Fundamentals — Cheat Sheet

OSI Model

// OSI Layers — top to bottom
// ┌─────────────────────────────────────────────────────────────────────┐
// │ Layer 7 — Application   │ HTTP, HTTPS, FTP, SMTP, DNS, WebSocket   │
// │ Layer 6 — Presentation  │ TLS/SSL, JPEG, MPEG, ASCII encoding      │
// │ Layer 5 — Session       │ NetBIOS, RPC, session establishment      │
// │ Layer 4 — Transport     │ TCP, UDP — ports, reliability, flow ctrl │
// │ Layer 3 — Network       │ IP, ICMP, ARP, routing                   │
// │ Layer 2 — Data Link     │ Ethernet, MAC, switches, frames          │
// │ Layer 1 — Physical      │ Cables, fibre, Wi-Fi, bits               │
// └─────────────────────────────────────────────────────────────────────┘

// Java socket sits at Layer 4/5 boundary:
ServerSocket server = new ServerSocket(8080);   // Transport layer (TCP port)
Socket client = server.accept();               // Session layer (connection)
InputStream in = client.getInputStream();      // Layer 5–7 data exchange

TCP/IP Model

// TCP/IP vs OSI mapping:
// ┌──────────────────────┬──────────────────────────────────────┐
// │ TCP/IP Layer         │ OSI Equivalent      │ Protocols      │
// ├──────────────────────┼──────────────────────────────────────┤
// │ Application          │ Layers 5, 6, 7      │ HTTP, DNS, SSH │
// │ Transport            │ Layer 4             │ TCP, UDP       │
// │ Internet             │ Layer 3             │ IP, ICMP, ARP  │
// │ Network Access/Link  │ Layers 1, 2         │ Ethernet, Wi-Fi│
// └──────────────────────┴──────────────────────────────────────┘

// A Java HTTP request flows through:
// 1. Application:    HttpClient sends GET /api/users HTTP/1.1
// 2. Transport:      TCP segments the request, adds port (443), handles reliability
// 3. Internet:       IP adds source/destination IP, routes across routers
// 4. Network Access: Ethernet frames carry IP packets over the physical network

Network Types — LAN, WAN, MAN & VPN

// Network types at a glance:
// ┌────────┬─────────────────┬─────────┬──────────┬──────────────────────┐
// │ Type   │ Scope           │ Speed   │ Latency  │ Example              │
// ├────────┼─────────────────┼─────────┼──────────┼──────────────────────┤
// │ PAN    │ < 10 metres     │ ~3 Mbps │ < 1ms    │ Bluetooth, USB       │
// │ LAN    │ Building/Campus │ 1 Gbps+ │ < 1ms    │ Office Wi-Fi         │
// │ MAN    │ City            │ 100Mbps │ 1–10ms   │ ISP city ring        │
// │ WAN    │ Global          │ Varies  │ 50–300ms │ Internet, MPLS links │
// └────────┴─────────────────┴─────────┴──────────┴──────────────────────┘

// In a microservices context:
// Service A → Service B (same datacenter, LAN): ~0.5ms
// Service A → Service B (different region, WAN): ~150ms
// This is why you keep chatty services co-located and use async
// messaging for cross-region communication.

Bandwidth, Latency & Throughput

// Bandwidth-Delay Product (BDP):
// BDP = Bandwidth × RTT
// = the amount of data "in flight" at any moment

// Example: 1 Gbps link, 100ms RTT
// BDP = 1,000,000,000 bits/s × 0.1s = 100,000,000 bits = 12.5 MB
// To fully utilise this link, the TCP window must be ≥ 12.5 MB

// Latency components:
// 1. Propagation delay = distance / speed of light in medium
//    London → New York ≈ 5,570 km, ~28ms one-way
// 2. Transmission delay = packet size / bandwidth
//    1 KB packet on 1 Gbps link = 8,000 bits / 1e9 bps = 0.008ms
// 3. Queuing delay    = time waiting in router buffers (variable)
// 4. Processing delay = time for router to inspect headers

// Rule of thumb:
// < 1ms   — same datacenter (LAN)
// 1–10ms  — same region / CDN edge
// 50–100ms — same continent
// 100–300ms — cross-continent

Packets, Frames & Encapsulation

// Ethernet Frame structure (Layer 2):
// ┌──────────────┬──────────────┬──────┬──────────────────────┬─────┐
// │ Dst MAC (6B) │ Src MAC (6B) │ Type │ Payload (IP Packet)  │ FCS │
// └──────────────┴──────────────┴──────┴──────────────────────┴─────┘
// Max payload = 1500 bytes (standard Ethernet MTU)
// Jumbo frames = up to 9000 bytes (datacenter NICs)

// IPv4 Packet structure (Layer 3):
// ┌────────────────────────────────────────────────────────────────┐
// │ Version │ IHL │ DSCP │ Total Length │ ID │ Flags │ Fragment   │
// │ TTL     │ Protocol  │ Checksum     │ Source IP (4B)          │
// │ Destination IP (4B) │ Options (variable)                      │
// │ Payload (TCP Segment / UDP Datagram)                          │
// └────────────────────────────────────────────────────────────────┘

// Protocol field identifies Layer 4 protocol:
// 6 = TCP, 17 = UDP, 1 = ICMP