HTTP/2 is a binary, multiplexed version of HTTP that reduces latency through header compression, stream prioritization, and server push, as defined in RFC 7540.

What is HTTP/2? Definition, examples and how it works

HTTP/2 is a major revision of the HTTP protocol, standardized in May 2015 as RFC 7540 and updated by RFC 9113 in 2022. It replaces HTTP/1.1's text-based, serial request-response model with a binary framing layer. This change allows multiple concurrent exchanges (streams) to be multiplexed over a single TCP connection, eliminating head-of-line blocking at the application layer.

The protocol organizes data into frames within streams. A single connection can hold hundreds of streams, with each stream carrying request and response messages broken into frames. Streams can be prioritized using a weighted dependency tree, so the server allocates bandwidth to important resources first. Header compression via HPACK (RFC 7541) reduces redundant header data, shrinking overhead from hundreds of bytes to a few. Server push enables the server to send resources the client has not yet requested, anticipating its needs. However, push is often disabled in practice due to caching complications.

HTTP/2 is a direct replacement for the HTTP/1.1 wire format but remains semantically compatible: methods, status codes, URIs, and header fields are unchanged. Most web browsers support it via TLS (with ALPN negotiation, using h2 identifier). It is the default protocol for HTTPS traffic on major web servers such as Apache, Nginx, and IIS. The key tradeoff is that its multiplexing benefits degrade under TCP packet loss, where a single lost packet can block all streams, a limitation addressed by HTTP/3 over QUIC.

Key facts

Defined in RFC 7540 (2015) and updated by RFC 9113 (2022).
Uses binary framing, with frames grouped into streams over one TCP connection.
HPACK header compression reduces redundant header overhead.
Server push allows the server to preemptively send resources before the client requests them.
Requires TLS with ALPN h2 in browsers; plaintext upgrade via h2c is rarely used.

How it works in practice

A web browser opens a single TCP connection to a server and requests an index.html file. The server begins sending the HTML and uses server push to send style.css and app.js simultaneously. As the browser parses the HTML, it finds links to those resources, but they are already arriving. Meanwhile, additional requests for images are multiplexed on stream IDs 5 and 7, all without opening new connections. HPACK compresses the repeated User-Agent and Cookie headers across these requests, saving hundreds of bytes per exchange.

Related terms

HTTP/3 QUIC HPACK ALPN SPDY Server Push

References

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