TCP/IP and Internet Protocols

The TCP/IP model (Transmission Control Protocol / Internet Protocol) is the practical protocol suite that powers the Internet. Unlike the theoretical OSI model, TCP/IP was designed by implementation — it grew out of real-world networking needs and is the foundation of all modern network communication.

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TCP/IP vs OSI

TCP/IP Layer	OSI Layers	Key Protocols
Application	Application, Presentation, Session	HTTP, HTTPS, DNS, SMTP, FTP, SSH
Transport	Transport	TCP, UDP
Internet	Network	IP (v4/v6), ICMP, ARP
Network Access	Data Link, Physical	Ethernet, Wi-Fi, PPP

The TCP/IP model is simpler (4 layers vs 7) and maps directly to the protocols in use today.

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The Internet Protocol (IP)

IP is the workhorse of the Internet layer. It handles addressing and routing packets from source to destination.

IPv4

• 32-bit address written as four octets: 192.168.1.100
• Approximately 4.3 billion unique addresses (2^32)
• Address exhaustion led to the development of IPv6

IPv6

• 128-bit address written in hexadecimal: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
• Approximately 340 undecillion unique addresses (2^128)
• Simplified header format for faster processing
• Built-in IPsec support for security
• No need for NAT in most cases

IP Packet Structure

┌──────────────────────────────────┐
│          IP Header               │
│  ┌────────┬───────┬────────────┐ │
│  │Version │  TTL  │  Protocol  │ │
│  ├────────┼───────┼────────────┤ │
│  │  Source IP Address           │ │
│  ├──────────────────────────────┤ │
│  │  Destination IP Address      │ │
│  └──────────────────────────────┘ │
├──────────────────────────────────┤
│          Payload (Data)          │
└──────────────────────────────────┘

Key IP Concepts

Concept	Description
TTL (Time to Live)	Limits the number of hops a packet can take; prevents infinite routing loops
Fragmentation	Breaking large packets into smaller ones to fit the MTU of a link
MTU	Maximum Transmission Unit — the largest packet size a link can handle (typically 1500 bytes for Ethernet)

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TCP — Transmission Control Protocol

TCP provides reliable, ordered, error-checked delivery of data between applications.

The Three-Way Handshake

TCP establishes a connection before sending data:

Client                Server
  │                      │
  │──── SYN ────────────>│  (1) Client sends SYN
  │                      │
  │<──── SYN-ACK ────────│  (2) Server responds with SYN-ACK
  │                      │
  │──── ACK ────────────>│  (3) Client confirms with ACK
  │                      │
  │    Connection is now established

TCP Features

Feature	Description
Sequencing	Each byte of data is numbered so the receiver can reassemble in order
Acknowledgements	The receiver confirms receipt of data segments
Retransmission	Lost segments are automatically resent
Flow control	The receiver advertises how much data it can handle (window size)
Congestion control	TCP slows down when network congestion is detected

Connection Termination (Four-Way Teardown)

Client                Server
  │                      │
  │──── FIN ────────────>│  (1) Client wants to close
  │<──── ACK ────────────│  (2) Server acknowledges
  │<──── FIN ────────────│  (3) Server is ready to close
  │──── ACK ────────────>│  (4) Client confirms
  │                      │
  │    Connection is now closed

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UDP — User Datagram Protocol

UDP is a connectionless protocol that prioritises speed over reliability.

UDP Characteristics