HTTP Versions

Introduction to HTTP

The Hypertext Transfer Protocol (HTTP) is the foundation of data communication for the World Wide Web. It defines how messages are formatted and transmitted, and what actions web servers and browsers should take in response to various commands. Over the years, HTTP has evolved significantly to improve performance, efficiency, and security.

HTTP/1.0 (1996)

HTTP/1.0 was the first widely adopted version of the protocol. It was a stateless, request-response protocol where each request required a new TCP connection. This led to inefficiencies, especially for pages with multiple embedded resources (images, CSS, JavaScript).

• Connection Model: Short-lived connections (one request per connection).
• Header Fields: Minimalistic, often relying on implicit understanding.
• Content Type: Primarily for HTML, with basic support for images and other media.
• Limitations: High latency due to repeated connection setup, poor handling of multiple resources.

A typical HTTP/1.0 request looked something like this:

GET /index.html HTTP/1.0
Host: www.example.com
User-Agent: Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0)

HTTP/1.1 (1997, updated 1999, 2014)

HTTP/1.1 introduced significant improvements to address the limitations of HTTP/1.0. It brought persistent connections, pipelining, and more robust header handling, making web browsing much more efficient.

• Connection Model: Persistent connections (Keep-Alive) by default, allowing multiple requests over a single TCP connection.
• Pipelining: Allows clients to send multiple requests without waiting for each response, though head-of-line blocking remained an issue.
• Host Header: Essential for virtual hosting on web servers.
• Caching: Enhanced caching mechanisms.
• Chunked Transfer Encoding: Allows for dynamic content generation without knowing the content length upfront.

Key features introduced or standardized:

• Connection: Keep-Alive
• Host header
• ETag, If-Modified-Since for conditional requests
• Range requests for partial content

An HTTP/1.1 request example:

GET /resource.css HTTP/1.1
Host: www.example.com
User-Agent: Chrome/91.0.4472.124
Accept: */*
Connection: Keep-Alive

HTTP/2 (2015)

HTTP/2 represents a major leap forward, designed to address the performance bottlenecks of HTTP/1.1, particularly head-of-line blocking. It achieves this through binary framing, multiplexing, header compression, and server push.

• Binary Protocol: Replaces the text-based protocol with a binary one for more efficient parsing.
• Multiplexing: Allows multiple requests and responses to be sent concurrently over a single TCP connection without blocking each other.
• Header Compression (HPACK): Reduces the overhead of redundant header information.
• Server Push: Allows servers to proactively send resources the client might need before the client requests them.
• Stream Prioritization: Clients can indicate the priority of different requests.

HTTP/2 is typically used over TLS (HTTPS) for security, though it's not strictly required.

HTTP/3 (2022)

HTTP/3 is the latest major version, built on top of QUIC (Quick UDP Internet Connections), a new transport protocol developed by Google. QUIC aims to further reduce latency and improve reliability, especially on lossy networks.

• Transport Protocol: Uses UDP instead of TCP, leveraging QUIC.
• Reduced Head-of-Line Blocking: QUIC's stream multiplexing is more robust than TCP's, preventing packet loss in one stream from affecting others.
• Faster Connection Establishment: QUIC combines the transport and TLS handshake, significantly reducing round trips.
• Connection Migration: QUIC connections are identified by a connection ID, allowing clients to retain their connection even if their IP address or port changes (e.g., switching from Wi-Fi to cellular).

Key Differences and Evolution

Feature	HTTP/1.0	HTTP/1.1	HTTP/2	HTTP/3
Transport Protocol	TCP	TCP	TCP (typically)	QUIC (over UDP)
Connection Model	Short-lived	Persistent (Keep-Alive)	Persistent, Multiplexed	Persistent, Multiplexed, Connection Migration
Data Format	Text	Text	Binary	Binary
Multiplexing	No	Limited (Pipelining)	Yes (per connection)	Yes (per connection)
Header Compression	No	No	HPACK	HPACK
Head-of-Line Blocking	Yes (TCP level)	Yes (TCP level & Pipelining)	Reduced (HTTP level), but TCP HOL remains	Eliminated (QUIC level)
Server Push	No	No	Yes	Yes
Connection Establishment	TCP handshake + TLS handshake	TCP handshake + TLS handshake	TCP handshake + TLS handshake	QUIC (combined transport/TLS handshake)