OSI(ISO) and TCP-IP Model

Introduction

There are two primary networking models that define how data is transmitted between devices:

1. OSI Model (ISO/OSI Model)
2. TCP/IP Model

Both models provide a layered approach to communication, breaking data transmission into structured steps for better understanding and troubleshooting.

1. OSI/ISO Model (Open Systems Interconnection Model)

• Standard reference model for communication between systems.
• Developed by International Telecommunication Union (ITU) and the International Organization for Standardization (ISO), hence also called the ISO/OSI model.
• Divides networking into 7 distinct layers, each with a specific role.

7 Layers of the OSI Model (Top to Bottom)

Layer	Function	Example Protocols/Technologies
7. Application	UserVim user interaction with network services	HTTP, FTP, SMTP, DNS
6. Presentation	Data formatting & encryption	SSL/TLS, JPEG, ASCII
5. Session	Manages communication sessions	NetBIOS, RPC, PPTP
4. Transport	Reliable/unreliable data delivery	TCP, UDP
3. Network	Logical addressing & routing	IP, ICMP, ARP
2. Data Link	Physical addressing (MAC)	Ethernet, Wi-Fi
1. Physical	Transmission of raw bits	Cables, Radio Waves

Key Feature: Each layer only interacts with the layer above and below it.

2. TCP/IP Model (Transmission Control Protocol/Internet Protocol)

• Foundation of the Internet and real-world networking.
• A practical model used for communication on public and private networks.
• Includes multiple protocols, not just TCP and IP.

TCP/IP Layers (Compared to OSI Model)

TCP/IP Layer	Equivalent OSI Layers	Function
Application	Application, Presentation, Session	Provides network services (HTTP, FTP, DNS)
Transport	Transport	Manages end-to-end communication (TCP, UDP)
Internet	Network	Routes data across networks (IP, ICMP, ARP)
Network Access	Data Link, Physical	Physical transmission of data (Ethernet, Wi-Fi)

Key Feature: More flexible than OSI, allowing variations as long as general rules are followed.

OSI vs. TCP/IP - Key Differences

Feature	OSI Model	TCP/IP Model
Structure	7 Layers	4 Layers
Usage	Theoretical, used for teaching and troubleshooting	Practical, used in real-world networks
Flexibility	Strict protocol guidelines	More adaptable
Development	Developed by ISO	Developed by DARPA
Example Protocols	Ethernet, PPP, ATM	TCP, IP, UDP

TCP/IP is used in practice, but OSI is useful for detailed analysis and troubleshooting.

3. Packet Transfers & Encapsulation

How Data Travels Through the Layers

1. Sender’s device processes data through each layer, adding a header at each step (Encapsulation).
2. Data moves through the Physical Layer (or Network Layer) for transmission.
3. Receiver’s device reverses the process, removing headers at each layer (Decapsulation).
4. The final data is received and used by the application.

Key Concept: Each layer adds a Protocol Data Unit (PDU), which contains control information for transmission.

A Protocol Data Unit (PDU) is a formatted data block exchanged between network layers. Each layer adds specific headers for communication.

Example: At the Network Layer, a PDU is called a Packet (IP header + data).

Why Understanding These Models is Important for Penetration Testers

• TCP/IP Model helps in understanding how connections are established and managed.
• OSI Model helps in breaking down network traffic and identifying vulnerabilities.
• Packet Analysis (e.g., Wireshark) relies on these models for deep network investigation.

Both models are valuable—TCP/IP is used in real-world networks, while OSI helps in detailed analysis.