What is the difference between HTTP/2 and HTTP/3?

HTTP/2 runs over TCP with multiplexing and HPACK header compression. HTTP/3 runs over QUIC (UDP-based) with 0-RTT handshake, no TCP head-of-line blocking, mandatory TLS 1.3, and QPACK header compression — lower latency especially on lossy networks.

When should you use WebSockets vs SSE?

WebSockets for bidirectional real-time — chat, gaming, collaborative editing. SSE for one-way server-to-client streams — live feeds, notifications, progress updates. SSE is simpler (plain HTTP), WebSockets have lower latency for two-way communication.

What is DNS prefetching?

Browser resolves domain names before the user navigates or requests resources. Use for external APIs/CDNs to eliminate DNS lookup latency on first request.

How does a CDN improve performance?

CDN caches static assets on edge servers globally. Users fetch from the nearest PoP — lower latency (e.g. 50ms vs 200ms to origin), reduced origin bandwidth, high availability, and DDoS absorption.

Advanced Networking

HTTP/3, QUIC, DNS, TLS, CDN, WebSockets, and the protocols SDE-2 interviews expect.

Module 11 · Advanced Networking

HTTP/2 vs HTTP/3, TCP vs UDP vs QUIC, DNS, TLS, connection pooling, load balancing, CDN, WebSockets, SSE, and Service Workers.

Advanced networking for frontend engineers: HTTP methods, caching headers, CDN, CORS, TLS, DNS prefetching, Long-Polling vs WebSockets vs SSE, and the full journey from typing a URL to page load.

About this module

Advanced networking goes beyond fetch() — SDE-2 interviews expect you to explain HTTP/2 multiplexing, why HTTP/3 uses QUIC over UDP, the TCP 3-way handshake, DNS resolution flow, and when to pick WebSockets vs SSE.

This module covers protocol comparisons, TLS handshakes, CDN architecture, load balancing strategies, real-time communication patterns, and Service Worker offline caching.

From typing a URL to receiving a response, you will trace DNS resolution, TCP/TLS handshakes, HTTP caching headers, and when to choose WebSockets, SSE, or long-polling for live data.

Deep dives include HTTP methods, Cache-Control and ETag, CDN benefits, CORS preflight, PUT vs PATCH, symmetric vs asymmetric encryption, and the full browser-to-server request lifecycle.

Why this matters

Networking questions appear in SDE-2 frontend and system design loops. You must explain HTTP caching with ETag and Cache-Control, why CDNs improve LCP, CORS preflight behavior, and real-time protocol trade-offs — not just name protocols. Every API call your React app makes depends on this stack.

Key topics covered

HTTP/2
HTTP/3
QUIC
TCP vs UDP
DNS
TLS
Connection Pooling
Load Balancing
CDN
WebSockets
SSE
Service Workers

Where you'll use it

Choosing WebSockets vs SSE for a real-time dashboard
Adding DNS prefetch for third-party API domains
Explaining why HTTP/3 reduces latency on mobile networks
Implementing Service Worker cache-first for offline support
Debugging CORS preflight failures in production

What Is Advanced Networking?

Secure, intelligent foundations beyond simple connectivity.

Advanced Networking goes beyond simple connectivity — creating a secure, intelligent, agile foundation that adapts to rapidly changing needs.

•Software-Defined Networking (SDN) — network controlled by software
•Automation — self-configuring, self-healing networks
•AI-driven management — predict issues, optimize performance
•Cloud integration — seamless connection with cloud environments
•Multi-region solutions — distributed data centers

•Prioritize critical applications
•Automatically detect and remediate issues
•Scale capacity on demand
•Protect sensitive data

HTTP/2 vs HTTP/3

Multiplexing, QUIC, and the evolution of web protocols.

HTTP/2 (2015)

•Multiplexing — multiple requests over single connection
•Header compression — HPACK reduces latency
•Server push — server sends resources before client requests
•Binary protocol — more efficient than HTTP/1.1 text

HTTP/3 (2022)

•QUIC protocol — UDP-based, faster connection setup
•0-RTT handshake — no round-trip for repeated connections
•Improved multiplexing — no TCP head-of-line blocking
•Encryption — built-in TLS 1.3 (mandatory)

Aspect	HTTP/1.1	HTTP/2	HTTP/3
Protocol	TCP	TCP	UDP (QUIC)
Multiplexing	No	Yes	Yes
Header Compression	No	HPACK	QPACK
Connection Setup	3 RTT	2 RTT	0–1 RTT
Encryption	Optional	Optional	Mandatory
Latency	High	Medium	Lowest

TCP vs UDP

Reliability vs speed — pick the right transport layer.

Aspect	TCP	UDP
Reliability	Reliable — retransmits lost packets	Unreliable — fire-and-forget
Ordering	Ordered	Unordered
Connection	3-way handshake	None — send immediately
Speed	Slower	Faster
Overhead	Higher	Lower
Use Case	HTTP, FTP, email	Video streaming, VoIP, gaming

Client → SYN → Server

Server → SYN-ACK → Client

Client → ACK → Server

Connection established

DNS (Domain Name System)

Domain names to IP addresses — resolution flow and optimization.

DNS converts domain names (google.com) to IP addresses (142.250.80.4).

1. Browser cache

2. OS cache

3. Router cache

4. ISP recursive resolver

5. Root DNS → TLD DNS

6. Authoritative DNS → IP returned

Record	Purpose
A	Domain → IPv4
AAAA	Domain → IPv6
CNAME	Domain → Domain (alias)
MX	Email server
TXT	Verification (SPF, DKIM)

•DNS prefetching — resolve before user clicks
•DNS caching — TTL (Time to Live)
•DNS over HTTPS (DoH) — encrypted DNS queries

<link rel="dns-prefetch" href="https://api.example.com">
<link rel="dns-prefetch" href="https://cdn.example.com">

TLS (Transport Layer Security)

Encrypting data in transit — HTTPS handshake.

Client → ClientHello (cipher suites)

Server → ServerHello + certificate

Client → verify cert, generate session key

Both → encrypt data with session key

Version	Status
TLS 1.0 / 1.1	Deprecated — insecure
TLS 1.2	Current standard
TLS 1.3	Latest — faster, more secure

•Encryption — prevent MITM attacks
•Authentication — verify server identity
•Integrity — detect tampered data

Connection Pooling

Reuse TCP connections — skip the handshake.

•Reduced latency — no handshake per request
•Lower resource usage — fewer open connections
•Better performance — parallel requests on pooled connections

import http from "node:http";

const agent = new http.Agent({ maxSockets: 50 });

http.get("https://api.com/data", { agent }, (res) => {
  // Connection reused from pool
});

Load Balancing

Distribute traffic across multiple servers.

Type	Description	Best For
Round Robin	Sequential 1 → 2 → 3 → 1	Equal capacity servers
Least Connections	Fewest active connections	Variable capacity
Weighted	Priority by weight	Different server sizes
IP Hash	Same client IP → same server	Session persistence

•Hardware — F5, Cisco
•Software — Nginx, HAProxy
•Cloud — AWS ALB, Azure Load Balancer

CDN (Content Delivery Network)

Global edge caching for faster loads.

User requests example.com/image.jpg

CDN checks nearby edge cache

Cache hit → return in ~50ms

Cache miss → fetch from origin (~200ms)

•Faster load — content from nearby server
•Reduced origin bandwidth
•High availability — multiple edge servers
•DDoS protection — CDN absorbs attack traffic

•AWS CloudFront
•Google Cloud CDN
•Azure CDN
•Netlify, Vercel

WebSockets

Persistent bidirectional real-time communication.

Client — HTTP request with Upgrade: websocket

Server — 101 Switching Protocols

Bidirectional WebSocket connection open

•Chat apps
•Live notifications
•Real-time dashboards
•Gaming

const ws = new WebSocket("wss://socket.example.com");

ws.onopen = () => console.log("Connected");
ws.onmessage = (e) => console.log(e.data);
ws.send("Hello");
ws.close();

Server-Sent Events (SSE)

One-way server → client streaming over HTTP.

•Live feeds
•Notifications
•Progress updates

const eventSource = new EventSource("https://api.com/updates");

eventSource.onmessage = (e) => console.log(e.data);

Aspect	WebSockets	SSE
Direction	Bidirectional	One-way (server → client)
Protocol	WebSocket	HTTP
Use Case	Chat, gaming	Feeds, notifications

Service Workers

Background scripts intercepting network requests.

•Offline support — cache responses
•Push notifications — background messages
•Background sync — queue requests when offline

self.addEventListener("fetch", (event) => {
  event.respondWith(
    caches.match(event.request).then((response) => {
      return response || fetch(event.request);
    })
  );
});

Advanced Networking Comparison Tables

TCP vs UDP vs QUIC, protocols, load balancing, real-time.

TCP vs UDP vs QUIC

Aspect	TCP	UDP	QUIC
Reliability	Reliable	Unreliable	Reliable
Connection	3-way handshake	None	0–1 RTT
Latency	High	Low	Lowest
Use Case	HTTP/1.1, HTTP/2	Video, gaming	HTTP/3

WebSockets vs SSE vs HTTP

Aspect	WebSockets	SSE	HTTP
Direction	Bidirectional	One-way	One-way
Persistent	Yes	Yes	No
Latency	Lowest	Low	High
Use Case	Chat, gaming	Feeds	Regular requests

Key Interview Takeaways

Ten points to nail advanced networking questions.

•HTTP/2 — multiplexing over TCP, HPACK, no head-of-line blocking at HTTP layer
•HTTP/3 — QUIC over UDP, 0-RTT, mandatory TLS 1.3, no TCP HOL blocking
•TCP — reliable, ordered, 3-way handshake — HTTP, databases
•UDP — fast, unreliable — video, VoIP, gaming; QUIC adds reliability on UDP
•DNS — browser → OS → ISP → root → TLD → authoritative
•DNS prefetch — <link rel="dns-prefetch"> for external domains
•TLS — ClientHello → ServerHello + cert → session key → encrypted
•CDN — edge cache, lower latency, DDoS absorption
•WebSockets — bidirectional persistent; SSE — one-way over HTTP
•Service Workers — intercept fetch, cache-first offline support

Frequently Asked Questions

Common interview questions about advanced networking.

Aspect

HTTP/1.1

HTTP/2

HTTP/3

Protocol

TCP

UDP (QUIC)

Multiplexing

Yes

Header Compression

HPACK

QPACK

Connection Setup

3 RTT

2 RTT

0–1 RTT

Encryption

Optional

Mandatory

Latency

High

Medium

Lowest

Aspect

TCP

UDP

Reliability

Reliable — retransmits lost packets

Unreliable — fire-and-forget

Ordering

Ordered

Unordered

Connection

3-way handshake

None — send immediately

Speed

Slower

Faster

Overhead

Higher

Lower

Use Case

HTTP, FTP, email

Video streaming, VoIP, gaming

Record

Purpose

Domain → IPv4

AAAA

Domain → IPv6

CNAME

Domain → Domain (alias)

Email server

TXT

Verification (SPF, DKIM)

Version

Status

TLS 1.0 / 1.1

Deprecated — insecure

TLS 1.2

Current standard

TLS 1.3

Latest — faster, more secure

Type

Description

Best For

Round Robin

Sequential 1 → 2 → 3 → 1

Equal capacity servers

Least Connections

Fewest active connections

Variable capacity

Weighted

Priority by weight

Different server sizes

IP Hash

Same client IP → same server

Session persistence

Aspect

WebSockets

SSE

Direction

Bidirectional

One-way (server → client)

Protocol

WebSocket

HTTP

Use Case

Chat, gaming

Feeds, notifications

Aspect

TCP

UDP

QUIC

Reliability

Reliable

Unreliable

Reliable

Connection

3-way handshake

None

0–1 RTT

Latency

High

Low

Lowest

Use Case

HTTP/1.1, HTTP/2

Video, gaming

HTTP/3

Aspect

WebSockets

SSE

HTTP

Direction

Bidirectional

One-way

Persistent

Yes

Latency

Lowest

Low

High

Use Case

Chat, gaming

Feeds

Regular requests

About this chapter

About this module

Why this matters

Key topics covered

Where you'll use it

What Is Advanced Networking?

HTTP/2 vs HTTP/3

TCP vs UDP

DNS (Domain Name System)

TLS (Transport Layer Security)

Connection Pooling

Load Balancing

CDN (Content Delivery Network)

WebSockets

Server-Sent Events (SSE)

Service Workers

Advanced Networking Comparison Tables

Key Interview Takeaways

Frequently Asked Questions

About this chapter

About this module

Why this matters

Key topics covered

Where you'll use it

What Is Advanced Networking?

HTTP/2 vs HTTP/3

TCP vs UDP

DNS (Domain Name System)

TLS (Transport Layer Security)

Connection Pooling

Load Balancing

CDN (Content Delivery Network)

WebSockets

Server-Sent Events (SSE)

Service Workers

Advanced Networking Comparison Tables

Key Interview Takeaways

Frequently Asked Questions