Common Network Protocols

Protocols are the "rules" that govern how devices communicate. Different protocols solve different problems.

Protocol Basics

A protocol is a standardized set of rules for communication:

• Define how data is formatted
• Define how devices initiate, maintain, and end communication
• Allow different vendors' equipment to interoperate

Transport Layer Protocols

The most important protocols for DevOps engineers are at the Transport Layer (Layer 4).

TCP (Transmission Control Protocol)

Purpose: Reliable, ordered delivery of data

TCP Three-Way Handshake (Connection Setup):

Client                Server
  |------ SYN ------->|   "I want to connect"
  |<----- SYN-ACK ----|   "OK, I'm ready"
  |------ ACK ------->|   "Great, connected"
  |======= DATA =====>|   Connection established

Key Features:

• Reliable — all data arrives
• Ordered — data arrives in sequence
• Flow Control — prevents overwhelming receiver
• Error Checking — detects and corrects errors
• Slower — overhead of reliability

TCP Flags:

TCP State Machine:

CLOSED → LISTEN (server)
         ↓
Client initiates SYN →
         ↓
    SYN_RECEIVED
         ↓
    ESTABLISHED ← ACK received
         ↓
FIN sent → FIN_WAIT_1
         ↓
    TIME_WAIT
         ↓
    CLOSED

When to use TCP:

• Web browsing (HTTP/HTTPS)
• Email (SMTP, POP3, IMAP)
• File transfer (FTP, SFTP)
• SSH (remote access)
• Database connections
• Anything where data integrity matters

UDP (User Datagram Protocol)

Purpose: Fast, best-effort delivery of data

UDP Communication (No Handshake):

Client                Server
  |====== DATA ======>|   One-way, no connection
                      |   Server may or may not receive

Key Features:

• Unreliable — packets may be lost
• Unordered — packets may arrive out of order
• No Connection — no setup/teardown
• Fast — minimal overhead
• Lower Latency — important for real-time apps

When to use UDP:

• DNS queries (quick lookup)
• Voice over IP / Video calls (can tolerate packet loss)
• Online gaming (latency more important than perfection)
• Live video streaming (occasional dropped frames acceptable)
• Network monitoring (SNMP)
• IoT sensors (volume matters more than individual readings)

ICMP (Internet Control Message Protocol)

Purpose: Send error messages and diagnostic information

Common ICMP Types:

Ping Example:

ping google.com
# Sends ICMP Echo Request
# Google replies with Echo Reply
# Shows latency and packet loss

Traceroute Example:

traceroute google.com
# Sends UDP packets with increasing TTL
# Each router responds when TTL expires
# Shows path from your machine to destination

When to use ICMP:

• Diagnostics (ping, traceroute)
• Network troubleshooting
• Path MTU discovery
• Not for application data

Protocol Comparison

IP (Internet Protocol)

Purpose: Route packets between networks

IPv4 Header Contains:

• Source IP address
• Destination IP address
• TTL (Time To Live) — prevents infinite loops
• Protocol (TCP=6, UDP=17, ICMP=1)
• Checksum — error detection
• Flags and fragment offset — packet fragmentation info

TTL (Time To Live):

• Decrements by 1 at each router
• When TTL reaches 0, packet is discarded
• Prevents routing loops
• Typical initial TTL: 64 or 255

Application Layer Protocols

These protocols sit on top of TCP/UDP:

HTTP (Hypertext Transfer Protocol)

• Built on TCP port 80
• Stateless request-response protocol
• Plain text (unencrypted)

HTTPS (Hypertext Transfer Protocol Secure)

• Built on TCP port 443
• HTTP + TLS encryption
• Secure version of HTTP

DNS (Domain Name System)

• Built on UDP port 53
• Translates domain names to IP addresses
• Can use TCP for large responses

SMTP (Simple Mail Transfer Protocol)

• Built on TCP port 25
• Sends emails

SSH (Secure Shell)

• Built on TCP port 22
• Remote login + command execution
• Encrypted

FTP (File Transfer Protocol)

• Built on TCP ports 20-21
• File transfer (unencrypted)

Protocol Layering (Encapsulation)

When you send data over the network, it gets wrapped in multiple headers:

Example: HTTP request over HTTPS

┌─────────────────────────────────┐
│ HTTP Request (Application)      │
│ "GET /index.html"               │
├─────────────────────────────────┤
│ TLS Record (Encryption Layer)   │
├─────────────────────────────────┤
│ TCP Header (Transport Layer)    │
│ Source Port: 54321              │
│ Dest Port: 443                  │
├─────────────────────────────────┤
│ IP Header (Network Layer)       │
│ Source IP: 203.0.113.50         │
│ Dest IP: 142.251.41.14          │
├─────────────────────────────────┤
│ Ethernet Header (Link Layer)    │
│ Source MAC: aa:bb:cc:dd:ee:01  │
│ Dest MAC: aa:bb:cc:dd:ee:02    │
└─────────────────────────────────┘

Port + Protocol Combinations

Web Services:

HTTP:           TCP 80
HTTPS:          TCP 443

Email:

SMTP:           TCP 25
POP3:           TCP 110
IMAP:           TCP 143

Remote Access:

SSH:            TCP 22
Telnet:         TCP 23 (deprecated)

Database:

MySQL:          TCP 3306
PostgreSQL:     TCP 5432
MongoDB:        TCP 27017
Redis:          TCP 6379

Monitoring & Management:

SNMP:           UDP 161
Syslog:         UDP 514
NTP:            UDP 123

Protocol Selection Decision Tree

Is data integrity critical?

• YES → Use TCP
• NO → Check next question

Is real-time response more important than perfection?

• YES → Use UDP
• NO → Use TCP

Is this diagnostic?

• YES → Use ICMP (ping/traceroute)
• NO → Use TCP or UDP

DevOps Considerations

Network Firewalls:

• Must allow specific protocol + port combinations
• Example: Open TCP 443 for HTTPS, but block TCP 23 (Telnet)

Load Balancers:

• TCP load balancing (Layer 4) — can handle any TCP protocol
• UDP load balancing (Layer 4) — stateless, more complex
• Application load balancing (Layer 7) — understands HTTP/HTTPS

Monitoring:

• TCP connections easily monitored
• UDP harder to track (stateless)
• ICMP used for availability monitoring

Summary

• TCP — reliable, ordered, slower; use for important data
• UDP — fast, unreliable, best-effort; use for real-time apps
• ICMP — diagnostic; used by ping and traceroute
• Protocols layer — each layer adds headers to the previous layer
• Port selection — choose standard ports when possible for compatibility
• Understand these protocols to design, troubleshoot, and monitor networks