10. Networking Basics
Understanding networking fundamentals is essential for system design. Every distributed system relies on networks for communication between clients, servers, services, and databases.
OSI Model
The Open Systems Interconnection model defines 7 layers of networking:
| Layer | Name | Protocol Examples | System Design Relevance |
|---|---|---|---|
| 7 | Application | HTTP, HTTPS, DNS, gRPC, WebSocket | API design, load balancing (L7) |
| 6 | Presentation | SSL/TLS, JPEG, JSON encoding | Encryption, data serialization |
| 5 | Session | Session management | WebSocket connections |
| 4 | Transport | TCP, UDP | Reliability, load balancing (L4) |
| 3 | Network | IP, ICMP | Routing, subnets, VPCs |
| 2 | Data Link | Ethernet, ARP | LAN communication |
| 1 | Physical | Cables, fiber, wireless | Hardware infrastructure |
In practice, the TCP/IP model (4 layers) is more commonly referenced:
┌─────────────────┐
│ Application │ HTTP, DNS, gRPC
├─────────────────┤
│ Transport │ TCP, UDP
├─────────────────┤
│ Internet │ IP
├─────────────────┤
│ Network │ Ethernet, WiFi
│ Access │
└─────────────────┘
IP (Internet Protocol)
Every device on a network has an IP address — a unique identifier for routing packets.
IPv4
- 32-bit address:
192.168.1.100 - ~4.3 billion addresses (exhausted).
- Private ranges:
10.0.0.0/8,172.16.0.0/12,192.168.0.0/16.
IPv6
- 128-bit address:
2001:0db8:85a3:0000:0000:8a2e:0370:7334 - ~
- Gradually replacing IPv4.
CIDR (Classless Inter-Domain Routing)
Notation for IP ranges:
10.0.0.0/24 = 10.0.0.0 to 10.0.0.255 (256 addresses)
10.0.0.0/16 = 10.0.0.0 to 10.0.255.255 (65,536 addresses)
10.0.0.0/8 = 10.0.0.0 to 10.255.255.255 (16.7 million addresses)
The number after / is the subnet mask bits. The remaining bits define the host range.
TCP (Transmission Control Protocol)
A connection-oriented, reliable transport protocol.
Three-Way Handshake
Client ──[SYN]──→ Server # Client requests connection
Client ←──[SYN-ACK]── Server # Server acknowledges
Client ──[ACK]──→ Server # Client confirms → Connection established
Connection Teardown (Four-Way Handshake)
Client ──[FIN]──→ Server # Client wants to close
Client ←──[ACK]── Server # Server acknowledges
Client ←──[FIN]── Server # Server wants to close
Client ──[ACK]──→ Server # Client acknowledges → Connection closed
TCP Features
| Feature | Description |
|---|---|
| Ordered delivery | Packets arrive in sequence |
| Reliable delivery | Lost packets are retransmitted |
| Flow control | Receiver controls send rate (sliding window) |
| Congestion control | Adapts to network congestion (slow start, AIMD) |
| Error detection | Checksums detect corruption |
TCP Use Cases
- HTTP/HTTPS (web traffic)
- Database connections
- File transfers (FTP, SFTP)
- Email (SMTP, IMAP)
- SSH
UDP (User Datagram Protocol)
A connectionless, unreliable transport protocol.
Client ──[Datagram]──→ Server # No handshake, no ACK, no ordering
UDP Characteristics
| Feature | Description |
|---|---|
| No connection setup | No handshake overhead |
| No guaranteed delivery | Packets may be lost |
| No ordering | Packets may arrive out of order |
| No congestion control | Sender sends at whatever rate |
| Low overhead | Minimal header (8 bytes vs TCP's 20+ bytes) |
| Fast | Much lower latency than TCP |
UDP Use Cases
- DNS queries
- Video/audio streaming (RTP)
- Online gaming
- VoIP (Voice over IP)
- IoT sensor data
- QUIC (HTTP/3 transport layer)
DNS (Domain Name System)
DNS translates domain names to IP addresses — the "phone book" of the internet.
DNS Resolution Flow
Browser → "What is google.com?"
│
├─→ Browser cache (check first)
├─→ OS cache (/etc/hosts)
├─→ Local DNS Resolver (ISP or 8.8.8.8)
│ │
│ ├─→ Resolver cache (check first)
│ ├─→ Root DNS Server → "Ask .com TLD server"
│ ├─→ TLD Server (.com) → "Ask ns1.google.com"
│ └─→ Authoritative Server → "142.250.80.46"
│
└─→ Browser caches result, connects to 142.250.80.46
DNS Record Types
| Record | Purpose | Example |
|---|---|---|
| A | Map domain to IPv4 | example.com → 93.184.216.34 |
| AAAA | Map domain to IPv6 | example.com → 2606:2800:220:1:248:... |
| CNAME | Alias one domain to another | www.example.com → example.com |
| MX | Mail server | example.com → mail.example.com |
| NS | Nameserver for the domain | example.com → ns1.example.com |
| TXT | Arbitrary text (verification, SPF) | example.com → "v=spf1 ..." |
| SRV | Service discovery (host + port) | _sip._tcp.example.com → sipserver.example.com:5060 |
| PTR | Reverse DNS (IP → domain) | 34.216.184.93 → example.com |
DNS in System Design
| Pattern | Description |
|---|---|
| GeoDNS | Return different IPs based on client location |
| DNS load balancing | Return multiple A records; client picks one (round-robin) |
| Failover | Health-checked DNS records; remove unhealthy IPs |
| Low TTL | Short cache time allows fast IP changes (failover) |
| High TTL | Long cache time reduces DNS lookup overhead |
DNS TTL Trade-offs
| Low TTL (e.g., 60s) | High TTL (e.g., 86400s) |
|---|---|
| Fast failover | Slow failover |
| More DNS queries | Fewer DNS queries |
| Higher DNS load | Lower DNS load |
| Quick IP changes | Stale IPs possible |
HTTP/HTTPS
HTTP/1.1
- One request per TCP connection (or keep-alive with pipelining).
- Text-based headers.
- Head-of-line blocking: second request waits for first to complete.
HTTP/2
- Multiplexing: Multiple requests over a single TCP connection.
- Header compression (HPACK): Reduces header overhead.
- Server push: Server sends resources before client requests them.
- Binary framing: More efficient than text.
- Stream prioritization: Important resources first.
HTTP/3 (QUIC)
- Built on UDP instead of TCP.
- Eliminates TCP head-of-line blocking.
- Built-in TLS 1.3 (encrypted by default).
- Faster connection setup (0-RTT).
- Better performance on lossy/mobile networks.
HTTP/1.1: [TCP handshake] + [TLS handshake] + [Request] = 3+ RTTs
HTTP/2: [TCP handshake] + [TLS handshake] + [Multiplexed requests] = 2+ RTTs
HTTP/3: [QUIC handshake (includes TLS)] + [Request] = 1 RTT (0-RTT on reconnect)
HTTPS (TLS/SSL)
Client ──[ClientHello]──→ Server
Client ←──[ServerHello + Certificate]── Server
Client ──[Key Exchange]──→ Server
Client ←→ [Encrypted Application Data] ←→ Server
TLS versions:
| Version | Status | Notes |
|---------|--------|-------|
| SSL 3.0 | Deprecated | Insecure (POODLE attack) |
| TLS 1.0 | Deprecated | Insecure |
| TLS 1.1 | Deprecated | Insecure |
| TLS 1.2 | Active | Widely used, secure |
| TLS 1.3 | Active | Fastest, most secure, recommended |
Network Design for Distributed Systems
VPC (Virtual Private Cloud)
An isolated network within a cloud provider:
┌──────────────────── VPC (10.0.0.0/16) ──────────────────────┐
│ │
│ ┌──── Public Subnet (10.0.1.0/24) ────┐ │
│ │ [Load Balancer] [Bastion Host] │ ← Internet access │
│ └──────────────────────────────────────┘ │
│ │ │
│ ┌──── Private Subnet (10.0.2.0/24) ───┐ │
│ │ [App Server 1] [App Server 2] │ ← No direct │
│ └──────────────────────────────────────┘ internet access │
│ │ │
│ ┌──── Private Subnet (10.0.3.0/24) ───┐ │
│ │ [Database Primary] [Database Replica] │ ← Most restricted │
│ └──────────────────────────────────────┘ │
│ │
└──────────────────────────────────────────────────────────────┘
Latency Reference (Datacenter to Datacenter)
| Route | Approximate Latency |
|---|---|
| Same rack | < 0.5 ms |
| Same datacenter | 0.5-1 ms |
| Same region (different AZ) | 1-2 ms |
| Cross-region (US East ↔ US West) | 40-70 ms |
| Cross-continent (US ↔ Europe) | 80-120 ms |
| US ↔ Asia | 150-200 ms |
Bandwidth vs Throughput vs Latency
| Concept | Definition | Analogy |
|---|---|---|
| Bandwidth | Maximum data transfer rate of a link | Width of a highway |
| Throughput | Actual data transfer rate achieved | Number of cars passing per hour |
| Latency | Time for one unit of data to travel from A to B | Time for one car to drive end-to-end |
Bandwidth-Delay Product (BDP):
The amount of data "in flight" on the network at any given time.
Summary
| Concept | Key Point |
|---|---|
| TCP | Reliable, ordered — use for most application communication |
| UDP | Fast, unreliable — use for streaming, gaming, DNS |
| DNS | Domain → IP resolution; use for load balancing and failover |
| HTTP/2 | Multiplexing over single TCP connection |
| HTTP/3 | UDP-based (QUIC), fastest connection setup |
| VPC | Isolated cloud network with public/private subnets |
| Latency | Within DC: <1ms; cross-continent: 100-200ms |