First, What Is It
- HTTP (HyperText Transfer Protocol): the "ordering process" of the web — the client sends a request (what I want), the server returns a response (here it is).
- Request/response model: HTTP is one question, one answer; servers never push data unsolicited.
- Runs over TCP: HTTP runs on TCP (port 80); HTTPS adds a TLS encryption layer (port 443).
Breaking Down the Principle
1. Complete Flow of One HTTP Request
2. Request Message Structure
GET /data HTTP/1.1 ← request line: method path version
Host: example.com ← headers: key-value pairs
Content-Type: application/json
← blank line separates headers from body
{"temp": 25} ← body (common with POST)Common methods:
| Method | Semantics | Typical use |
|---|---|---|
| GET | Fetch a resource | Read sensor data, web pages |
| POST | Submit/create | Report data, login |
| PUT | Replace/update | Update configuration |
| DELETE | Remove | Delete a resource |
3. Status Codes: How the Server Answers
| Code | Meaning | Common cases |
|---|---|---|
| 2xx | Success | 200 OK |
| 3xx | Redirect | 301/302 |
| 4xx | Client error | 404 not found, 401 unauthorized |
| 5xx | Server error | 500 internal server error |
4. HTTPS: A Sealed Envelope on HTTP
HTTPS = HTTP + TLS: first a handshake exchanges keys and verifies the certificate, then traffic is encrypted. SDK HTTPS examples involve certificate configuration; a failed server check means no connection.
How the SDK Implements It
- Related pages: HTTP Intro, GET, POST, HTTPS, RESTful API Server
An SDK HTTP client GET flow: connect Wi-Fi, get IP, socket to port 80, send the request text, read the response:
/* Pseudocode: GET = TCP connection + send request line/headers + read response */
char req[] = "GET /api/data HTTP/1.1\r\nHost: example.com\r\nConnection: close\r\n\r\n";
send(sock, req, strlen(req), 0);
while ((len = recv(sock, buf, sizeof(buf), 0)) > 0) { /* process response body */ }POST differs by adding Content-Type and a body; the RESTful API server example implements the "server" role on the board, receiving and parsing requests.
Common Exam & Interview Questions
Difference between HTTP and HTTPS?
HTTPS adds a TLS encryption layer between HTTP and TCP, preventing eavesdropping/tampering and verifying the server identity — at the cost of handshake overhead.
Essential difference between GET and POST?
GET fetches resources, parameters usually in the URL; POST submits data, parameters in the body. Semantically GET is "read", POST is "write".
What do 404 and 500 mean?
404: the client asked for a resource that does not exist. 500: an internal server error. Check the path for 404; check the server program for 500.
Why is the blank line in a request important?
It separates headers from the body; the server knows headers end there. Without it, the two cannot be distinguished.
Why should a dev board know HTTP?
Interfacing with cloud platforms/web services, reporting data, fetching configuration, and OTA firmware downloads all use HTTP/S — the basic protocol for IoT devices on the internet.
Have questions?
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