Day 3: OSI Model and TCP/IP Suite

OSI Model & TCP/IP Suite

Networking Models: Overview

A networking model categorizes and provides structure for networking protocols and standards.

Protocol = A set of logical rules defining how network devices and software communicate and function.

Networking models help engineers understand which layer is responsible for which functions, making troubleshooting and design much easier.

OSI Model

What is the OSI Model?

OSI = Open Systems Interconnection Model

Conceptual model created by ISO (International Organization for Standardization)
Standardizes different functions in a network across 7 layers
Purpose: Provide a reference framework for understanding how networks operate
These 7 layers work together hierarchically to enable end-to-end communication

Key Concept: Data Movement

As data moves through the layers, it follows two processes:

Encapsulation (Top → Bottom, Layers 7 → 1)

Data travels DOWN the OSI stack
Each layer adds its own header/trailer to the data
Headers contain layer-specific information needed for that layer's function
Happens at the sender's device

De-encapsulation (Bottom → Top, Layers 1 → 7)

Data travels UP the OSI stack
Each layer removes (strips) its header/trailer
Extracts the information needed at that layer
Happens at the receiver's device

Key Concept: Layer Interactions

Adjacent-Layer Interaction

Communication between different layers on the same device
Example: Layer 5 sends data to Layer 4, Layer 4 adds header and sends to Layer 3

Same-Layer Interaction

Communication between the same layer on different devices
Example: The HTTP layer (Layer 7) on your PC browser talks to HTTP layer on a web server
You can analyze one layer's interaction in isolation, ignoring what other layers do

The 7 Layers of the OSI Model

Layer 7 - APPLICATION

What it does:

Closest to the end user
Interacts directly with user applications and software
Provides services that enable network communication for applications

Key Functions:

Identifying and locating communication partners
Synchronizing communication between applications
Providing user-visible network services

Common Layer 7 Protocols:

HTTP / HTTPS (web browsing)
FTP (file transfer)
SMTP / POP3 (email)
DNS (domain name resolution)
Telnet / SSH (remote login)

Who uses it: Application developers and end users

Layer 6 - PRESENTATION

What it does:

Translates data into an appropriate format for transmission
Converts between application format and network transmission format
Handles data encryption, compression, and translation

Key Functions:

Data formatting and conversion
Encryption/Decryption of data
Data compression
Character encoding (ASCII, Unicode, etc.)

Who uses it: Application developers

Layer 5 - SESSION

What it does:

Controls "dialogues" (sessions) between communicating applications
Manages the connection lifecycle between local and remote applications

Key Functions:

Establishing connections
Managing (maintaining) connections
Terminating connections
Dialog control and synchronization

Common Session Layer Examples:

Remote Desktop Protocol (RDP)
X Window System

Who uses it: Application developers

⚠️ Important Note: Top 3 Layers

Network engineers rarely work directly with Layers 5-7.

These layers are the domain of application developers who design software to use networks. When troubleshooting network issues, most work happens at Layers 1-4.

Layer 4 - TRANSPORT

What it does:

Segments (breaks up) and reassembles data for communication between end hosts
Provides HOST-TO-HOST (end-to-end) communication

Why segmentation matters:

Large pieces of data are broken into smaller segments
Smaller segments are easier to send across the network
If an error occurs, only the affected segment needs retransmission (not the entire message)

Key Protocols:

TCP (Transmission Control Protocol) — reliable, connection-oriented
UDP (User Datagram Protocol) — fast, connectionless

PDU (Protocol Data Unit) at Layer 4:

When Layer 7-5 data arrives at Layer 4, Layer 4 adds a Layer 4 Header
Result: SEGMENT
Format: [DATA + L4 Header]

Common Layer 4 Port Numbers (CCNA exam-relevant):

TCP 80 (HTTP)
TCP 443 (HTTPS)
TCP 22 (SSH)
UDP 53 (DNS)

Layer 3 - NETWORK

What it does:

Provides connectivity between different networks (outside the local LAN)
Provides logical addressing using IP addresses (unlike Layer 2's MAC addresses)
Performs path selection — determines the best route for data to travel from source to destination
Works with routers to forward data between networks

Key Functions:

IP addressing and routing
Path selection using routing protocols
Logical addressing (not physical)

Device operating at Layer 3:

Routers — forward packets between networks based on IP addresses

PDU (Protocol Data Unit) at Layer 3:

When Layer 4 header arrives at Layer 3, Layer 3 adds a Layer 3 Header
Result: PACKET
Format: [DATA + L4 Header + L3 Header]
Contains: Source/destination IP addresses

Common Layer 3 Protocols:

IP (IPv4, IPv6)
ICMP (ping)
IGMP

Layer 2 - DATA LINK

What it does:

Provides NODE-TO-NODE connectivity and data transfer
Examples: PC to Switch, Switch to Router, Router to Router
Defines how data is formatted for transmission over physical medium (e.g., UTP cables)
Detects (and sometimes corrects) physical layer errors
Uses Layer 2 addressing (MAC addresses) — separate from Layer 3 IP addresses

Key Functions:

Node-to-node delivery
Physical addressing (MAC addresses)
Framing and error detection
Media access control (determining who uses the network when)

Device operating at Layer 2:

Switches — forward frames between ports based on MAC addresses

PDU (Protocol Data Unit) at Layer 2:

When Layer 3 packet arrives at Layer 2, Layer 2 adds both a Header AND a Trailer
Result: FRAME
Format: [L2 Header + DATA + L4 Header + L3 Header + L2 Trailer]
Contains: Source/destination MAC addresses

Common Layer 2 Protocols:

Ethernet
PPP (Point-to-Point Protocol)
Frame Relay

Layer 1 - PHYSICAL

What it does:

Defines the physical characteristics of the network medium
Converts digital bits into electrical or radio signals for transmission
Receives signals and converts them back to digital bits

Key Functions:

Physical medium specifications (copper, fiber, wireless)
Voltage levels and signal timing
Maximum transmission distances
Physical connectors and cable specifications
Bit transmission (0s and 1s)

Examples of Layer 1:

RJ-45 connectors
Ethernet cables (UTP, fiber-optic)
Hub devices (simple repeaters)
The actual wires and wireless frequencies

All content from your Networking Devices and Interfaces & Cables notes is Layer 1 related

OSI Model: PDU Summary

A PDU (Protocol Data Unit) is the formatted data at each layer of the OSI model. As data moves down the stack, each layer adds information to the PDU.

Encapsulation Process (Sender, Top to Bottom):

Layer 7-5: [DATA]

↓

Layer 4: [DATA + L4 Header] = SEGMENT

↓

Layer 3: [DATA + L4 Header + L3 Header] = PACKET

↓

Layer 2: [L2 Header + DATA + L4 Header + L3 Header + L2 Trailer] = FRAME

↓

Layer 1: Convert Frame to bits (0s and 1s) = BITS

De-encapsulation Process (Receiver, Bottom to Top):

Layer 1 converts bits to frame
Layer 2 strips L2 header/trailer, checks for errors
Layer 3 strips L3 header, checks IP address
Layer 4 strips L4 header, hands data to upper layers
Layers 5-7 process the original data

Quick Reference Table:

OSI Layer	PDU Name	Headers/Trailers Added	Key Information
7-5	DATA	None (origination)	User data
4	SEGMENT	Layer 4 Header	Port numbers (TCP/UDP)
3	PACKET	Layer 3 Header	IP addresses
2	FRAME	L2 Header + L2 Trailer	MAC addresses
1	BITS	None (transmission)	0s and 1s

TCP/IP Suite (Modern Model)

What is TCP/IP?

TCP/IP Suite:

Actual set of communications protocols used in the modern Internet and networks
Named after two foundational protocols: TCP and IP
Developed by U.S. Department of Defense through DARPA (Defense Advanced Research Projects Agency)
This is the model actually deployed and in use today

How is it Different from OSI?

Similarities:

Both are layered models
Both help organize network functions

Key Differences:

Aspect	OSI Model	TCP/IP Model
Layers	7 layers	4-5 layers (varies by source)
Status	Conceptual reference	Practical, in-use model
Development	International standards body (ISO)	U.S. Department of Defense (DARPA)
Protocol specificity	Generic, protocol-agnostic	Built around TCP/IP protocols
Use	Teaching and troubleshooting framework	Actual implementation

TCP/IP Model Layers (Simplified)

Most commonly shown as 4 layers:

Application Layer (Layers 5-7 in OSI)
- Protocols: HTTP, HTTPS, FTP, SMTP, DNS, SSH
Transport Layer (Layer 4 in OSI)
- Protocols: TCP, UDP
Internet Layer (Layer 3 in OSI)
- Protocols: IP, ICMP
Link Layer (Layers 1-2 in OSI)
- Protocols: Ethernet, PPP
- Physical medium and hardware

OSI vs. TCP/IP: Why OSI Still Matters

Although TCP/IP is the working model:

OSI model still influences how network engineers think and talk about networks
CCNA exam heavily emphasizes OSI model concepts
OSI's 7-layer breakdown is clearer for teaching individual functions
Both models coexist in professional environments

Quick Reference: Which Device Operates at Which Layer?

Device	OSI Layer	Function
Hub	Layer 1	Repeats signals
Switch	Layer 2	Forwards frames by MAC address
Router	Layer 3	Forwards packets by IP address
Firewall	Layer 3-7	Filters traffic based on multiple criteria
PC / Server	Layers 1-7	All layers

Key Takeaways for CCNA

✅ Understand encapsulation/de-encapsulation — This is fundamental to how networks work

✅ Know what each layer does — Particularly Layers 1-4 for network engineers

✅ Understand PDU names — Segment, Packet, Frame, Bits (test questions often ask "what is this called?")

✅ Know device-to-layer mapping — Switches at Layer 2, Routers at Layer 3

✅ Understand adjacent vs. same-layer interaction — Helps with troubleshooting concepts

✅ TCP/IP is the real model — OSI is the teaching model; they're used together in practice