Networking Essentials

Why This Matters

Every system design answer involves network communication. Understanding protocols, latency, and how data moves between services is foundational.

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OSI Model (Simplified for Interviews)

Focus on layers 4 and 7 — that's where 95% of interview questions live.

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TCP vs UDP

TCP (Transmission Control Protocol)

• Connection-oriented — 3-way handshake (SYN, SYN-ACK, ACK)
• Reliable — guarantees delivery, ordering, no duplicates
• Flow control — receiver advertises window size
• Congestion control — slow start, congestion avoidance
• Use cases: HTTP, database connections, file transfers, APIs

UDP (User Datagram Protocol)

• Connectionless — no handshake
• Unreliable — no delivery guarantee, no ordering
• Lightweight — minimal overhead (8 byte header vs 20+ for TCP)
• Use cases: Video streaming, gaming, DNS lookups, VoIP

When to Choose What

Need reliability + ordering? → TCP
Need speed + low latency + can tolerate loss? → UDP
Need reliability + speed? → TCP with optimizations, or UDP + application-level reliability (QUIC)

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HTTP Protocol

HTTP/1.1

• One request per TCP connection (or keep-alive for reuse)
• Head-of-line blocking — requests queue behind each other
• Text-based headers (verbose)

HTTP/2

• Multiplexing — multiple streams on single TCP connection
• Header compression (HPACK)
• Server push — server can send resources proactively
• Binary protocol — more efficient parsing
• Still has TCP head-of-line blocking at transport layer

HTTP/3 (QUIC)

• Built on UDP instead of TCP
• No head-of-line blocking — streams are independent
• 0-RTT connection — faster handshake
• Built-in encryption (TLS 1.3)
• Used by Google, Cloudflare, Meta

HTTP Methods for API Design

HTTP Status Codes (Must Know)

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DNS (Domain Name System)

How DNS Works

User types google.com
  → Browser cache
  → OS cache
  → Router cache
  → ISP's recursive resolver
  → Root nameserver (.)
  → TLD nameserver (.com)
  → Authoritative nameserver (google.com)
  → Returns IP: 142.250.80.46

DNS Record Types

DNS in System Design

• TTL (Time to Live) — how long to cache DNS records
• DNS-based load balancing — return different IPs (round-robin)
• GeoDNS — return IP based on client location (global LB)
• DNS failover — update records to point to healthy servers
• Low TTL for services that need fast failover (30-60s)
• High TTL for stable services (3600s+)

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CDN (Content Delivery Network)

How CDNs Work

Client request → Nearest CDN Edge (PoP)
  → Cache HIT → Return cached content (fast!)
  → Cache MISS → Fetch from origin → Cache → Return

Pull vs Push CDN

CDN Use Cases in Interviews

• Static assets — images, CSS, JS, videos
• API responses — cache GET responses at edge
• Video streaming — Netflix/YouTube use CDN heavily
• Security — DDoS protection, WAF at edge

Key CDN Providers

• CloudFront (AWS), Cloud CDN (GCP), Azure CDN, Cloudflare, Akamai, Fastly

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WebSockets

What

• Full-duplex, persistent connection between client and server
• Starts as HTTP upgrade, then switches to WS protocol
• Both sides can send messages at any time

When to Use

• Chat applications — real-time messaging
• Live notifications — push updates
• Collaborative editing — Google Docs
• Gaming — real-time game state
• Live dashboards — stock tickers, monitoring

When NOT to Use

• Simple request-response APIs (use HTTP)
• Infrequent updates (use polling or SSE)
• High fan-out broadcast (consider SSE or pub/sub)

WebSocket vs Alternatives

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Proxies

Forward Proxy

• Sits between client and internet
• Client knows it's using a proxy
• Use cases: privacy, access control, caching

Reverse Proxy

• Sits between internet and servers
• Client doesn't know it exists
• Use cases: load balancing, SSL termination, caching, compression, security
• Examples: Nginx, HAProxy, Envoy, Traefik

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Key Networking Numbers

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Resources

• 📖 DDIA Chapter 8: The Trouble with Distributed Systems (networking section)
• 📖 "High Performance Browser Networking" by Ilya Grigorik (free online)
• 🔗 How DNS Works (comic)
• 🎥 Hussein Nasser — HTTP/1 vs HTTP/2 vs HTTP/3
• 🔗 Cloudflare Learning Center — CDN

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