January 7, 2025 7 min to read

Understanding OSI 7 Layers and TCP/IP 4 Layers

A comprehensive comparison of OSI and TCP/IP network models

Overview

Let’s explore the OSI 7 Layer model and TCP/IP 4 Layer model, understanding their structures and differences.

Why Network Models Matter

Network models provide a standardized framework for understanding how data travels from one device to another. They help in:

• Troubleshooting network issues
• Designing network protocols
• Facilitating vendor interoperability
• Simplifying network complexity through modularity
• Providing a common language for network professionals

OSI 7 Layers

What is OSI?

The OSI (Open Systems Interconnection) model is a reference model for network protocol stacks developed by ISO (International Organization for Standardization).

Layer Structure

OSI Model (7 Layers):

1. Application : Interfaces with the user and provides network services (HTTP, FTP).
2. Presentation : Formats and encrypts data for the application layer.
3. Session : Manages sessions and connections between devices.
4. Transport : Ensures reliable data transfer (TCP, UDP).
5. Network : Handles logical addressing and routing (IP).
6. Data Link : Manages error detection and frame synchronization (Ethernet).
7. Physical : Transmits raw data over physical media (cables, radio waves).

Detailed Explanation of Each OSI Layer

Layer 7: Application Layer

Function: Provides network services directly to end-users and applications.

Key Protocols: HTTP/HTTPS (Web), FTP (File transfer), SMTP (Email), DNS (Domain name), SSH (Remote access), Telnet (Terminal)

Data Unit: Data

Analogy: The content of a letter (the actual message you want to send).

Layer 6: Presentation Layer

Function: Translates data between application format and network format.

Responsibilities: Data translation (ASCII, EBCDIC), Encryption/Decryption (SSL/TLS), Compression (JPEG, MPEG, GIF), Data formatting

Data Unit: Data

Analogy: Translation of your letter into a language the recipient understands.

Layer 5: Session Layer

Function: Establishes, manages, and terminates communication sessions.

Responsibilities: Session establishment/maintenance/termination, checkpointing, dialog control (full/half-duplex)

Protocols: NetBIOS, RPC, PPTP

Data Unit: Data

Analogy: A phone call - starting, maintaining, and ending a conversation.

Layer 4: Transport Layer

Function: Provides reliable data transfer services between end systems.

Key Protocols: TCP (reliable), UDP (fast, unreliable)

Responsibilities: Segmentation/reassembly, error recovery, flow control, connection management, port addressing

Data Unit: Segments (TCP) / Datagrams (UDP)

Analogy: A postal service that ensures your letters arrive in order and without damage.

Layer 3: Network Layer

Function: Routes data packets between different networks.

Key Protocols: IP (IPv4, IPv6), ICMP, IGMP, OSPF, BGP, RIP

Responsibilities: Logical addressing (IP), path determination, packet forwarding, fragmentation/reassembly

Data Unit: Packets

Devices: Routers, Layer 3 Switches

Analogy: A mail sorting facility that determines the best route for your letter.

Layer 2: Data Link Layer

Function: Provides reliable data transfer between directly connected nodes.

Sub-layers: MAC (hardware addressing), LLC (flow control, error checking)

Key Protocols: Ethernet, PPP, HDLC, Frame Relay, ATM

Responsibilities: Physical addressing (MAC), error detection/handling, frame sync, media access control

Data Unit: Frames

Devices: Switches, Bridges, NICs

Analogy: Putting your letter in an envelope with a specific address.

Layer 1: Physical Layer

Function: Transmits raw bit stream over physical medium.

Responsibilities: Bit transmission, medium specs, signal encoding, data rate, connectors

Standards: RS-232, V.35, RJ45, Ethernet physical, DSL, ISDN

Data Unit: Bits

Devices: Hubs, Repeaters, Cables, Connectors

Analogy: The actual transportation method (truck, plane) that carries your letter.

TCP/IP 4 Layers

What is TCP/IP?

TCP/IP (Transmission Control Protocol/Internet Protocol) is the reference model for internet protocol stacks.

Layer Structure

TCP/IP Model (4 Layers):

1. Application : Combines Application, Presentation, and Session layers (HTTP, DNS).
2. Transport : Provides end-to-end communication (TCP, UDP).
3. Internet : Handles addressing and routing (IP).
4. Network Interface : Combines Data Link and Physical layers, managing hardware transmission.

Detailed Explanation of TCP/IP Layers

Layer 4: Application Layer

Equivalent OSI Layers: Application, Presentation, Session

Key Protocols: HTTP/HTTPS, FTP, SMTP/POP3/IMAP, DNS, Telnet/SSH, SNMP

Function: Provides network services to applications and handles data formatting, encryption, and session management.

Layer 3: Transport Layer

Equivalent OSI Layer: Transport

Key Protocols: TCP (reliable), UDP (fast, unreliable)

Function: Manages end-to-end communication, segmentation, flow control, error recovery.

Port Ranges:

• Well-known: 0-1023
• Registered: 1024-49151
• Dynamic/Private: 49152-65535

Layer 2: Internet Layer

Equivalent OSI Layer: Network

Key Protocols: IP (IPv4, IPv6), ICMP, ARP, IGMP

Function: Handles logical addressing, routing, and packet forwarding between networks.

Layer 1: Network Interface Layer

Equivalent OSI Layers: Data Link, Physical

Technologies: Ethernet, Wi-Fi (802.11), PPP, FDDI, Token Ring

Function: Manages physical addressing, media access, and transmission of data over physical networks.

Key Differences: OSI vs TCP/IP Model

OSI vs TCP/IP Model Comparison

Data Encapsulation and Flow

Data encapsulation is the process of adding headers (and sometimes trailers) as data moves down the OSI layers:

Practical Application: Troubleshooting Using the OSI Model

The OSI model is invaluable for troubleshooting network issues by providing a systematic approach:

Real-World Example: Web Page Load Through OSI Layers

When you type https://example.com in a browser:

1. Application Layer: Browser initiates HTTP request
2. Presentation Layer: Data is formatted and possibly encrypted (HTTPS/TLS)
3. Session Layer: TCP session established with web server
4. Transport Layer: HTTP request packaged into TCP segments
5. Network Layer: IP routing to the server via IP addressing
6. Data Link Layer: Frames constructed with MAC addressing
7. Physical Layer: Bits transmitted over network medium

The server responds by sending data back up through the layers to your browser.

Key Points

1. OSI model is more detailed but theoretical
2. TCP/IP is more practical and widely implemented
3. Both models serve as reference frameworks
4. TCP/IP combines several OSI layers for efficiency
5. Understanding these models helps with network design and troubleshooting
6. Each layer has specific responsibilities and protocols

Reference

• Network Layers - RaonCTF
• TCP/IP Protocol Architecture
• OSI Model - ISO
• Cisco Networking Basics: Understanding the OSI Model
• RFC 1122 - Requirements for Internet Hosts