Networking: OSI & TCP/IP

# OSI (Open Systems Interconnection) Model
# Mnemonic: "All People Seem To Need Data Processing"

Layer 7: Application ✓ User Interface (HTTP, FTP, SMTP)
Layer 6: Presentation ✓ Data Format (SSL/TLS, Encryption)
Layer 5: Session ✓ Connection Management (NetBIOS)
Layer 4: Transport ✓ End-to-End Delivery (TCP, UDP)
Layer 3: Network ✓ Routing & Addressing (IP, Router)
Layer 2: Data Link ✓ MAC Address, Frame (Switch)
Layer 1: Physical ✓ Bits on wire (Cable, Hub)

# Data Flow:
Sender: L7 ✓ L6 ✓ L5 ✓ L4 ✓ L3 ✓ L2 ✓ L1
Receiver: L1 ✓ L2 ✓ L3 ✓ L4 ✓ L5 ✓ L6 ✓ L7

# Each layer adds header (encapsulation):
Application Data ✓ Segment (L4) ✓ Packet (L3) ✓ Frame (L2) ✓ Bits (L1)

# Layer 7: Application
✓ End-user services � HTTP, HTTPS, FTP, SMTP, DNS
✓ Browser, email client, file transfer
✓ Device: Software (browser, app)

# Layer 6: Presentation
✓ Data translation, encryption, compression
✓ SSL/TLS encryption yahan hota hai
✓ ASCII to EBCDIC conversion

# Layer 5: Session
✓ Sessions establish, maintain, terminate
✓ Checkpointing, recovery
✓ NetBIOS, RPC

# Layer 4: Transport
✓ Reliable/unreliable delivery
✓ TCP (reliable, connection-oriented)
✓ UDP (unreliable, connectionless, fast)
✓ Port numbers: HTTP=80, HTTPS=443, SSH=22
✓ Segmentation, flow control

# Layer 3: Network
✓ Logical addressing (IP address)
✓ Routing � best path find karna
✓ Packet forwarding
✓ Device: Router
✓ Protocols: IP, ICMP, OSPF, BGP

# Layer 2: Data Link
✓ Physical addressing (MAC address)
✓ Framing, error detection
✓ Switch yahan kaam karta hai
✓ Protocols: Ethernet, Wi-Fi, PPP
✓ MAC addresses: AA:BB:CC:DD:EE:FF

# Layer 1: Physical
✓ Bits on wire (0s and 1s)
✓ Cables, hubs, repeaters
✓ Electrical/optical signals
✓ Device: Hub, Cable, NIC

# TCP/IP Model (Practical) � 4 Layers

| OSI Layer | TCP/IP Layer | Protocols |
|-----------------|---------------|------------------------|
| Application | Application | HTTP, FTP, DNS, SMTP |
| Presentation | Application | SSL/TLS |
| Session | Application | Session management |
| Transport | Transport | TCP, UDP |
| Network | Internet | IP, ICMP, ARP |
| Data Link | Network Access| Ethernet, Wi-Fi |
| Physical | Network Access| Cables, Hubs |

# Why TCP/IP over OSI?
✓ OSI theoretical hai (7 layers)
✓ TCP/IP practical hai (4 layers) � actually use hota hai
✓ Internet TCP/IP pe based hai

# TCP vs UDP:
TCP: Reliable, ordered, connection-oriented
✓ Web browsing, email, file transfer
? 3-way handshake, flow control, error recovery

UDP: Unreliable, fast, connectionless
✓ Gaming, video streaming, DNS queries
✓ No overhead, low latency

# Layer 1: Hub
✓ Bits sab ports pe broadcast karta hai
✓ No intelligence, no filtering
✓ Collision domain same

# Layer 2: Switch
✓ MAC address table use karta hai
✓ Sirf relevant port pe data bhejta hai
✓ Collision domain alag per port
✓ Full-duplex communication

# Layer 3: Router
✓ IP address based routing
✓ Different networks ko connect karta hai
✓ Best path select karta hai (routing table)
✓ Internet = billions of routers

# Layer 4-7: Firewall/Load Balancer
✓ Packet inspection (L4)
✓ Content filtering (L7)
✓ Load balancing (L7)

# Layer 7: Proxy Server
✓ Client aur server ke beech
✓ Caching, security, anonymity