send

Layer 7 - Application: HTTP GET request created

Browser generates an HTTP GET request. This is raw application data - the payload that the user actually wants to send. PDU = Data. Protocol: HTTP.

💡 The Application layer is where user-facing protocols live: HTTP, FTP, DNS, SMTP. It creates the message but has no idea how it will travel across the network.

Line:

# No code - conceptual layer
# HTTP GET /index.html HTTP/1.1
# Host: www.example.com

💡 The Application layer only cares about 'what' to send, not 'how'. It hands the data down to Presentation.

send

Layer 6 - Presentation: encode/encrypt data

Presentation layer handles data formatting, encryption (SSL/TLS), and compression. For HTTPS it would encrypt the HTTP data here. PDU still = Data.

💡 The Presentation layer is the 'translator' - it ensures data is in a format both sides understand. Think of it as converting data to a common language and optionally encrypting it.

Line:

# SSL/TLS encryption happens here for HTTPS
# Data encoding: ASCII, UTF-8, JPEG, MPEG

💡 For plain HTTP the Presentation layer passes data through. For HTTPS it wraps data in TLS encryption here.

send

Layer 5 - Session: manage connection dialog

Session layer establishes, manages, and terminates sessions between applications. It adds synchronization checkpoints for long transfers and manages dialog control (who speaks when). PDU = Data.

💡 Think of the Session layer as the 'meeting coordinator' - it sets up the connection, manages the conversation flow, and tears it down when done.

Line:

# Session establishment, checkpointing
# NetBIOS, RPC operate here

💡 For HTTP, the Session layer is mostly transparent. For longer file transfers it adds checkpoints so transfers can resume after interruption.

send

Layer 4 - Transport: segment into TCP segments, add ports

Transport layer breaks data into segments and adds source/destination port numbers (e.g. src=54321, dst=80 for HTTP). TCP adds sequence numbers for ordering and reliability. PDU changes to Segment.

💡 Port numbers are the key addition at Layer 4. They identify which application receives the data on the destination (port 80=HTTP, 443=HTTPS, 22=SSH). TCP also adds sequence numbers to reassemble segments in order.

Line:

# TCP segment header:
# src_port=54321, dst_port=80
# seq_num=1, ack_num=0
# flags=SYN

💡 The Transport layer is the first layer that creates end-to-end communication between processes. Network layers below only see IP addresses, not ports.

send

Layer 3 - Network: add IP header, create packet

Network layer adds source and destination IP addresses. This is where routing decisions are made - how to get from client IP to server IP across potentially many routers. PDU changes to Packet.

💡 IP addresses are logical addresses that identify devices across different networks. Routers operate at this layer - they read the IP header and decide where to forward the packet next.

Line:

# IP packet header:
# src_ip=192.168.1.10
# dst_ip=93.184.216.34
# TTL=64, protocol=TCP

💡 MAC addresses (Layer 2) only work within a single network. IP addresses (Layer 3) work globally - that's why we need both layers.

send

Layer 2 - Data Link: add MAC addresses, create frame

Data Link layer adds source and destination MAC addresses for the local network segment. Also adds a trailer with CRC error-detection code. PDU changes to Frame.

💡 MAC addresses identify devices on the same local network (like room numbers in a building). The IP address is the city+street address; the MAC is the apartment number within that building.

Line:

# Ethernet frame:
# dst_mac=AA:BB:CC:11:22:33 (next hop router)
# src_mac=FF:EE:DD:44:55:66 (client NIC)
# + CRC trailer for error detection

💡 The MAC destination here is the router's MAC, not the server's MAC. Layer 2 is only for the current network hop; IP handles the full journey.

send

Layer 1 - Physical: convert to bits, transmit

Physical layer converts the Frame into electrical signals (copper cable), light pulses (fiber optic), or radio waves (WiFi). These bits travel across the physical medium to the receiver. PDU = Bits.

💡 The Physical layer has no concept of addresses or structure - it just transmits raw 0s and 1s. The actual voltage levels, frequencies, and encoding schemes are defined here.

Line:

# Electrical signals on copper wire
# Light pulses on fiber optic
# Radio waves on WiFi (802.11)

💡 Every other layer is software/firmware. The Physical layer is hardware - the actual wire, fiber, or air carrying the signal.

receive

Receiver Layer 2 - Data Link: check CRC, extract packet

Receiver's Physical layer collects bits and reconstructs the Frame. Data Link layer checks CRC for errors. If valid, strips the Ethernet header and trailer, extracts the IP Packet, passes it up to Network layer.

💡 This is decapsulation - the receiver undoes what the sender did, layer by layer in reverse order. Each layer removes its own header/trailer.

Line:

# Check CRC: if error → drop frame
# Strip Ethernet header + CRC trailer
# Extract IP packet → pass to Layer 3

💡 Decapsulation is the reverse of encapsulation. The receiver reconstructs the original message by stripping headers from outermost to innermost.

receive

Receiver Layer 3 - Network: read IP, extract segment

Network layer reads the IP header, verifies destination IP matches this host. Strips IP header, extracts TCP Segment, passes up to Transport layer.

💡 The Network layer checks 'Is this packet meant for me?' by comparing the destination IP to its own IP address. If yes, strip the IP header and pass up.

Line:

# Check dst_ip == my_ip
# Strip IP header
# Extract TCP segment → pass to Layer 4

💡 Routers along the path also operate at Layer 3 - they read the IP header, decide the next hop, and forward without going above Layer 3.

receive

Receiver Layer 4 - Transport: reassemble, deliver to port 80

Transport layer reads TCP header. Checks destination port (80 = HTTP server process). Uses sequence numbers to reassemble multiple segments in order. Sends TCP ACK. Strips TCP header, passes Data up.

💡 Port 80 tells the OS which application should receive this data - the web server. Sequence numbers ensure segments are assembled in the correct order even if they arrived out of order.

Line:

# Check dst_port → route to HTTP server process
# Use seq_num to reassemble segments in order
# Send TCP ACK back to sender
# Strip TCP header → extract Data

💡 TCP ACKs are sent back to the sender to confirm receipt. If ACK is not received, the sender retransmits - this is TCP's reliability guarantee.

receive

Receiver Layers 5,6,7 - deliver HTTP request to web server

Session layer manages the ongoing connection. Presentation layer decrypts (for HTTPS) and decodes data. Application layer (web server) receives the HTTP GET request and processes it to send back the webpage response.

💡 The last three layers undo what the sender's layers did: Session tracks the dialog, Presentation decrypts, Application processes the message. The web server now sees the original HTTP GET request.

Line:

# Session: maintain dialog context
# Presentation: decrypt TLS, decode format
# Application: HTTP server processes GET /index.html

💡 The response from the web server travels down all 7 layers on the server side and up all 7 layers on the client side - the exact same process in reverse.

OSI Model - Seven Layers, Functions & PDUs

Layer 7 - Application: HTTP GET request created

Layer 6 - Presentation: encode/encrypt data

Layer 5 - Session: manage connection dialog

Layer 4 - Transport: segment into TCP segments, add ports

Layer 3 - Network: add IP header, create packet

Layer 2 - Data Link: add MAC addresses, create frame

Layer 1 - Physical: convert to bits, transmit

Receiver Layer 2 - Data Link: check CRC, extract packet

Receiver Layer 3 - Network: read IP, extract segment

Receiver Layer 4 - Transport: reassemble, deliver to port 80

Receiver Layers 5,6,7 - deliver HTTP request to web server

Key Takeaways