Networking Basics

Fundamental Concepts

The Internet

A global network of networks connecting billions of devices.

• IP Address: Unique identifier (e.g., 192.168.1.1).
• Packet: Unit of data transmitted.
• Router: Device forwarding packets between networks.
• Latency: Time to travel (ms).
• Bandwidth: Data transfer rate (Mbps).

The OSI Model

7-layer conceptual model characterizing communication.

Mnemonic: Please Do Not Throw Sausage Pizza Away

Transport Layer: TCP vs UDP

TCP (Transmission Control Protocol)

• Connection-oriented: 3-way handshake.
• Reliable: ACKs, Retransmission, Ordering.
• Heavyweight: Header 20-60 bytes.
• Use: Web (HTTP), Email, File Transfer.
• Flow Control: Sliding Window.
• Congestion Control: Slow Start.

3-Way Handshake:

1. SYN: Client sends sequence number $x$.
2. SYN-ACK: Server ACKs $x+1$, sends $y$.
3. ACK: Client ACKs $y+1$.

UDP (User Datagram Protocol)

• Connectionless: No handshake.
• Unreliable: No ACKs, no order guarantee.
• Lightweight: Header 8 bytes.
• Use: Streaming, Gaming, DNS, VoIP.

Application Layer: HTTP

HTTP/1.1 vs HTTP/2 vs HTTP/3

• HTTP/1.1: Text-based. Keep-alive. Head-of-line blocking.
• HTTP/2: Binary. Multiplexing (multiple requests over one conn). Header compression (HPACK). Server Push.
• HTTP/3: Based on QUIC (UDP). Solves TCP head-of-line blocking.

Methods

• GET: Retrieve resource (Idempotent).
• POST: Create resource.
• PUT: Update/Replace resource (Idempotent).
• PATCH: Partial update.
• DELETE: Delete resource.

Status Codes

• 200: OK.
• 301: Moved Permanently.
• 400: Bad Request.
• 401: Unauthorized (No auth).
• 403: Forbidden (Has auth, no permission).
• 404: Not Found.
• 500: Internal Server Error.
• 502: Bad Gateway.

HTTPS & TLS (SSL)

Secure version of HTTP. Handshake:

1. ClientHello (Cipher suites, Random).
2. ServerHello (Selected cipher, Cert, Random).
3. Key Exchange (Diffie-Hellman or RSA).
4. Finished. Symmetric Encryption: Used for data transfer (AES). Asymmetric Encryption: Used for handshake (RSA/ECC).

Core Infrastructure

DNS (Domain Name System)

Phonebook of the internet. Maps google.com $\to$ 142.250.190.46. Resolution Steps:

1. Browser Cache.
2. OS Cache.
3. Resolver (ISP).
4. Root Server (.).
5. TLD Server (.com).
6. Authoritative Server (google.com).

Record Types:

• A: IPv4.
• AAAA: IPv6.
• CNAME: Alias (domain to domain).
• MX: Mail.
• NS: Name Server.

Load Balancers

Distributes traffic across servers.

• L4 LB: Transport layer. Routes based on IP/Port. Fast.
• L7 LB: Application layer. Routes based on URL, Cookies, Headers. Smarter.
• Algorithms: Round Robin, Least Connections, Consistent Hashing.

WebSockets

Persistent full-duplex connection.

• Handshake via HTTP Upgrade header.
• Low latency real-time (Chat, Feeds).

Security Concepts

• CORS (Cross-Origin Resource Sharing): Browser mechanism restricting requests to different domains.
• XSS (Cross-Site Scripting): Injecting scripts into client pages. Mitigation: CSP, Escaping.
• CSRF (Cross-Site Request Forgery): Tricking user to perform action. Mitigation: Anti-CSRF tokens.
• SQL Injection: Malicious SQL queries. Mitigation: Prepared Statements.
• DDOS: Overwhelming server. Mitigation: Rate Limiting, CDN.

Interview Problem Types

Type 1: Troubleshooting

Type 2: Protocol Design

Quick Reference

• Port 80: HTTP.
• Port 443: HTTPS.
• Port 22: SSH.
• Port 53: DNS.
• Localhost: 127.0.0.1.
• Subnet Mask: Defines network size (e.g., /24).

Practice Problem Categories

• Browser: What happens when you type a URL?
• Security: Explain HTTPS handshake.
• Design: API Rate Limiter, Chat System Protocol.