From HTTP/0.9 to HTTP/3: Evolution of Web Protocols Explained

HTTP/0.9

Initially defined informally, HTTP/0.9 only supported a single-line GET request and returned raw HTML without headers or status codes.

<code>GET /index.html</code>

<code>&lt;html&gt;
  &lt;body&gt;HELLO WORLD!&lt;/body&gt;
&lt;/html&gt;</code>

HTTP/1.0

Standardized in 1996, HTTP/1.0 introduced version numbers in requests, status lines in responses, headers (including Content‑Type and Content‑Encoding), support for POST and HEAD methods, and optional persistent connections.

Version number added to request line

Status line added to response

Headers introduced

Content‑Type for non‑HTML files

Content‑Encoding for compression (gzip, compress, deflate)

POST and HEAD methods

Optional keep‑alive connections

<code>GET /index.html HTTP/1.0
User-Agent: NCSA_Mosaic/2.0 (Windows 3.1)

200 OK
Date: Tue, 15 Nov 1994 08:12:31 GMT
Server: CERN/3.0 libwww/2.17
Content-Type: text/html; charset=utf-8
&lt;HTML&gt;
A page with an image
  &lt;IMG src="/image.gif"&gt;
&lt;/HTML&gt;</code>

HTTP/1.1

Released months after 1.0, HTTP/1.1 became the dominant version for a decade. Major improvements include persistent connections (keep‑alive), pipelining, chunked transfer encoding, enhanced caching controls, content negotiation, and the Host header for virtual hosting.

Keep‑alive connections by default

Pipelining to send multiple requests without waiting for each response

Chunked transfer encoding for dynamic content

Advanced cache‑control directives

Content negotiation (language, encoding, type)

Host header enables multiple domains on one IP

Cache Mechanisms

HTTP/1.1 adds strong cache (Pragma, Expires) and flexible cache‑control directives such as

no-store

,

no-cache

, and

max-age

. Conditional requests use

Last-Modified

and

ETag

to return 304 when resources are unchanged.

Additional Methods

Five new request methods were introduced: OPTIONS, PUT, DELETE, TRACE, and CONNECT.

HTTP/2

Standardized in 2015, HTTP/2 (based on Google’s SPDY) replaces the textual format with binary frames, enabling header compression (HPACK), multiplexing multiple streams over a single TCP connection, server push, and improved performance.

Binary frame layer

Multiplexed streams eliminate head‑of‑line blocking at the HTTP layer

HPACK header compression reduces overhead

Server push allows the server to proactively send resources

HTTP/3 (HTTP‑over‑QUIC)

HTTP/3, standardized in 2022, runs over QUIC, a UDP‑based transport that provides low‑latency handshakes, 0‑RTT data, connection migration, and independent flow control per stream, addressing TCP‑level head‑of‑line blocking.

0‑RTT and 1‑RTT Handshakes

QUIC uses TLS 1.3 handshakes with optional 0‑RTT data, allowing clients to send encrypted application data on the first flight after caching server parameters.

Ordered Delivery

Each QUIC packet carries an offset field; the receiver reorders packets based on this offset to ensure in‑order delivery despite UDP’s unreliability.

Connection Migration

QUIC connections are identified by a 64‑bit Connection ID, so changing network interfaces (IP address) does not break the connection.