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2.1 HTTP Versions

HTTP/1.0, HTTP/1.1, HTTP/2, and HTTP/3: persistent connections, framing, multiplexing, header compression, TCP head-of-line blocking, and QUIC.

Computer NetworksCreated Updated 2 min readhistorical

This is a historical learning note and may contain outdated or incomplete understanding.

1. HTTP/1.0

HTTP is a request/response application protocol. HTTP itself is stateless in the sense that each request carries the information needed to interpret it; applications add state through cookies, tokens, sessions, or other mechanisms.

HTTP/1.0 commonly used one TCP connection per request/response, although persistent connections were also deployed through extensions such as Connection: keep-alive.

2. HTTP/1.1

Persistent connections became the default model, reducing repeated TCP/TLS setup cost.

Message boundaries are represented with mechanisms such as:

  • Content-Length;
  • chunked transfer coding for responses/requests where length is not known in advance;
  • connection close in specific legacy cases.

Pipelining

HTTP/1.1 pipelining allows multiple requests to be sent without waiting for each response, but responses must remain ordered on the connection. A slow earlier response can therefore create application-level head-of-line blocking, and pipelining was never broadly successful in browsers.

3. HTTP/2

HTTP/2 introduces binary framing and multiplexes multiple logical streams over one TCP connection.

Major features include:

  • stream multiplexing;
  • HPACK header compression;
  • stream priorities (whose practical semantics evolved across implementations);
  • server push in the original design, though browser support has largely been deprecated.

HTTP/2 solves HTTP/1.x response-order head-of-line blocking between streams, but all streams still share one TCP connection. A lost TCP packet can temporarily stall delivery for every stream until TCP recovers it.

4. HTTP/3

HTTP/3 runs over QUIC rather than TCP.

QUIC provides independent reliable streams over UDP, integrates cryptographic handshaking with TLS 1.3, and supports connection migration.

A lost packet affecting one QUIC stream does not impose TCP-level head-of-line blocking across unrelated streams in the same way as HTTP/2 over one TCP connection.

5. Practical View

The evolution can be summarized as:

HTTP/1.x: textual messages, limited connection-level concurrency
HTTP/2: binary multiplexed streams over TCP
HTTP/3: HTTP semantics over QUIC with transport-level multiplexed streams

HTTP semantics—methods, status codes, headers, caching—remain conceptually separate from the transport version underneath.