The Journey of a Single Click: How Data Travels Through the 7-Layer OSI Model

What really happens in the milliseconds between clicking a link and seeing a webpage appear?

In under 200 milliseconds, your request may traverse dozens of routers, cross multiple countries, switch physical media, and pass through several abstraction layers — long before a single pixel is rendered on your screen.

It feels instantaneous.

But beneath the surface, a precisely coordinated, highly engineered process is unfolding.

To understand how data reliably travels across your room or around the world, engineers use a conceptual framework called the OSI (Open Systems Interconnection) Model. It breaks networking into seven distinct layers, each with a clearly defined responsibility. While modern networks implement the TCP/IP model in practice, the OSI model remains invaluable for understanding, designing, and debugging networked systems.

In this article, we’ll trace a simple network request — loading https://example.com—as it travels down through the seven OSI layers, across the network, and back up again on the receiving server.

The Fundamental Concept: Encapsulation

Before diving into the layers, we must understand Encapsulation.

Think of it like sending a delicate letter.

1. You write the letter (data).
2. You put it in an envelope (header added)
3. You put that envelope in a courier bag (header added)
4. The courier puts the bag in shipping container (header added)

Networking works the same way.

As data moves down the OSI layers on the sender’s side, each layer wraps the data from the layer above with its own header (and sometimes a trailer). This wrapping process is called encapsulation.

When the data reaches its destination, the process reverses. Each layer removes its corresponding header — this is decapsulation — until the original data reaches the receiving application.

The Journey Down: Sender Side

Let’s trace what happens when you type https://example.com into your browser and press Enter.

At each layer, the data has a specific name called a Protocol Data Unit (PDU).

Layer 7: Application Layer

This is the layer closest to the user. It doesn’t include the application itself (like Chrome or curl), but rather the protocols that applications use to communicate over the network.

Action:

• Your browser generates an HTTP request asking the server for a resource.

PDU: Raw application data. Below code block showing layer 7 data when sending get request through curl.

#Layer 7: HTTP Request

GET / HTTP/1.1
Host: example.com
User-Agent: curl/8.4.0
Accept: */*

Common protocols: HTTP, HTTPS, SMTP, POP3, IMAP, DNS, FTP

Layer 6: Presentation Layer

This layer ensures data is readable and secure for the receiving system.

Action:

• Translates data formats (e.g., ASCII vs Unicode)
• Encrypts data when using HTTPS (via TLS)
• Optionally compresses data

In real-world TCP/IP stacks, TLS spans multiple layers, but it maps conceptually to the Presentation layer in OSI.

PDU: Still referred as Data but now its formatted, compressed and secured.

#Layer 6: Encrypted & Compressed Binary data of HTTP Request (conceptually)

9f a3 4b 7c 91 e2 ...

Common protocols/formats: SSL, TLS, ASCII, EBCDIC

Layer 5: Session Layer

This layer manages sessions — logical conversations between applications.

Action:

• Establishes, maintains, and terminates communication sessions
• Ensures the request belongs to the correct conversation

In modern networking, Session-layer responsibilities are often handled by the Application or Transport layers, but OSI keeps it separate for conceptual clarity.

PDU: Still referred as Data. (No modification in payload)

Layer 4: Transport Layer

Now we get to the heavy lifting of ensuring data arrives correctly. This layer receives the large chunk of data from the upper layers and prepares it for transport.

Action:

• It breaks the large amount of data into smaller manageable chunks called Segments.
• Adds port numbers to ensure the message gets to the right application on the computer.
• Handles reliability, ordering, and retransmission (TCP)

PDU: Referred as a Segment (for TCP) or Datagram (for UDP)

#Layer 4: PDU generated by upper layers converted into one/many segments 
┌────────────────────────────────────────┐
│ Source Port      : 52344               │
│ Destination Port : 443                 │
│ Sequence Number  : 1029384756          │
│ Acknowledgment   : 5566778899          │
│ Flags            : PSH, ACK            │
│ Window Size      : 64240               │
│ Checksum         : 0xA91C              │
└────────────────────────────────────────┘
│ Payload Partition-0(Data).             │
│ [Encrypted / Encoded / Raw bytes]      │
└────────────────────────────────────────┘

Common protocols: TCP (Transmission Control Protocol), UDP (User Datagram Protocol)

Layer 3: Network Layer

This layer is responsible for routing data across different networks to its final destination.

Action:

• Adds logical addressing (IP addresses)
• Determines the best path to the destination
• Routers operate primarily at this layer

It does logical addressing, basically it wraps the TCP Segment into an IP Packet. It adds as header containing the Source IP address and the Destination IP address. Routers examine the destination IP address and consult their routing tables to determine the best path.

PDU: Referred as a Packet

#Layer 3: Wrapped Segment referred as Packet
┌───────────────────────────────────────────┐
│ Version          : 4                      │
│ Header Length    : 20 bytes               │
│ Total Length     : 552 bytes              │
│ Identification   : 0x1C46                 │
│ Flags / Offset   : DF                     │
│ Time To Live     : 64                     │
│ Protocol         : 6 (TCP)                │
│ Header Checksum  : 0xB861                 │
│ Source IP        : 192.168.1.10           │
│ Destination IP   : 93.184.216.34          │
└───────────────────────────────────────────┘
│ Payload (Layer 4 Segment)                 │
│ [TCP Header + Data]                       │
└───────────────────────────────────────────┘

Common protocols : IPv4, IPv6, ICMP

Layer 2: Data Link Layer

The Network layer handles the long-distance journey; the Data Link layer is responsible for the immediate next hop — getting the data from your computer’s network card to your Wi-Fi router.

Action:

• Wraps packets into frames
• Uses MAC addresses
• Performs error detection using CRC/FCS

It does physical addressing, basically it wraps the IP Packet into a Frame. It adds a header containing the MAC (Media Access Control) addresses. The destination MAC is not the final server, but the immediate next device(your router). It also does error detection, it adds a trailer (FCS — Frame check sequence) to the end of the frame to ensure the data wasn’t corrupted during the hop to the router.

At every router hop, the Layer 2 frame is stripped and rebuilt with new MAC addresses, while the IP packet remains intact.

PDU: Referred as a Frame

#Layer 2: Wrapped Packet, now referred as Frame
┌──────────────────────────────────────────────┐
│ Destination MAC : 00:1A:2B:3C:4D:5E          │
│ Source MAC      : 08:00:27:AA:BB:CC          │
│ EtherType       : 0x0800 (IPv4)              │
└──────────────────────────────────────────────┘
│ Payload (Layer 3 Packet)                     │
│ [IP Header + TCP Segment + Data]             │
└──────────────────────────────────────────────┘
│ Frame Check Sequence (CRC)                   │
└──────────────────────────────────────────────┘

Common protocols : Ethernet (IEEE 802.3), Wi-Fi (IEEE 802.11), PPP (Point-to-Point Protocol)

Layer 1: Physical Layer

This is where bits meet physics.

Action:

• Converts binary data into physical signals
• No headers, no addresses — just signalsThe Network Interface Card (NIC) takes the binary 1s and 0s that make up the Frame and converts them into physical signals suitable for the medium.
• If Ethernet: Electrical voltage pulses over copper wire.
• If Wi-Fi: Radio frequency waves through the air.
• If Fiber: Pulses of light down a glass strand.

At this layer, there are no addresses, no headers, and no protocols to interpret — just raw bits transmitted according to electrical, optical, or radio standards.

PDU: Stream of Bits

#Layer 1: Conceptual Bit stream
10110010 11001001 00101101 ...

Common standards: RJ45, USB, Bluetooth, DSL, ISDN

The Journey Back Up: Receiver Side

The physical signals travel across the internet, hopping from router to router. At the destination server, the entire process happens in exact reverse order (Decapsulation).

The server’s physical layer receives signals and turns them into bits. The Data Link layer checks the MAC address and strips the frame header. The Network layer checks the IP and strips the IP header. This continues up to the Application layer, which finally receives the raw “Get me the homepage” request and processes it.

The server then generates a response, encapsulates it through the same OSI layers in reverse order, sends it back across the network, and the sender side decapsulates the response to finally render the webpage in the browser.

Why the OSI Model Still Matters

The OSI model isn’t about memorizing layers — it’s about knowing where to look when things break.

• TLS handshake failing? → Layer 6 / 7
• Packet loss or retransmissions? → Layer 4
• Wrong IP or routing issue? → Layer 3
• Network cable unplugged? → Layer 1

Even though real networks use TCP/IP, the OSI model remains one of the most powerful mental tools for understanding and debugging how the internet works.

Every click you make still follows this journey — quietly, reliably, and incredibly fast.

Thanks for reading…