The TCP/IP Model and Layers

This lesson covers the TCP/IP model (also known as the Internet Protocol Suite) and its four layers. This is part of OCR J277 Section 1.3.2. The TCP/IP model describes how data is structured, transmitted, and received across networks.

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What Is the TCP/IP Model?

The TCP/IP model is a conceptual framework that describes the layers of protocols used for internet communication. It divides the process of sending data across a network into four layers, each with specific responsibilities.

The model is called TCP/IP because TCP (Transmission Control Protocol) and IP (Internet Protocol) are the two most important protocols in the suite, although many other protocols operate within the model.

The following diagram shows the four layers of the TCP/IP model:

graph TD
    L4["Layer 4: Application\nHTTP, FTP, SMTP, DNS"]
    L3["Layer 3: Transport\nTCP, UDP"]
    L2["Layer 2: Internet\nIP, ICMP"]
    L1["Layer 1: Network Interface\nEthernet, WiFi"]
    L4 --> L3
    L3 --> L2
    L2 --> L1

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The Four Layers

Layer	Name	Protocols	Purpose
4	Application layer	HTTP, HTTPS, FTP, SMTP, IMAP, DNS	Provides network services directly to applications; handles user-facing protocols
3	Transport layer	TCP, UDP	Splits data into packets (segments); ensures reliable delivery and reassembly
2	Internet layer	IP, ICMP	Addresses packets with IP addresses; routes packets across networks
1	Network interface layer (Link layer)	Ethernet, WiFi (802.11)	Handles the physical transmission of data over the network medium (cables, wireless)

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Layer 4: Application Layer

The application layer is the topmost layer — it is the layer that the user interacts with (through applications like web browsers and email clients).

Protocol	Function at This Layer
HTTP/HTTPS	Requesting and displaying web pages
FTP	Transferring files
SMTP	Sending emails
IMAP/POP3	Retrieving emails
DNS	Resolving domain names to IP addresses

When you type a URL in your browser, the browser uses HTTP/HTTPS (application layer) to request the web page.

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Layer 3: Transport Layer

The transport layer manages the end-to-end delivery of data between two devices.

TCP (Transmission Control Protocol)

Feature	Detail
Reliable	Guarantees all data arrives correctly and in order
Connection-oriented	Establishes a connection before data transfer (three-way handshake)
Error checking	Detects lost or corrupted packets and requests retransmission
Sequencing	Numbers each packet so they can be reassembled in the correct order
Use cases	Web browsing, email, file transfer — where accuracy matters

UDP (User Datagram Protocol)

Feature	Detail
Unreliable	Does not guarantee delivery or order
Connectionless	No connection is established before sending
Fast	Less overhead than TCP, so data is transmitted faster
Use cases	Live streaming, online gaming, VoIP — where speed matters more than perfect accuracy

OCR Exam Tip: If asked to compare TCP and UDP: TCP is reliable but slower (used for web and email); UDP is faster but unreliable (used for streaming and gaming).