HTTP & HTTPS — The Web Protocol

Every webpage, API call, and file download uses HTTP. Understanding its structure, headers, and the TLS layer that makes it secure lets you debug anything from a broken API to a performance bottleneck.

HTTP Basics

HTTP is a request-response protocol. A client sends a request; the server replies with a response. Each exchange is independent — HTTP is stateless (the server doesn't remember previous requests by default).

HTTP Methods

Method	Semantics	Idempotent	Safe
GET	Retrieve resource	Yes	Yes
POST	Create resource / trigger action	No	No
PUT	Replace resource entirely	Yes	No
PATCH	Partial update	No	No
DELETE	Remove resource	Yes	No
HEAD	GET but only headers, no body	Yes	Yes
OPTIONS	List allowed methods (CORS preflight)	Yes	Yes

Idempotent = calling it multiple times has the same effect as calling it once.
Safe = doesn't modify server state.

Status Codes

Range	Meaning	Common codes
1xx	Informational	100 Continue, 101 Switching Protocols
2xx	Success	200 OK, 201 Created, 204 No Content
3xx	Redirect	301 Moved Permanently, 302 Found, 304 Not Modified
4xx	Client error	400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found, 429 Too Many Requests
5xx	Server error	500 Internal Server Error, 502 Bad Gateway, 503 Service Unavailable

Important Headers

Request headers:

Response headers:

Cookies

Cookies are key-value pairs the server sets on the client (Set-Cookie). The browser sends them back on every subsequent request to the same domain.

Flag	Meaning
HttpOnly	JavaScript cannot access this cookie — XSS protection
Secure	Only sent over HTTPS
SameSite=Strict	Not sent with cross-site requests — CSRF protection
Max-Age=N	Expires in N seconds

HTTP Versions

HTTP/1.0

New TCP connection per request — extremely slow for pages with many resources.

HTTP/1.1

Persistent connections: TCP connection reused for multiple requests. Pipelining: send multiple requests without waiting for responses — but responses must arrive in order (head-of-line blocking).

HTTP/2

• Multiplexing: multiple requests/responses interleaved over one TCP connection, no head-of-line blocking
• Header compression (HPACK): reduces overhead for large/repeated headers
• Server push: server proactively sends resources the client will need
• Binary framing instead of plain text

HTTP/3 (QUIC)

• Runs over QUIC (UDP-based) instead of TCP
• QUIC bakes TLS 1.3 into the handshake — 1 round-trip instead of 2
• Per-stream loss recovery: a lost packet in stream A doesn't block stream B
• Built-in connection migration (survive IP address changes, e.g., WiFi → 5G)

HTTPS = HTTP + TLS

TLS (Transport Layer Security) encrypts the connection. Without it, anyone on the network can read your passwords and data.

TLS 1.3 Handshake

Certificate Chain

The browser verifies each signature up to a trusted root. If any step fails, you see the "Your connection is not secure" warning.

CORS — Cross-Origin Resource Sharing

Browsers block JavaScript from making requests to a different origin (protocol + domain + port) than the page — the Same-Origin Policy.

CORS lets servers whitelist trusted origins:

Python: Making HTTP Requests

Key Takeaways

• HTTP is stateless and request-response; headers carry metadata about requests and responses
• Methods encode semantics (GET=read, POST=create, PUT=replace, DELETE=remove)
• Status codes: 2xx=success, 3xx=redirect, 4xx=client error, 5xx=server error
• HTTP/2 multiplexes requests; HTTP/3 eliminates TCP head-of-line blocking with QUIC
• HTTPS = HTTP + TLS; the certificate chain proves the server's identity
• Use HttpOnly + Secure + SameSite=Strict cookies to harden auth sessions