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

// Data units:
// Application layer → Message / Data
// Transport layer   → Segment (TCP) or Datagram (UDP)
// Internet layer    → Packet
// Network Access    → Frame (Layer 2) / Bits (Layer 1)

// When you call:
HttpClient client = HttpClient.newHttpClient();
HttpResponse<String> response = client.send(
    HttpRequest.newBuilder(URI.create("https://api.example.com/users")).build(),
    HttpResponse.BodyHandlers.ofString()
);

// The "GET /users HTTP/1.1" is a Message (Application layer)
// TCP breaks it into one or more Segments
// IP wraps each Segment into a Packet with IP header
// Ethernet wraps each Packet into a Frame with MAC header