Introduction
Network protocols are the foundation of internet communication. Understanding how data moves across networks helps you build better applications and troubleshoot issues. This guide covers essential network protocols.
OSI Model
Seven Layers
| Layer | Protocol | Examples |
|---|---|---|
| 7. Application | HTTP, DNS | Browser, email |
| 6. Presentation | TLS, SSL | Encryption |
| 5. Session | RPC, NetBIOS | Session management |
| 4. Transport | TCP, UDP | Port-to-port |
| 3. Network | IP, ICMP | Router-to-router |
| 2. Data Link | Ethernet, WiFi | Frame delivery |
| 1. Physical | Fiber, Cable | Bits over wire |
Transport Layer
TCP vs UDP
| Feature | TCP | UDP |
|---|---|---|
| Connection | Yes | No |
| Reliability | Reliable | Best effort |
| Order | Guaranteed | Not guaranteed |
| Speed | Slower | Faster |
| Overhead | More | Less |
When to Use
- TCP: Web, email, file transfer (reliability)
- UDP: Video streaming, gaming, DNS (speed)
TCP Handshake
Client → SYN → Server
Client ← SYN-ACK ← Server
Client → ACK → Server
HTTP/HTTPS
HTTP Methods
| Method | Description | Idempotent |
|---|---|---|
| GET | Read | Yes |
| POST | Create | No |
| PUT | Replace | Yes |
| PATCH | Modify | No |
| DELETE | Remove | Yes |
HTTP Status Codes
1xx: Informational
2xx: Success
3xx: Redirection
4xx: Client Error
5xx: Server Error
Common codes:
- 200 OK
- 201 Created
- 301 Moved Permanently
- 400 Bad Request
- 401 Unauthorized
- 404 Not Found
- 500 Internal Server Error
HTTPS
# TLS handshake
Client → ClientHello →
← ServerHello + Certificate ←
→ Key Exchange →
← Finished ←
Encrypted communication begins
TLS/SSL
- Encrypts data in transit
- Server authentication
- Data integrity
- Uses certificates
DNS
How DNS Works
1. Browser asks: "What is example.com?"
2. Local DNS resolver checks cache
3. Root server: "Ask .com TLD server"
4. TLD server: "Ask authoritative server"
5. Authoritative server: "IP is 93.184.216.34"
DNS Records
| Type | Purpose |
|---|---|
| A | IPv4 address |
| AAAA | IPv6 address |
| CNAME | Alias |
| MX | Mail server |
| TXT | Text info |
| NS | Name server |
Common Protocols
FTP
File Transfer Protocol:
- Port 21 (control)
- Port 20 (data)
- Commands: GET, PUT, LIST
SSH
Secure Shell:
- Port 22
- Encrypted remote access
- SFTP, SCP
SMTP
Simple Mail Transfer Protocol:
- Port 25, 587
- Send email between servers
IMAP/POP3
- IMAP (143/993): Server-based
- POP3 (110/995): Download and delete
Ports
Common Ports
| Port | Service |
|---|---|
| 21 | FTP |
| 22 | SSH |
| 25 | SMTP |
| 53 | DNS |
| 80 | HTTP |
| 443 | HTTPS |
| 3306 | MySQL |
| 5432 | PostgreSQL |
| 6379 | Redis |
Ephemeral Ports
- 32768-60999 (typically)
- Used for client-side connections
Troubleshooting
Tools
- ping: Check reachability
- traceroute: Path to destination
- netstat: Connection status
- tcpdump/Wireshark: Packet analysis
- dig/nslookup: DNS queries
Common Issues
- DNS resolution failure
- Firewall blocking
- Port not open
- High latency
Security
Common Attacks
- DDoS: Flooding with requests
- Man-in-the-middle: Intercepting traffic
- DNS spoofing: Fake DNS responses
Best Practices
- Use HTTPS
- Firewall rules
- Keep software updated
- Monitor traffic
Conclusion
Understanding network protocols helps you build better applications and diagnose issues. Start with TCP vs UDP, learn HTTP thoroughly, and understand DNS. These fundamentals apply everywhere.
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