IndianTechnoEra: Network Models & Standards | Computer Networks: From Scratch to Mastery

Chapter 2: Network Models & Standards

Understanding the OSI and TCP/IP models, networking standards, and data encapsulation

Introduction

Building on our introduction to computer networks, this chapter explores the conceptual models that define how networks operate. Understanding these models is essential for network design, troubleshooting, and communication between networking professionals.

By the end of this chapter, you will be able to:

• Describe all seven layers of the OSI model and their functions
• Compare the TCP/IP model with the OSI model
• Explain the roles of IEEE and IETF standards organizations
• Understand the encapsulation and decapsulation process
• Analyze network traffic using these models

The OSI Model: Understanding the 7 Layers

The Open Systems Interconnection (OSI) model is a conceptual framework that standardizes network communication functions into seven distinct layers. Developed by ISO in 1984, it provides a universal language for networking professionals.

Figure 1: The seven layers of the OSI model with example protocols and devices

Layer 7: Application Layer

Function: Provides network services directly to user applications

Protocols: HTTP, FTP, SMTP, DNS

Devices/Software: Web browsers, email clients

Example:

When you enter a URL in your browser, the HTTP request is initiated at this layer.

Layer 6: Presentation Layer

Function: Data translation, encryption, and compression

Protocols: SSL/TLS, JPEG, MPEG

Devices/Software: Encryption software, compression tools

Example:

When you visit a secure website (HTTPS), TLS encryption occurs here.

Layer 5: Session Layer

Function: Establishes, manages, and terminates connections

Protocols: NetBIOS, RPC

Devices/Software: Session management software

Example:

When you log in to a remote server, this layer maintains your session.

Layer 4: Transport Layer

Function: End-to-end communication, error recovery, and flow control

Protocols: TCP, UDP

Devices/Software: Firewalls, load balancers

Example:

TCP ensures your file download arrives complete and in order.

Layer 3: Network Layer

Function: Logical addressing and routing

Protocols: IP, ICMP, OSPF

Devices: Routers, layer 3 switches

Example:

Your router uses IP addresses to determine where to send packets.

Layer 2: Data Link Layer

Function: Physical addressing and access to media

Protocols: Ethernet, PPP, MAC

Devices: Switches, bridges, network interface cards

Example:

Your switch uses MAC addresses to forward frames to the correct device.

Layer 1: Physical Layer

Function: Transmission of raw bits over physical medium

Protocols: Ethernet, USB, Bluetooth physical layers

Devices: Hubs, repeaters, cables, connectors

Example:

The electrical signals traveling through your Ethernet cable operate here.

The TCP/IP Model: Structure and Comparison with OSI

While the OSI model is theoretical, the TCP/IP model (also called the Internet Protocol Suite) is the practical implementation that powers the modern Internet. It combines some OSI layers into four broader categories:

TCP/IP Layer	OSI Layers	Key Protocols	Function
Application	Application, Presentation, Session	HTTP, FTP, DNS, SMTP	End-user services
Transport	Transport	TCP, UDP	End-to-end communication
Internet	Network	IP, ICMP, ARP	Logical addressing and routing
Network Access	Data Link, Physical	Ethernet, Wi-Fi	Physical transmission

Key Differences Between OSI and TCP/IP

OSI Model

• 7 distinct layers
• More theoretical framework
• Clear separation between presentation and session functions
• Developed by ISO as a universal standard

TCP/IP Model

• 4 combined layers
• Practical implementation
• No formal session or presentation layers
• Evolved from ARPANET research

Common Pitfall

Many beginners confuse the layers between models. Remember that TCP/IP's Application layer encompasses three OSI layers, while its Network Access layer combines two OSI layers.

IEEE and IETF Standards: Shaping Network Protocols

IEEE Standards

The Institute of Electrical and Electronics Engineers (IEEE) develops standards primarily for the physical and data link layers:

• IEEE 802.3: Ethernet (wired networking)
• IEEE 802.11: Wi-Fi (wireless networking)
• IEEE 802.1Q: VLAN tagging
• IEEE 802.1X: Network access control

IEEE standards ensure hardware interoperability. For example, an 802.11ac Wi-Fi router will work with any 802.11ac-compatible device.

IETF Standards

The Internet Engineering Task Force (IETF) develops Internet standards through Request for Comments (RFC) documents:

• RFC 791: Internet Protocol (IP)
• RFC 793: Transmission Control Protocol (TCP)
• RFC 826: Address Resolution Protocol (ARP)
• RFC 2616: Hypertext Transfer Protocol (HTTP/1.1)

IETF standards evolve through community consensus, with new RFCs updating or obsoleting previous ones.

Standards Development Process

IEEE Process

1. Project authorization
2. Working group formation
3. Draft development
4. Balloting and approval
5. Publication

IETF Process

1. Internet Draft submission
2. Working group discussion
3. RFC Editor review
4. Publication as Proposed Standard
5. Progression to Internet Standard

Encapsulation and Decapsulation: Data Flow Across Layers

As data moves through the network layers, it undergoes encapsulation (adding headers) when sending and decapsulation (removing headers) when receiving. This process ensures each layer can communicate with its peer layer on another device.

Figure 2: Data encapsulation process through the OSI model layers

Protocol Data Units (PDUs)

OSI Layer	PDU Name	Contents
Application	Data	User data
Presentation	Data	Encrypted/compressed data
Session	Data	Session information + data
Transport	Segment (TCP) or Datagram (UDP)	Transport header + data
Network	Packet	IP header + segment
Data Link	Frame	MAC header + packet + trailer
Physical	Bits	Binary transmission

Encapsulation Example: HTTP Request

1. Application: Browser creates HTTP GET request
2. Presentation: Data may be compressed (gzip) or encrypted (TLS)
3. Session: Session ID added if using cookies
4. Transport: TCP header added with source/destination ports
5. Network: IP header added with source/destination IPs
6. Data Link: Ethernet header added with MAC addresses
7. Physical: Converted to electrical/optical signals

Practical Example: Analyzing Packet Flow with Wireshark

Wireshark is a powerful network protocol analyzer that lets us see encapsulation in action. Here's how to examine a simple HTTP request:

Step 1: Capture Traffic

1. Install Wireshark from wireshark.org
2. Select your network interface (e.g., Wi-Fi or Ethernet)
3. Start capturing packets

Step 2: Filter HTTP Traffic

# In Wireshark's filter bar:
http and ip.dst == example.com
                    

This shows only HTTP traffic to example.com

Step 3: Examine Layers

Select a packet and expand sections in the packet details pane:

Frame (Physical layer - bits)
Ethernet II (Data Link - MAC addresses)
Internet Protocol Version 4 (Network - IP addresses)
Transmission Control Protocol (Transport - ports)
Hypertext Transfer Protocol (Application - HTTP)
                    

Troubleshooting Tip

If you can't see higher layer protocols, check if encryption is preventing inspection. For HTTPS traffic, you'll only see TCP segments unless you configure Wireshark with server keys.

Visualizing the OSI Model

Figure 3: Interactive OSI model showing encapsulation process

How to Read This Visualization

The animation demonstrates how data moves through the OSI model layers:

• Top-down (sending): Data is encapsulated with headers at each layer
• Bottom-up (receiving): Headers are removed during decapsulation
• Peer communication: Each layer communicates with its counterpart on the other device

Chapter Summary

Key Concepts

• The OSI model's seven layers provide a framework for understanding network communication
• The TCP/IP model is the practical implementation with four layers
• IEEE standards govern physical and data link layers, while IETF RFCs define Internet protocols
• Encapsulation adds headers as data moves down the stack; decapsulation removes them

Best Practices

• When troubleshooting, identify which layer the problem occurs at
• Use tools like Wireshark to analyze actual protocol behavior
• Refer to official IEEE and IETF standards for implementation details
• Remember that real-world networks often mix models (e.g., TCP/IP with OSI terminology)

Further Reading

• Books: "Computer Networking: A Top-Down Approach" by Kurose and Ross (Chapter 1)
• RFCs: RFC 791 (IP), RFC 793 (TCP)
• IEEE Standards: 802.3 (Ethernet), 802.11 (Wi-Fi)
• Tools: Wireshark for packet analysis, Cisco Packet Tracer for simulation