)
┌──────────────────┐ ┌──────────────────┐
│ Application │──────────────│ │
├──────────────────┤ │ Application │
│ Presentation │──────────────│ │
├──────────────────┤ │ │
│ Session │──────────────│ │
├──────────────────┤ ├──────────────────┤
│ Transport │──────────────│ Transport │
├──────────────────┤ ├──────────────────┤
│ Network │──────────────│ Internet │
├──────────────────┤ ├──────────────────┤
│ Data Link │──────────────│ │
├──────────────────┤ │ Network Access │
│ Physical │──────────────│ │
└──────────────────┘ └──────────────────┘
Theoretical Model Practical Implementation
(Reference) (Real-world Internet)
Key Differences
| Aspect | OSI Model | TCP/IP Model |
|---|---|---|
| Layers | 7 | 4 |
| Development | ISO (theoretical) | DARPA (practical) |
| Approach | Protocol-independent | Protocol-specific |
| Usage | Reference/teaching | Real-world networking |
| Session/Presentation | Separate layers | Combined in Application |
1.5 Data Encapsulation Process
When data travels down the OSI stack, each layer adds its own header (and sometimes trailer). This process is called encapsulation.
SENDER (Encapsulation - Adding Headers)
Layer 7 ┌─────────────────────────────────────────────┐
App │ DATA │
└─────────────────────────────────────────────┘
│
▼
Layer 4 ┌───────┬─────────────────────────────────────┐
Transport │TCP/UDP│ DATA │
│Header │ │
└───────┴─────────────────────────────────────┘
│
▼
Layer 3 ┌───────┬───────┬─────────────────────────────┐
Network │ IP │TCP/UDP│ DATA │
│Header │Header │ │
└───────┴───────┴─────────────────────────────┘
│
▼
Layer 2 ┌────────┬───────┬───────┬────────────────┬─────┐
Data Link │Ethernet│ IP │TCP/UDP│ DATA │ FCS │
│ Header │Header │Header │ │ │
└────────┴───────┴───────┴────────────────┴─────┘
│
▼
Layer 1 ┌────────────────────────────────────────────────┐
Physical │ 1010110101001011010101010101010110101010... │
│ (Electrical Signals / Bits) │
└────────────────────────────────────────────────┘
1.6 Protocol Data Units (PDUs)
Each layer has its own name for the data it handles:
| Layer | PDU Name | Description |
|---|---|---|
| 7-5 | Data (Message) | Application data (payload) |
| 4 | Segment (TCP) / Datagram (UDP) | Transport layer unit, includes port numbers |
| 3 | Packet | Network layer unit, includes IP addresses |
| 2 | Frame | Data link layer unit, includes MAC addresses |
| 1 | Bit | Physical layer unit, binary 1s and 0s |
Visual Representation
DATA ← Layer 7-5 (Application Data)
│
▼
┌────┬────────┐
│HDR │ DATA │ ← Layer 4 (Segment/Datagram)
└────┴────────┘
│
▼
┌────┬────┬────────┐
│HDR │HDR │ DATA │ ← Layer 3 (Packet)
└────┴────┴────────┘
│
▼
┌────┬────┬────┬────────┬─────┐
│HDR │HDR │HDR │ DATA │ FCS │ ← Layer 2 (Frame)
└────┴────┴────┴────────┴─────┘
│
▼
10101010101010101010101010101010 ← Layer 1 (Bits)
1.7 Layer Functions Summary
Layer 7 - Application
Function: Provides network services directly to applications
- User interface for network access
- Email, file transfer, web browsing
- Network virtual terminal
Protocols: HTTP, HTTPS, FTP, SMTP, DNS, SSH, Telnet, SNMP, SIP, LDAP, NFS, SMB
Layer 6 - Presentation
Function: Data translation, encryption, compression
- Character encoding (ASCII, Unicode)
- Data compression (reduces size)
- Encryption/Decryption (security)
Examples: SSL/TLS, JPEG, MPEG, GIF, ASCII, EBCDIC
Layer 5 - Session
Function: Manages sessions between applications
- Establishes, maintains, terminates sessions
- Synchronization (checkpoints)
- Dialog control (half/full duplex)
Protocols: NetBIOS, RPC, PPTP, PAP, SIP (session aspects)
Layer 4 - Transport
Function: End-to-end communication and data integrity
- Segmentation and reassembly
- Flow control
- Error detection and correction
- Connection-oriented (TCP) or connectionless (UDP)
Protocols: TCP, UDP, SCTP, RTP, RTCP
Layer 3 - Network
Function: Logical addressing and routing
- IP addressing (source and destination)
- Path determination (routing)
- Packet forwarding
- Fragmentation
Protocols: IPv4, IPv6, ICMP, IGMP, IPsec, OSPF, BGP
Layer 2 - Data Link
Function: Physical addressing and frame transmission
- MAC addressing
- Frame creation and error detection (FCS)
- Media access control
- Flow control (local)
Protocols: Ethernet (802.3), Wi-Fi (802.11), PPP, HDLC, ARP
Sub-layers:
- LLC (Logical Link Control) - Error/flow control
- MAC (Media Access Control) - Physical addressing
Layer 1 - Physical
Function: Physical transmission of raw bits
- Electrical/optical signal transmission
- Bit synchronization
- Physical topology (bus, star, ring)
- Transmission mode (simplex, half/full duplex)
Devices: Cables, Hubs, Repeaters, NICs, Connectors
Standards: RS-232, RJ-45, 100BASE-TX, 1000BASE-T
1.8 Real-World Example: Web Request
Let’s trace an HTTP request through all layers:
Layer 7 (Application)
Browser creates HTTP GET request:
GET / HTTP/1.1\r\nHost: www.example.com\r\n\r\n
Layer 6 (Presentation)
- If HTTPS: TLS encrypts the data
- Character encoding: UTF-8
Layer 5 (Session)
- Establishes session with server
- Manages TLS session if HTTPS
Layer 4 (Transport) TCP segments data, adds:
- Source Port: 52431 (random ephemeral)
- Destination Port: 80 (HTTP) or 443 (HTTPS)
- Sequence numbers for ordering
Layer 3 (Network) IP packet created with:
- Source IP: 192.168.1.100 (your PC)
- Destination IP: 93.184.216.34 (example.com)
- TTL: 64
Layer 2 (Data Link) Ethernet frame created with:
- Source MAC: AA:BB:CC:DD:EE:FF (your NIC)
- Destination MAC: 11:22:33:44:55:66 (router/gateway)
- EtherType: 0x0800 (IPv4)
- FCS (Frame Check Sequence) for error detection
Layer 1 (Physical) Electrical signals transmitted over:
- Ethernet cable (Cat5e/Cat6)
- Or Wi-Fi radio waves
- Bits: 10101010101010101010101010…
1.9 Wireshark and OSI Layers
In Wireshark, you can see each layer’s information:
WIRESHARK PACKET DETAILS PANE
▼ Frame 1: 342 bytes on wire ← Physical (L1)
▼ Ethernet II, Src: aa:bb:cc:dd:ee:ff, Dst: ... ← Data Link (L2)
▼ Internet Protocol Version 4, Src: 192.168.1.100 ← Network (L3)
▼ Transmission Control Protocol, Src Port: 52431 ← Transport (L4)
▼ Hypertext Transfer Protocol ← Application (L7)
GET / HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0...
Each expandable section corresponds to a layer’s header information.
1.10 Chapter Summary
Key Takeaways
- OSI Model has 7 layers (Application → Physical)
- Each layer has specific functions and protocols
- Data is encapsulated with headers as it moves down the stack
- PDU names: Data → Segment → Packet → Frame → Bits
- TCP/IP model is the practical implementation (4 layers)
- Understanding layers helps troubleshoot network issues
- Wireshark displays information organized by layers
Next: Section 2: Layer 2 - Data Link - Ethernet, ARP, MAC Addressing