Module 8 — APIs

Networking Basics

TCP, UDP, HTTP, DNS—the foundation of all networked systems.

1The Postal System Analogy

💡 Simple Analogy
IP Address: Your street address
Port: Apartment number at that address
DNS: Phone book that converts "John's House" → "123 Main St"
TCP: Registered mail—guaranteed delivery, in order
UDP: Postcards—fast, no guarantees

2OSI Model (Simplified)

7
Application
HTTP, FTP, SMTP
User-facing protocols
4
Transport
TCP, UDP
Reliable vs fast delivery
3
Network
IP
Routing between networks
2
Data Link
Ethernet, WiFi
Local network communication
1
Physical
Cables, Radio
Physical transmission

3TCP vs UDP

TCP

Connection-oriented, reliable, ordered.

  • ✓ Guaranteed delivery
  • ✓ In-order packets
  • ✓ Error checking
  • ✗ Higher latency

Use: HTTP, email, file transfer

UDP

Connectionless, fast, no guarantees.

  • ✓ Low latency
  • ✓ No connection setup
  • ✗ No delivery guarantee
  • ✗ Packets can arrive out of order

Use: Video streaming, gaming, DNS

4DNS Resolution

1
Browser: Do I have google.com cached? No.
2
OS: Check /etc/hosts, then OS cache. No.
3
Recursive Resolver: Check cache, ask root if not.
4
Root DNS: I don't know google.com, try .com servers.
5
.com TLD: google.com is at these nameservers.
6
Google's NS: google.com → 142.250.190.78
7
Browser: Cache it, connect to that IP.

5HTTP Overview

HTTP/1.1
One request per connection (or keep-alive). Text-based.
HTTP/2
Multiplexing (multiple requests on one connection). Binary, header compression.
HTTP/3 (QUIC)
Based on UDP. Faster connection setup, better mobile performance.

6Key Takeaways

1DNS converts domain names to IP addresses.
2TCP: reliable, ordered. UDP: fast, no guarantees.
3HTTP is application layer protocol built on TCP.
4HTTP/2 multiplexes requests; HTTP/3 uses UDP (QUIC).
5Know the OSI layers—helps debug network issues.
6In interviews: understand TCP handshake, DNS resolution, HTTP versions.