Tell us about the versions of HTTP and their differences.

Main versions of HTTP:

• HTTP/0.9: Basic version for transmitting simple HTML documents. Supported only the GET method, lacked headers and response statuses.
• HTTP/1.0: Introduced support for headers (Content-Type, User-Agent), methods POST and HEAD, response statuses. Connection was closed after each request/response.
• HTTP/1.1: Significant improvements:
  • Persistent connections - reuse TCP connection for multiple requests.
  • Pipelining - sending multiple requests without waiting for the previous response (though poorly implemented and rarely used).
  • Virtual hosts (Host header).
  • Partial GET requests (Range header).
  • Caching (Cache-Control, ETag, Last-Modified).
  • Methods PUT and DELETE.
• HTTP/2: Protocol based on Google's SPDY. Main differences:
  • Binary protocol instead of text - more efficient parsing.
  • Multiplexing - parallel transmission of multiple requests/responses over one TCP connection. Eliminates the "head-of-line blocking" problem of HTTP/1.1 pipelining.
  • Header compression (HPACK) - reduces data size.
  • Prioritization (Server Push) - server can send resources likely needed by the client before it requests them.
• HTTP/3: Based on UDP protocol (using QUIC - Quick UDP Internet Connections) instead of TCP. Aimed at improving performance, especially in unstable networks:
  • Eliminates "head-of-line blocking" at the TCP connection level. If one packet is lost in TCP, all subsequent packets are blocked until retransmission. In QUIC (and HTTP/3), only the stream to which the lost packet belongs is blocked.
  • Faster connection establishment (0-RTT or 1-RTT).
  • QUIC includes its own flow control and reliability mechanisms.

Main differences in the table:

Feature	HTTP/1.0	HTTP/1.1	HTTP/2	HTTP/3
Protocol	TCP	TCP	TCP	UDP (QUIC)
Format	Text	Text	Binary	Binary
Default connection	Short-lived	Persistent	Persistent (multiplexing)	Persistent (QUIC Streams)
Multiplexing	No	Pipelining (imitation)	Yes	Yes
Header compression	No	No	HPACK	QPACK (in QUIC)
Server Push	No	No	Yes	Yes
HOL Blocking (TCP)	Yes (at request level)	Yes (at request level)	Yes (at connection level)	No (at stream level)

Example of an HTTP/1.1 request:

GET /index.html HTTP/1.1
Host: www.example.com
User-Agent: curl/7.64.1
Accept: */*

Conceptual example of an HTTP/2 request (binary):

// Request 1: Get /index.html
// :method = GET
// :path = /index.html
// :authority = www.example.com

// Request 2: Get /style.css
// :method = GET
// :path = /style.css
// :authority = www.example.com
// ...transmitted in parallel over one connection