3. Protocols

TCP vs UDP

# TCP 3-way handshake
Client → SYN            → Server
Client ← SYN+ACK        ← Server
Client → ACK            → Server
# Connection established, data transfer begins

HTTP / HTTPS

HTTP is the foundational protocol of the web. HTTPS adds TLS encryption on top of HTTP. Key concepts:

• Methods: GET (read), POST (create), PUT (replace), PATCH (partial update), DELETE
• Status codes: 2xx (success), 3xx (redirect), 4xx (client error), 5xx (server error)
• Headers: Metadata like Content-Type, Authorization, Cache-Control
• HTTP/1.1 vs HTTP/2: HTTP/2 introduces multiplexing, header compression, server push
• HTTP/3: Built on QUIC (UDP-based), reduces head-of-line blocking, faster connection establishment

# HTTP Request
GET /api/users/123 HTTP/1.1
Host: example.com
Authorization: Bearer token123
Accept: application/json

# HTTP Response
HTTP/1.1 200 OK
Content-Type: application/json

{"id": 123, "name": "Alice"}

REST (Representational State Transfer)

REST is an architectural style for designing networked applications. Key principles:

• Stateless: Each request contains all necessary information; no client context stored on the server.
• Resource-based: URLs represent resources (/users/123), not actions (/getUser).
• Standard methods: GET, POST, PUT, PATCH, DELETE map to CRUD operations.
• Representation: Resources can be represented as JSON, XML, or other formats.

Real-Time Protocols

• WebSocket: Full-duplex persistent connection over TCP. Used for chat, live notifications, real-time dashboards.
• SSE (Server-Sent Events): Unidirectional server-to-client streaming over HTTP. Simpler than WebSocket; good for live feeds, notifications.
• WebRTC: Peer-to-peer real-time communication (audio, video, data). Used for video calls, conferencing.

Modern API Protocols

• gRPC: High-performance RPC framework using Protocol Buffers and HTTP/2. Supports streaming, bidirectional communication. Used heavily in microservices.
• GraphQL: Query language that lets clients request exactly the data they need. Reduces over-fetching and under-fetching. Single endpoint, complex queries.

# gRPC service definition (Protocol Buffers)
service UserService {
  rpc GetUser (GetUserRequest) returns (User);
  rpc ListUsers (ListUsersRequest) returns (stream User);
}

message GetUserRequest { string user_id = 1; }