The TCP/IP Protocol Stack

The TCP/IP model (also called the Internet Protocol Suite) is the layered architecture that underpins all communication on the Internet. At A-Level you must know the four layers, their functions, the key protocols at each layer, and how data is encapsulated as it passes through the stack.

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Why Use Layers?

Layered networking models break the complex task of communication into smaller, manageable sub-tasks. Benefits include:

• Modularity: Each layer can be developed and updated independently.
• Interoperability: Devices from different manufacturers can communicate as long as they follow the same protocols at each layer.
• Abstraction: Higher layers do not need to know the details of how lower layers work.
• Troubleshooting: Problems can be isolated to a specific layer.

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The Four Layers

Layer	Name	Function	Key Protocols
4	Application	Provides network services directly to user applications	HTTP, HTTPS, FTP, SMTP, POP3, IMAP, DNS, DHCP, SSH
3	Transport	End-to-end communication, segmentation, flow control, error recovery	TCP, UDP
2	Internet	Logical addressing and routing of packets across networks	IP (IPv4, IPv6), ICMP, ARP
1	Network Interface (Link)	Physical addressing, framing, transmission over the physical medium	Ethernet, Wi-Fi (802.11), PPP

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Layer 4: Application Layer

The application layer is where user-facing network services operate. It defines the protocols that applications use to exchange data.

Protocol	Purpose
HTTP / HTTPS	Web page requests and responses (HTTPS adds encryption via TLS)
FTP	File transfer between client and server
SMTP	Sending email
POP3 / IMAP	Retrieving email
DNS	Resolving domain names to IP addresses
DHCP	Automatically assigning IP addresses to devices
SSH	Secure remote terminal access

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Layer 3: Transport Layer

The transport layer provides end-to-end communication between processes on different hosts. It segments data from the application layer and reassembles it at the destination.

TCP (Transmission Control Protocol)

Feature	Detail
Connection	Connection-oriented — establishes a connection before sending data (three-way handshake: SYN → SYN-ACK → ACK)
Reliability	Guaranteed delivery — uses acknowledgements and retransmission
Ordering	Data arrives in order — segments are numbered
Flow control	Uses windowing to prevent the sender overwhelming the receiver
Error checking	Checksums verify data integrity
Use	Web browsing, email, file transfer — any application requiring reliable delivery

UDP (User Datagram Protocol)

Feature	Detail
Connection	Connectionless — no handshake, data is sent immediately
Reliability	No guaranteed delivery — no acknowledgements or retransmission
Ordering	No ordering guarantee
Flow control	None
Error checking	Optional checksum
Use	Video/audio streaming, online gaming, DNS queries — applications where speed matters more than reliability