🌐 Network & Operating Systems

"Understanding the infrastructure helps you build better applications."

Networks and operating systems form the foundation that all applications run on. Knowledge here helps debug complex issues and optimize performance.

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🔗 Network Protocols

TCP/IP Stack

TCP Three-Way Handshake

TCP vs UDP

Feature	TCP	UDP
Connection	Connection-oriented	Connectionless
Reliability	Guaranteed delivery	Best effort
Ordering	Maintained	Not guaranteed
Speed	Slower (overhead)	Faster
Use Case	HTTP, FTP, Email	DNS, Video streaming, Gaming

HTTP/HTTPS

Version	Features
HTTP/1.1	Persistent connections, pipelining
HTTP/2	Multiplexing, header compression, server push
HTTP/3	QUIC (UDP-based), reduced latency

HTTPS = HTTP + TLS/SSL
- Encrypts data in transit
- Verifies server identity
- Prevents man-in-the-middle attacks

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🐧 Linux Essentials

Essential Commands

Category	Commands
File System	ls, cd, cp, mv, rm, find, grep
Process	ps, top, htop, kill, nohup, &
Network	netstat, ss, curl, wget, ping, traceroute
Disk	df, du, mount, lsblk
System	systemctl, journalctl, dmesg

Process Management

# View all processes
ps aux | grep java

# Real-time process monitoring
top -u $(whoami)

# Run in background
nohup java -jar app.jar > app.log 2>&1 &

# View open files/ports by process
lsof -i :8080
lsof -p <pid>

# Network connections
ss -tlnp  # TCP listening ports
netstat -an | grep ESTABLISHED

File Permissions

# Format: rwx rwx rwx
#         user group others

chmod 755 script.sh  # rwxr-xr-x
chmod 600 secret.key # rw-------

# Ownership
chown user:group file

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💾 Memory & Process

Process States

Memory Concepts

Concept	Description
Virtual Memory	Abstraction over physical memory
Paging	Fixed-size memory blocks
Page Fault	Accessing memory not in RAM
Swap	Using disk as extended memory
Memory-mapped I/O	File access via memory addresses

Concurrency Primitives

Primitive	Description	Use Case
Mutex	Mutual exclusion lock	Critical sections
Semaphore	Counting resource access	Resource pools
Condition Variable	Wait for condition	Producer-consumer
Read-Write Lock	Multiple readers, single writer	Read-heavy scenarios

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📝 Detailed Topics

• DNS Resolution
• TLS/SSL Handshake
• Linux Performance Tuning
• Container Networking
• System Calls

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🔧 Debugging Toolkit

# Network debugging
tcpdump -i eth0 port 80           # Capture HTTP traffic
curl -v https://api.example.com    # Verbose HTTP request
dig example.com                     # DNS lookup

# System debugging
strace -p <pid>                    # Trace system calls
vmstat 1                           # Virtual memory stats
iostat -x 1                        # I/O statistics

# Log analysis
tail -f /var/log/syslog            # Follow system logs
journalctl -u nginx -f             # Follow service logs

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Key Takeaways

1. TCP for reliability, UDP for speed
2. HTTPS should be default for all web traffic
3. Master grep, awk, sed for log analysis
4. Understand process states for debugging hangs
5. Use strace when all else fails