Networking Fundamentals

Understanding networking is essential for designing globally distributed, highly available systems. This guide covers the core concepts every senior engineer must know.

The OSI Model


Layer 7 - Application   │ HTTP, HTTPS, WebSocket, gRPC
Layer 6 - Presentation  │ SSL/TLS, Encryption, Compression
Layer 5 - Session       │ Session management, Authentication
Layer 4 - Transport     │ TCP, UDP, Ports
Layer 3 - Network       │ IP, Routing, ICMP
Layer 2 - Data Link     │ Ethernet, MAC addresses, Switches
Layer 1 - Physical      │ Cables, Radio waves, Hardware

DNS (Domain Name System)

Translates human-readable domain names to IP addresses.

DNS Hierarchy


                    ┌─────────────┐
                    │ Root DNS    │  (.)
                    │ 13 servers  │
                    └──────┬──────┘
                           │
         ┌─────────────────┼─────────────────┐
         │                 │                 │
    ┌────▼────┐      ┌─────▼────┐      ┌────▼────┐
    │  .com   │      │  .org    │      │  .io    │
    │   TLD   │      │   TLD    │      │   TLD   │
    └────┬────┘      └──────────┘      └─────────┘
         │
    ┌────▼──────────┐
    │ example.com   │
    │ Authoritative │
    └───────────────┘

DNS Resolution Flow


┌────────┐    1. Query     ┌──────────────┐
│ Client │────────────────▶│   Recursive  │
│Browser │                 │   Resolver   │
└────────┘                 │ (ISP/8.8.8.8)│
    ▲                      └──────┬───────┘
    │                             │
    │ 8. IP Address               │ 2. Query Root
    │                             ▼
    │                      ┌─────────────┐
    │                      │  Root DNS   │
    │                      └──────┬──────┘
    │                             │ 3. Referral to TLD
    │                             ▼
    │                      ┌─────────────┐
    │                      │   TLD DNS   │
    │                      │   (.com)    │
    │                      └──────┬──────┘
    │                             │ 4. Referral to Auth
    │                             ▼
    │                      ┌───────────────┐
    │                      │ Authoritative │
    │                      │    DNS        │
    │                      └───────┬───────┘
    │                              │ 5. IP Address
    └──────────────────────────────┘

DNS Record Types

Record	Purpose	Example
A	IPv4 address	`example.com → 93.184.216.34`
AAAA	IPv6 address	`example.com → 2606:2800:220:1:...`
CNAME	Alias to another domain	`www.example.com → example.com`
MX	Mail server	`example.com → mail.example.com`
TXT	Text data (SPF, DKIM)	Verification, email auth
NS	Name server	`example.com → ns1.example.com`
SOA	Zone authority	Primary NS, admin email

DNS Caching

Cache Level	TTL	Location
Browser	60s-few mins	Client
OS	Based on TTL	Client
Recursive Resolver	Based on TTL	ISP/Public DNS
CDN	Varies	Edge

DNS Performance Tips

Lower TTL before migrations (5 min vs 24 hours)
Use anycast DNS (Route53, Cloudflare)
Implement DNS prefetching in browsers
Consider GeoDNS for regional routing

CDN (Content Delivery Network)

Caches content at edge locations globally for lower latency.

CDN Architecture


                         ┌─────────────┐
                         │   Origin    │
                         │   Server    │
                         └──────┬──────┘
                                │
            ┌───────────────────┼───────────────────┐
            │                   │                   │
      ┌─────▼─────┐       ┌─────▼─────┐       ┌─────▼─────┐
      │ Edge PoP  │       │ Edge PoP  │       │ Edge PoP  │
      │ US-West   │       │  Europe   │       │   Asia    │
      └─────┬─────┘       └─────┬─────┘       └─────┬─────┘
            │                   │                   │
      ┌─────▼─────┐       ┌─────▼─────┐       ┌─────▼─────┐
      │   Users   │       │   Users   │       │   Users   │
      │  (~20ms)  │       │  (~20ms)  │       │  (~20ms)  │
      └───────────┘       └───────────┘       └───────────┘
      
      Without CDN: 200-300ms (to origin)
      With CDN: 20-50ms (to edge)

How CDN Works

User requests content → DNS resolves to nearest edge
Cache HIT → Edge serves directly (~20ms)
Cache MISS → Edge fetches from origin, caches, serves

CDN Caching Strategies


# Cache static assets aggressively
Cache-Control: public, max-age=31536000, immutable
 
# Cache API responses briefly
Cache-Control: public, max-age=60, s-maxage=300
 
# Don't cache sensitive data
Cache-Control: private, no-store

CDN Use Cases

Content Type	Strategy
Static assets (JS, CSS, images)	Long cache, versioned URLs
HTML pages	Short cache or no-cache
API responses	Varies by endpoint
Video/streaming	Chunked delivery, adaptive bitrate

TCP/IP Fundamentals

TCP Three-Way Handshake


Client                              Server
   │                                   │
   │────── SYN (seq=100) ─────────────▶│
   │                                   │
   │◀───── SYN-ACK (seq=300,ack=101) ──│
   │                                   │
   │────── ACK (ack=301) ─────────────▶│
   │                                   │
   │         Connection Established    │
   │                                   │

TCP vs UDP

Feature	TCP	UDP
Connection	Connection-oriented	Connectionless
Reliability	Guaranteed delivery	Best effort
Ordering	Ordered	Unordered
Speed	Slower (overhead)	Faster
Use Cases	HTTP, SSH, Email	DNS, Video, Gaming

TCP Congestion Control


Slow Start → Congestion Avoidance → Fast Recovery

Window Size
    │
    │           Congestion
    │              ▼
    │         ┌────┐
    │        /│    │\
    │       / │    │ \
    │      /  │    │  \
    │     /   │    │   \
    │    /    │    │    \
    └───/─────┴────┴─────\────▶ Time
      Slow    CA   Loss  Recovery
      Start

Algorithm	Description
Slow Start	Exponential growth until threshold
Congestion Avoidance	Linear growth after threshold
Fast Retransmit	Retransmit on 3 duplicate ACKs
BBR	Model-based, better for modern networks

TLS (Transport Layer Security)

Encrypts communication between client and server.

TLS Handshake (TLS 1.3)


Client                                Server
   │                                     │
   │─── ClientHello (supported ciphers) ─▶│
   │                                     │
   │◀── ServerHello + Certificate + ─────│
   │    Finished                         │
   │                                     │
   │─── Finished ───────────────────────▶│
   │                                     │
   │     Encrypted Communication         │
   │◀────────────────────────────────────▶│

TLS 1.3: 1-RTT handshake (vs 2-RTT in TLS 1.2)

Certificate Chain


┌─────────────────────────┐
│     Root CA             │  (Trusted, in browser/OS)
│  (DigiCert, Let's      │
│   Encrypt, etc.)        │
└───────────┬─────────────┘
            │ Signs
            ▼
┌─────────────────────────┐
│   Intermediate CA       │  (Issues end-entity certs)
└───────────┬─────────────┘
            │ Signs
            ▼
┌─────────────────────────┐
│   Server Certificate    │  (example.com)
└─────────────────────────┘

TLS Best Practices

Practice	Recommendation
TLS Version	TLS 1.3 (or 1.2 minimum)
Cipher Suites	ECDHE + AES-GCM
Certificate	2048-bit RSA or ECDSA
HSTS	Enable with long max-age
Certificate Transparency	Monitor for mis-issuance

HTTP/2 and HTTP/3

HTTP Evolution


HTTP/1.1                HTTP/2                  HTTP/3
┌────────┐             ┌────────┐             ┌────────┐
│Request1│             │  Req1  │             │  Req1  │
├────────┤             │  Req2  │             │  Req2  │
│Request2│             │  Req3  │             │  Req3  │
├────────┤             │(single │             │(QUIC)  │
│Request3│             │ conn)  │             │        │
└────────┘             └────────┘             └────────┘
 6 connections          1 connection           1 connection
 Head-of-line           Multiplexing           No HOL blocking
 blocking               Binary framing         0-RTT resumption

HTTP/2 Features

Feature	Benefit
Multiplexing	Multiple requests over single connection
Header Compression	HPACK reduces header size
Server Push	Server sends resources proactively
Stream Priority	Prioritize important resources
Binary Protocol	Efficient parsing

HTTP/3 (QUIC)

Built on UDP instead of TCP
0-RTT connection resumption
No head-of-line blocking (stream-level)
Connection migration (change networks without reconnect)

Load Balancing

L4 vs L7 Load Balancing


Layer 4 (Transport):
┌────────┐                      ┌────────┐
│ Client │──TCP packet─────────▶│  L4 LB │──Forward──▶ Server
└────────┘                      └────────┘
    Routes based on IP + Port (fast, simple)

Layer 7 (Application):
┌────────┐                      ┌────────┐
│ Client │──HTTP request───────▶│  L7 LB │──Route──▶ Server
└────────┘                      └────────┘
    Routes based on URL, headers, cookies (flexible)

Load Balancing Algorithms

Algorithm	Description	Use Case
Round Robin	Sequential distribution	Equal servers
Least Connections	Route to least busy	Variable load
IP Hash	Consistent routing by IP	Session affinity
Weighted	Based on server capacity	Mixed hardware

Global Server Load Balancing (GSLB)


User Request
     │
     ▼
┌─────────────┐
│  GeoDNS     │  → Returns nearest region IP
└──────┬──────┘
       │
   ┌───┴───┐───────┐
   │       │       │
   ▼       ▼       ▼
US-East  EU-West  AP-South
   │       │       │
   ▼       ▼       ▼
  LB       LB      LB

Interview Quick Reference

”What happens when you type google.com?”

Browser cache → Check local DNS cache
OS resolver → Check system DNS cache
Recursive resolver → ISP/public DNS (8.8.8.8)
Root → TLD → Authoritative → Get IP address
TCP handshake → SYN, SYN-ACK, ACK
TLS handshake → Certificates, key exchange
HTTP request → GET /
Server processing → Generate response
Response → HTML, then JS/CSS/images
Rendering → DOM, CSSOM, paint

Numbers to Know

Metric	Value
Speed of light (fiber)	~200,000 km/s
US coast-to-coast RTT	~60-80ms
Transatlantic RTT	~80-100ms
DNS lookup	20-120ms
TLS handshake	1-2 RTT
TCP handshake	1 RTT
CDN edge latency	10-50ms

Debugging Checklist

DNS resolving correctly? (dig, nslookup)
TCP connection working? (telnet, nc)
TLS handshake successful? (openssl s_client)
HTTP response correct? (curl -v)
Latency source? (traceroute, mtr)