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Networking Fundamentals

Understanding networking is the foundation of system design. Every distributed system communicates across networks using standardized protocols and architectures.

1. What Happens When You Type a URL?

This is one of the most common core questions in system design interviews.

URL to Page

Step-by-Step Breakdown:

  1. Browser parses the URL: Extracts protocol (https), domain (google.com), and path (/search).
  2. DNS Lookup: Resolves google.com to an IP address (e.g., 142.250.183.14) via Browser Cache → OS Cache → Router → ISP Resolver → Root/TLD/Authoritative DNS.
  3. TCP Connection Establishment: Three-way handshake (SYN → SYN-ACK → ACK) between client and server IP.
  4. TLS Handshake: If HTTPS, client and server negotiate encryption ciphers, verify certificates, and exchange session keys.
  5. HTTP Request Sent: Browser sends GET /search?q=hello HTTP/2 along with headers, cookies, and tokens.
  6. Server Processing: Load balancer routes the request to an API server; server queries DB/cache and constructs HTML/JSON response.
  7. HTTP Response Returned: Server returns status 200 OK with response headers and payload.
  8. Browser Rendering: Browser parses HTML to construct the DOM tree, fetches CSS/JS/images, and paints the screen.

2. HTTP vs HTTPS

HTTP (HyperText Transfer Protocol) is the fundamental protocol for transferring data over the web.

Feature HTTP HTTPS
Port 80 443
Encryption None (plaintext) TLS/SSL encrypted
Speed Slightly faster Slight initial handshake overhead
Security Vulnerable to Man-in-the-Middle (MITM) End-to-end encrypted and authenticated
SEO Not preferred Search ranking priority
Use Case Internal secure private VPCs All public-facing traffic

HTTP Methods & Idempotency:

Method Purpose Idempotent Safe
GET Retrieve data ✅ Yes ✅ Yes
POST Create resource ❌ No ❌ No
PUT Replace entire resource ✅ Yes ❌ No
PATCH Partial update ❌ No ❌ No
DELETE Remove resource ✅ Yes ❌ No

HTTP Status Code Ranges:

Range Category Common Examples
1xx Informational 100 Continue, 101 Switching Protocols
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, 403 Forbidden, 404 Not Found, 429 Too Many Requests
5xx Server Error 500 Internal Server Error, 502 Bad Gateway, 503 Service Unavailable, 504 Gateway Timeout

3. TCP vs UDP

Feature TCP (Transmission Control Protocol) UDP (User Datagram Protocol)
Connection Connection-oriented (3-way handshake) Connectionless
Reliability Guaranteed delivery, ordering, retransmission Best-effort, packets may drop or reorder
Speed & Overhead Slower (20-60 byte header, ACK overhead) Faster (8 byte header, no ACKs)
Flow & Congestion Control Yes (sliding window, congestion algorithms) None
Use Cases Web (HTTP/HTTPS), Email (SMTP), File Transfer (FTP), SSH Live streaming, DNS, VoIP, Real-time multiplayer gaming

TCP Three-Way Handshake:

sequenceDiagram participant C as 💻 Client participant S as 🖥️ Server C->>S: SYN (seq=100) S->>C: SYN-ACK (seq=300, ack=101) C->>S: ACK (ack=301) Note over C,S: ✅ Connection Established

4. DNS (Domain Name System)

DNS maps human-friendly domain names to machine-routable IP addresses.

Resolution Hierarchy:

flowchart TD B["🌐 Client Browser"] --> OSC["OS Cache"] OSC --> RC["Router Cache"] RC --> ISP["🌐 ISP Resolver"] ISP --> Root["Root DNS Server (.)"] Root --> TLD["TLD DNS Server (.com)"] TLD --> Auth["Authoritative DNS Server (google.com)"] Auth --> IP["✅ IP: 142.250.183.14"]

DNS Record Types:

Record Purpose Example
A Domain → IPv4 address example.com → 93.184.216.34
AAAA Domain → IPv6 address example.com → 2606:2800:220:1::
CNAME Canonical Name (alias to another domain) www.example.com → example.com
MX Mail exchange server for domain example.com → mail.google.com
NS Authoritative Name Server for zone example.com → ns1.cloudflare.com
TXT Text records (SPF, DKIM verification) v=spf1 include:_spf.google.com ~all

5. REST APIs

REST (Representational State Transfer) is an architectural style based on stateless, client-server communication using standard HTTP verbs and status codes.

flowchart LR GET1["GET /api/users"] -->|"List users"| S["🖥️ Server"] GET2["GET /api/users/123"] -->|"Retrieve user 123"| S POST["POST /api/users"] -->|"Create new user"| S PUT["PUT /api/users/123"] -->|"Replace user 123"| S PATCH["PATCH /api/users/123"] -->|"Partial update 123"| S DELETE["DELETE /api/users/123"] -->|"Delete user 123"| S

6. Real-Time Communication Protocols

sequenceDiagram participant C as 💻 Client participant S as 🖥️ Server Note over C,S: HTTP Polling C->>S: Request S-->>C: Response (connection closes) Note over C,S: Long Polling C->>S: Request (server holds until event) S-->>C: Response Note over C,S: WebSockets C<<->>S: Persistent Bidirectional Stream Note over C,S: Server-Sent Events (SSE) C->>S: Request S-->>C: Unidirectional Stream
Feature HTTP Polling Long Polling Server-Sent Events (SSE) WebSockets
Direction Client → Server Client → Server Server → Client Full Duplex (Bidirectional)
Connection New request every interval Connection held open Persistent HTTP connection Persistent TCP socket
Overhead High (header overhead) Medium Low Very Low (2-10 bytes/frame)
Use Cases Infrequent checks Basic alerts/notifications Stock tickers, news feeds Chat apps (Slack), collaborative docs, gaming

7. Latency vs Bandwidth & Key Numbers

  • Latency: Time taken for a single data packet to travel from source to destination.
  • Bandwidth: Maximum data transfer capacity per unit of time.
  • Throughput: Actual data successfully delivered per unit of time.

Latency Numbers Every Systems Engineer Should Know:

Operation Approximate Time
L1 cache reference 0.5 ns
L2 cache reference 7 ns
Main memory (RAM) reference 100 ns
SSD random read 150 µs
Read 1 MB sequentially from memory 250 µs
Read 1 MB sequentially from SSD 1 ms
HDD random seek 10 ms
Read 1 MB sequentially from HDD 20 ms
Round trip packet within same datacenter 0.5 ms
Round trip packet CA → Netherlands → CA 150 ms