🌐 Computer Networks — Master Interview & Revision Guide

Targeted for: Juspay Elimination Round, Hackathon Part B System Discussion & Backend Engineering Interviews.
Core Focus: High-scale payment systems, network security, protocol internals, handshake mechanisms, idempotency, and client identification.

📑 Complete CN Module Index

#	Document	Core Topics Covered	Juspay High-Yield Relevance
01	01-dns-deep-dive.md	Recursive vs Iterative resolution, Root/TLD/Authoritative hierarchy, Record types (`A`, `AAAA`, `CNAME`, `MX`, `NS`, `TXT`, `SOA`, `PTR`), Caching & TTL, DNSSEC	⭐⭐⭐ Asked directly in Part B final phase
02	02-tcp-vs-udp-transport-layer.md	3-Way Handshake, 4-Way Termination, `TIME_WAIT` vs `CLOSE_WAIT` leaks, Sliding Window (Flow Control), Congestion Control (Slow Start, AIMD, Fast Retransmit), UDP vs QUIC	⭐⭐⭐ Critical for transport reliability & low-latency payments
03	03-http-https-evolution.md	Methods (GET, POST, PUT, PATCH, DELETE, OPTIONS), Status codes (200, 201, 301, 400, 401, 403, 404, 409, 429, 500, 502, 503, 504), HTTP/1.1 vs HTTP/2 vs HTTP/3 (QUIC)	⭐⭐⭐ REST API semantics & microservice communication
04	04-ssl-tls-and-certificates.md	TLS 1.2 vs 1.3 Handshake, Symmetric vs Asymmetric crypto, Certificate Authorities (CA), Local OS Trust Store validation, Digital Signatures, mTLS in Banking	⭐⭐⭐⭐ Direct Juspay Part B interview question
05	05-ip-addressing-and-routing.md	IPv4 Classes, Subnetting & CIDR calculations, IPv6, NAT / NAPT (PAT) translation tables, Public vs Private IP ranges (RFC 1918)	⭐⭐⭐ Essential for networking & infrastructure routing
06	06-idempotency-in-payments.md	Why payments demand idempotency, Double-debit prevention, Idempotency Keys (UUIDv4), Redis + DB locking state machines, In-flight concurrency & 409/429 handling	⭐⭐⭐⭐⭐ CORE JUSPAY QUESTION (Payment Gateway LLD)
07	07-session-management-and-identification.md	Client differentiation: 2 tabs on 1 laptop vs 2 laptops on 1 WiFi (NAT) vs 2 laptops on different networks; Cookies, JWT, SessionStorage, Security (CSRF, XSS, SameSite)	⭐⭐⭐⭐ Direct Juspay Part B interview question
08	08-osi-and-tcp-ip-models.md	All 7 OSI Layers, PDUs, Encapsulation/Decapsulation, OSI vs TCP/IP mapping, Hub vs Switch vs Router, L4 vs L7 Load Balancers, MAC vs IP	⭐⭐⭐⭐ Foundational Networking & Systems Architecture
09	09-vpn-tunneling-cryptography-and-encoding.md	VPN Internals (TUN/TAP, Encapsulation, WireGuard/IPsec), Symmetric vs Asymmetric Crypto, Encoding vs Encryption vs Hashing vs Tokenization, RBI Payment Card Tokenization, JWT Deep Dive, npm for frontend	⭐⭐⭐⭐⭐ Direct Juspay Systems & Security Interview questions
10	10-web-security-attacks-and-defenses.md	SOP & CORS Deep Dive (Preflight OPTIONS, Headers), XSS (Stored, Reflected, DOM), CSRF (Synchronizer tokens, SameSite), SSRF (AWS IMDS exploit), Clickjacking, Payment Replay attacks	⭐⭐⭐⭐⭐ Critical Web & Payment Gateway Security Gauntlet
11	11-rest-api-architecture-and-design.md	Roy Fielding's 6 Constraints, Richardson Maturity Model (L0-L3), HTTP Semantics (RFC 9110), Idempotency Filters & Lock, OCC (ETags), Keyset Pagination, Distributed Rate Limiting, Versioning & Webhooks	⭐⭐⭐⭐⭐ CORE JUSPAY QUESTION (REST API Architecture & Design)

⚡ 10-Minute Rapid Pre-Interview Cheat Sheet

1. DNS Record Quick Matrix

A: Maps hostname $\rightarrow$ IPv4 address (api.juspay.in $\rightarrow$ 13.235.12.45).
AAAA: Maps hostname $\rightarrow$ IPv6 address (api.juspay.in $\rightarrow$ 2404:6800:...).
CNAME: Alias mapping canonical name to canonical name (pay.merchant.com $\rightarrow$ secure.juspay.in). Cannot be on root zone (@).
MX: Mail exchange server with priority weighting (10 mail.juspay.in).
NS: Nameserver authority delegating DNS zone (ns1.awsdns.com).
TXT: Arbitrary text — domain verification, SPF (anti-spam), DKIM, SSL domain validation.

2. TCP Handshake & State Essentials

HANDSHAKE: [Client] --- SYN (seq=x) ---> [Server]
           [Client] <-- SYN-ACK (seq=y, ack=x+1) -- [Server]
           [Client] --- ACK (ack=y+1) ---> [Server] (Can send data!)

TERMINATION: [Client] --- FIN ---> [Server]
             [Client] <-- ACK --- [Server]  (Server enters CLOSE_WAIT)
             [Client] <-- FIN --- [Server]
             [Client] --- ACK ---> [Server] (Client enters TIME_WAIT: 2 * MSL)

Why TIME_WAIT (2MSL = ~60-120s)?
1. Ensure final ACK reaches server (if lost, server retransmits FIN).
2. Prevent old delayed duplicate packets from colliding with a new incarnation of the same socket (IP:Port).

3. How Computer Validates an SSL/TLS Certificate Locally

1. Server sends Leaf Certificate Chain (Leaf -> Intermediate -> Root CA).
2. Client inspects OS / Browser Root Trust Store to locate the Root CA Public Key.
3. Client verifies Digital Signature:
   - Decrypts issuer's signature using issuer's Public Key -> gets Hash(Cert).
   - Computes SHA-256 Hash of received certificate body independently.
   - If computed_hash == decrypted_hash -> Certificate is authentic and untampered!
4. Checks: Expiration dates (NotBefore / NotAfter), Domain Name match (SAN/CN), Revocation status (OCSP / CRL).

4. Idempotency in Payment Gateways

Definition: $f(f(x)) = f(x)$. Executing the same operation multiple times produces the exact same side-effect as executing it once.
Why Vital: Prevent double charging a customer when a bank timeout or mobile network retry occurs.
Implementation:
1. Client sends header Idempotency-Key: <unique-uuid>.
2. Backend acquires a distributed lock (Redis) or uses DB unique constraint UNIQUE(idempotency_key).
3. If key exists:
  - If state == COMPLETED: Return cached response immediately.
  - If state == PROCESSING: Return 409 Conflict or poll/wait.
4. If new: Create record in state PENDING $\rightarrow$ Execute payment with Bank/UPI $\rightarrow$ Update to SUCCESS $\rightarrow$ Cache response $\rightarrow$ Release lock.

5. Client Differentiation Scenarios

┌──────────────────────────────────────┬────────────────────────────────────────────────────────────────────────┐
│ Scenario                             │ How the Server Differentiates                                          │
├──────────────────────────────────────┼────────────────────────────────────────────────────────────────────────┤
│ Two Tabs on Same Laptop              │ Same Public IP, Same Client Port (if sharing HTTP/2 conn), Same Cookie │
│                                      │ -> Differentiated by: Unique Tab ID, sessionStorage, or custom header. │
├──────────────────────────────────────┼────────────────────────────────────────────────────────────────────────┤
│ Two Laptops on Same WiFi (NAT)       │ Same Public IP (seen by server). Differentiated by: Unique NAT Source  │
│                                      │ Ports (Transport Layer) and Distinct Session Cookies / JWTs (App Layer).│
├──────────────────────────────────────┼────────────────────────────────────────────────────────────────────────┤
│ Two Laptops on Different Networks    │ Differentiated at Layer 3: Distinct Public IP Addresses + Unique Auth. │
└──────────────────────────────────────┴────────────────────────────────────────────────────────────────────────┘

🎯 How to Use These Notes

Read each markdown file sequentially for conceptual mastery.
Study the "Verbatim Interview Scripts" to learn how to structure your spoken answers under pressure.
Review the "Juspay & Real-World Payment Context" callouts to stand out with production-grade architectural awareness.

00-master-cn-index.md

🌐 Computer Networks — Master Interview & Revision Guide

📑 Complete CN Module Index

⚡ 10-Minute Rapid Pre-Interview Cheat Sheet

1. DNS Record Quick Matrix

2. TCP Handshake & State Essentials

3. How Computer Validates an SSL/TLS Certificate Locally

4. Idempotency in Payment Gateways

5. Client Differentiation Scenarios

🎯 How to Use These Notes