Networking Basics

TL;DR (30-second summary)

The internet runs on protocols: TCP (reliable), UDP (fast), HTTP (web), WebSockets (real-time). DNS translates names to IPs. CDN caches content close to users. Load balancers distribute traffic.

Key concept: Network is unreliable and slow - design for it.

Why This Matters

In interviews: You'll design systems that communicate over networks. Understanding protocols helps you choose the right tool and discuss latency/reliability trade-offs.

At work: Every distributed system depends on networking. Poor choices cause cascading failures.

Core Concepts

1. OSI Model (Simplified)

What matters for system design:

• Layer 4 (Transport): TCP vs UDP
• Layer 7 (Application): HTTP, WebSockets, gRPC

2. TCP vs UDP

Feature	TCP	UDP
Reliability	Guaranteed delivery, ordered	Best effort, no guarantee
Connection	Connection-oriented (3-way handshake)	Connectionless
Speed	Slower (overhead for reliability)	Faster (no handshake)
Use Cases	HTTP, database connections, file transfer	Video streaming, DNS, gaming
Overhead	~40 bytes per packet	~8 bytes per packet

TCP Three-Way Handshake:

When to use:

• TCP: When you need reliability (web requests, APIs, databases)
• UDP: When speed matters more than reliability (live video, VoIP, gaming)

Interview Tip

Say: "We'll use TCP for API calls since we need reliability, but UDP for real-time video streaming where occasional packet loss is acceptable."

3. HTTP/HTTPS

HTTP (HyperText Transfer Protocol): The foundation of the web.

Request structure:

GET /api/users/123 HTTP/1.1
Host: api.example.com
Authorization: Bearer <token>
Content-Type: application/json

Response structure:

HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: 85

{"id": 123, "name": "John", "email": "john@example.com"}

HTTP Methods:

Method	Purpose	Idempotent	Safe
GET	Retrieve data	✅	✅
POST	Create resource	❌	❌
PUT	Update/replace	✅	❌
PATCH	Partial update	❌	❌
DELETE	Remove resource	✅	❌

HTTP Status Codes (memorize these):

• 2xx Success: 200 OK, 201 Created, 204 No Content
• 3xx Redirection: 301 Moved Permanently, 302 Found, 304 Not Modified
• 4xx Client Error: 400 Bad Request, 401 Unauthorized, 404 Not Found, 429 Too Many Requests
• 5xx Server Error: 500 Internal Server Error, 503 Service Unavailable

HTTP/1.1 vs HTTP/2 vs HTTP/3:

Feature	HTTP/1.1	HTTP/2	HTTP/3
Protocol	TCP	TCP	QUIC (UDP)
Multiplexing	❌ (1 request/connection)	✅ (many streams)	✅
Header Compression	❌	✅ (HPACK)	✅ (QPACK)
Latency	High (head-of-line blocking)	Better	Best
Adoption	Universal	Common (50% of web)	Growing

4. HTTPS (HTTP + TLS/SSL)

HTTPS = HTTP + Encryption

Why HTTPS:

• Encryption: Can't eavesdrop on traffic
• Authentication: Verify server identity via certificates
• Integrity: Detect tampering

Cost: ~100ms extra latency for TLS handshake (first request only)

5. WebSockets

Purpose: Full-duplex (two-way) real-time communication.

Use cases:

• Chat applications (WhatsApp, Slack)
• Live sports scores
• Collaborative editing (Google Docs)
• Real-time dashboards

Alternative: Server-Sent Events (SSE) - one-way (server → client) only, simpler than WebSockets

Feature	WebSockets	SSE	Polling
Direction	Bi-directional	Server → Client	Client → Server (repeated)
Protocol	Custom (ws://)	HTTP	HTTP
Overhead	Low (persistent)	Low (persistent)	High (reconnections)
Use Case	Chat, gaming	Live feeds, notifications	Simple updates

6. DNS (Domain Name System)

Purpose: Translate domain names to IP addresses.

DNS Record Types:

Type	Purpose	Example
A	Domain → IPv4	example.com → 93.184.216.34
AAAA	Domain → IPv6	example.com → 2606:2800:220:1...
CNAME	Alias	www.example.com → example.com
MX	Mail server	example.com → mail.example.com
TXT	Text data	SPF, DKIM for email

DNS Caching:

• Browser cache: ~1 minute
• OS cache: ~5 minutes
• ISP cache: ~TTL (Time To Live, often 24 hours)

TTL trade-off:

• Low TTL (5 min): Fast to update, higher DNS load
• High TTL (24 hr): Less DNS load, slow to propagate changes

Red Flag

Don't forget DNS when designing! It's a single point of failure. Use:

• Multiple DNS providers (Route53 + Cloudflare)
• Low TTL before migrations
• Health checks to automatically remove failed servers

7. CDN (Content Delivery Network)

Purpose: Cache static content close to users globally.

What CDNs cache:

• ✅ Static files (images, CSS, JS, videos)
• ✅ API responses (with cache headers)
• ❌ Dynamic, user-specific data (without special config)

Benefits:

1. Reduced latency: Serve from nearest edge server (50ms → 5ms)
2. Lower origin load: Most requests hit cache (90%+ cache hit rate)
3. DDoS protection: Absorb malicious traffic at edge

Popular CDNs:

• Cloudflare (free tier, great DDoS protection)
• AWS CloudFront (integrates with AWS)
• Fastly (real-time purging)
• Akamai (oldest, largest network)

8. Load Balancers

Purpose: Distribute incoming traffic across multiple servers.

Algorithms:

Algorithm	How It Works	Use Case
Round Robin	Rotate through servers in order	Uniform server capacity
Least Connections	Send to server with fewest active connections	Long-lived connections
Weighted	More traffic to powerful servers	Heterogeneous capacity
IP Hash	Same client → same server (consistent)	Session persistence

Health Checks:

• Load balancer periodically pings servers (e.g., every 10s)
• If server fails health check (timeout or error), remove from pool
• When server recovers, add back to pool

Layer 4 vs Layer 7 (covered in detail in Chapter 10):

• L4 (Transport): Faster, route based on IP/port
• L7 (Application): Slower, route based on HTTP headers, URL path

Common Interview Questions

Q1: "When would you use UDP instead of TCP?"

Answer:

• Real-time applications where low latency matters more than reliability
• Examples: Live video streaming, VoIP (Zoom, Skype), online gaming
• Reason: Packet loss in video just causes brief glitch; waiting for retransmission would cause stuttering

Q2: "How does HTTPS improve security?"

Answer:

1. Encryption: Traffic encrypted with TLS, can't be read by intermediaries
2. Authentication: Certificate proves server identity (prevents man-in-the-middle)
3. Integrity: Detects if data was modified in transit

Trade-off: Adds ~100ms latency for initial handshake

Q3: "Explain how a CDN works and when to use it."

Answer:

1. User requests cdn.example.com/image.jpg
2. DNS routes to nearest CDN edge server
3. If edge has cached copy → return immediately (cache hit)
4. If not → edge fetches from origin, caches, and returns (cache miss)
5. Subsequent requests hit cache

Use when:

• Serving static content globally
• High traffic (cost-effective)
• Need DDoS protection

Q4: "WebSockets vs HTTP polling for real-time updates?"

Answer:

Aspect	WebSockets	Polling
Latency	Instant (push)	Up to poll interval
Overhead	Low (persistent connection)	High (repeated handshakes)
Complexity	More (connection management)	Less (simple HTTP)
Scalability	Need to manage open connections	Stateless, easier to scale

Use WebSockets for: Chat, collaborative editing, gaming
Use Polling for: Less frequent updates, simpler systems

Trade-offs

Decision	Option A	Option B	Trade-off
Protocol	TCP (reliable)	UDP (fast)	Reliability vs latency
HTTP Version	HTTP/1.1 (simple)	HTTP/2 (multiplexing)	Compatibility vs performance
Real-time	WebSockets (push)	Polling (pull)	Complexity vs simplicity
DNS TTL	Low (5 min)	High (24 hr)	Agility vs DNS load
CDN	Use CDN (fast)	Direct (simple)	Cost vs latency

Real-World Examples

Netflix

• Protocol: HTTP/2 for API, adaptive bitrate streaming
• CDN: 90%+ traffic via Open Connect (own CDN)
• Result: Sub-second startup time globally

WhatsApp

• Protocol: Custom protocol over TCP (not HTTP)
• Real-time: Persistent connections (WebSocket-like)
• Result: Instant message delivery, billions of connections

Cloudflare

• Service: Global CDN + DDoS protection
• Scale: 25% of all web traffic passes through Cloudflare
• Result: Protects millions of sites from attacks

Quick Reference Card

Protocols:

• TCP: Reliable, ordered, slow(er) - use for APIs, databases
• UDP: Fast, unreliable - use for streaming, gaming
• HTTP: Request-response, stateless - use for web APIs
• WebSockets: Persistent, bi-directional - use for real-time

HTTP Status Codes:

• 2xx: Success
• 4xx: Client error (your fault)
• 5xx: Server error (their fault)

DNS TTL:

• Low (5 min): Fast updates, high load
• High (24 hr): Slow updates, low load

CDN Benefits:

• Lower latency (serve from edge)
• Reduce origin load (cache hit rate)
• DDoS protection

Further Reading

• MDN Web Docs - HTTP
• WebSockets vs Server-Sent Events
• Cloudflare Learning Center
• High Performance Browser Networking (Free book)

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Next: Storage Fundamentals - SQL vs NoSQL, ACID vs BASE.